The Paris Climate Conference and Forests: Ramifications of the Agreement

(click here to download a pdf of this article)

By Eric Walberg

The Paris Agreement was unanimously approved by delegates to COP21 and the role of forests in climate regulation is highlighted in several sections of the document. The Agreement is precedent setting in that it signals a global transition to a low carbon economy and establishes a working relationship among participating nations to move towards that goal. What are the implications for forests? What are the ramifications for forest owners and managers? Given the aspirational nature of the Agreement and the significant latitude provided to participating nations in how they meet their commitments, it is impossible to know in detail how things will play out. The following commentary outlines the impacts and opportunities for the forestry sector given what we know of the direction that has been established and the timing of the process.

Explicit References to Forests in the Agreement

Two sections of the Paris Agreement deal explicitly with forests.  Section 55 in the finance section acknowledges the importance of “adequate and predictable financial resources, including for results-based payments” … “for reducing emissions from deforestation and forest degradation”. This section also highlights “the role of conservation, sustainable management of forests and enhancement of forest carbon stocks, as well as alternative policy approaches, such as joint mitigation and adaptation approaches”. Finally, this section identifies “public and private, bilateral and multilateral sources such as the Green Climate Fund and alternative sources” as methods of funding forest protection.

Article 5 of the Agreement opens with a statement that “Parties should take action to conserve and enhance, as appropriate, sinks and reservoirs of greenhouse gases” … “including forests”. The following section of Article 5 acknowledges two approaches to forest preservation. The first reference is to pay-for-performance approaches such as REDD that monetize the carbon stored in forests. The second reference is to joint mitigation and adaptation approaches that attempt to link carbon and non-carbon benefits of forests in identifying preservation goals.

Likely Ramifications:
  • Increased global emphasis on protection and restoration of forests,
  • Increased funding to support forest protection and restoration in developing nations.
Possible Ramifications:
  • Establishment of new carbon credit markets and trading opportunities, possible increase in demand for forest products from geographic areas outside of the areas targeted for protection.

Sections of the Agreement That Will Also Impact the Role of Forests

Beyond the sections of the Agreement that explicitly reference forests, several sections of the document deal with issues that will influence the value and management of forests, such as land use tradeoffs, accounting and compliance with Intended Nationally Determined Contributions (INDCs), ratcheting of commitments over time, financial support of developing nations and opportunities for international credit trading.

Countervailing Land Use Goals

The Agreement frames forest protection in the context of the need to deal with a range of potentially conflicting efforts, including alleviation of poverty, food production, and adaptation to climate change. Article 2 sets the goal of “Increasing the ability to adapt to the adverse impacts of climate change and foster climate resilience and low greenhouse gas emissions development, in a manner that does not threaten food production”. Article 4 sites the need to “achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century, on the basis of equity, and in the context of sustainable development and efforts to eradicate poverty.”

Likely Ramifications:
  • Articles 2 and 4 of the Agreement highlight the need for efficient use of a finite land base to meet multiple objectives,
  • Opportunities will exist for:
    • Innovative low-carbon approaches to integration of forest and agricultural systems,
    • Economic development projects that maximize carbon sequestration and storage,
    • Forest revitalization and afforestation in geographic areas that bolster climate change adaptation, and,
    • Biomass energy production that contributes to meeting INDCs.

Regular Evaluation of Progress on Intended Nationally Determined Contributions

The Agreement establishes a schedule for participating nations to report progress towards achievement of the goals identified in their INDCs. Article 4, paragraph 9 states that “Each Party shall communicate a nationally determined contribution every five years

Likely Ramifications:
  • Development of a more complete and detailed global inventory of forest change over time.
Possible Ramifications:
  • Reduction in flow of illegal timber,
  • Reduction in conversion of forests to other uses.

Ratcheting of Intended Nationally Determined Contributions Over Time

The INDCs that were submitted prior to the Paris Conference are not sufficient to achieve the goal of limiting warming to 2 degrees C. The Agreement establishes a process for updating commitments over time to achieve the needed reductions in greenhouse gas emissions and/or increases in sequestration and storage. Article 15, paragraph 3 of the Agreement states that “The outcome of the global stocktake shall inform Parties in updating and enhancing, in a nationally determined manner, their actions and support in accordance with the relevant provisions of this Agreement”.

Likely Ramifications:
  • An increase in value of the carbon sequestration and storage services provided by forests as emission targets become more stringent,
  • Increased reliance on the services supplied by forests as an element of meeting INDCs.

International Offsets

Article 6 of the Agreement sets up a framework for cooperation among participating nations in meeting emission targets and allows for “internationally transferred mitigation outcomes”.

Possible Ramifications:
  • Another potential funding mechanism and/or carbon credit trading opportunity that could monetize forest services,
  • International agreements that seek to implement least cost solutions to meeting INDC targets.

Price on Carbon

Section 137 of the Agreement “recognizes the important role of providing incentives for emission reduction activities, including tools such as domestic policies and carbon pricing”.

Possible Ramifications:
  • If carbon pricing becomes a reality it would dramatically increase the value of the carbon sequestration and storage services provided by forests and could be a game changer for carbon credit markets.

Implications of the Gap in the U.S. INDC

Several recent studies indicate that the measures outlined in the INDC submitted by the United States are insufficient to meet the identified 2025 target of a 26 – 28% reduction in greenhouse gas emissions below the 2005 level. Enhanced efforts to meet the 2025 goal could take many forms.

Possible Ramifications:
  • Additional regulation of industrial sectors such as energy production, transportation, and manufacturing,
  • Additional regulation of non-industrial sectors including forestry and agriculture.

Closing Thoughts

The Paris Agreement lays the groundwork for a major transformation in the global economy. Achieving the goal of balance between greenhouse gas emissions and sequestration and storage by the second half of this century will require both a technological revolution and a transition to land use and development patterns that efficiently meet multiple objectives. Are we up to the challenge? Stay tuned, it’s going to be a wild ride!

Climate Change and Extreme Weather, Part 1: Trends & Projections

By Jennifer Hushaw

(click here to download a pdf of the full article or a one-page synopsis)

As the climate changes, we will not only experience a gradual change in average conditions, but also an increase in the frequency and intensity of some types of extreme weather and climate events. A number of these observed or anticipated changes have been highlighted in past bulletins, including extreme heat, heavy precipitation, and more intense drought. These extremes may pose the biggest climate change risk for forest ecosystems in the short-term. Extremes create conditions that are beyond what species have historically experienced and they can push natural systems past their threshold for resilience and recovery because there is generally little time or capacity for natural adaptation in those situations. This potential for irreversible impacts elevates extremes on the list of climate-related forest management concerns.

Climate is the statistics of weather. More specifically, the climate is the statistical distribution of any given weather variable (clouds, rainfall, temperature, wind speed, etc.), which generally resembles a bell curve (see figures, below). For instance, the average January temperature in New York City is 35⁰F, but it has been as warm as 70⁰F and as cold as -4⁰F. Climate change is, by definition, a change in that distribution.

There are a number of ways the distribution can change. As the climate warms and the average temperature increases, the entire curve will shift toward the right (see figure, below). This means more warm extremes, new record heat events, and conditions that used to be ‘extreme’ becoming more normal.shifted_mean

In some regions, the variability may also change (a.k.a. the width of the bell curve) due to changing dynamics that are also linked to climate change, such as changing sea ice conditions, shifts in the jet stream, vegetation change, etc. (see example in figure, below). The curve may become wider (more variability) or narrower (less variability), depending on the region.


In fact, a recent analysis of land temperatures over the northern hemisphere indicated that the bell curve is already shifting and widening as the planet warms—see this figure from Hansen et al. 2012 in New Scientist.


Importantly, the risk of extreme heat and heavy precipitation events goes up exponentially as average temperatures increase. A recent study quantified how the probability of extreme heat and heavy precipitation events changes in response to increasing global temperatures (see figure, below) and their results for the present-day agree well with what we have actually observed. They  found that the “probability of a hot extreme at 2⁰C [global] warming is almost double that at 1.5⁰C and more than five times higher than for present-day,” so the difference is “small in terms of global temperatures but large in terms of the probability of extremes” (Fischer and Knutti 2015).

Note: See Fischer and Knutti 2015, Figure 2 (a) and (b) for a graph showing how the probability of moderate extremes (blue; events that exceed the 99th percentile) and the probability of the largest extremes (red; events that exceed the 99.9th percentile) change relative to pre-industrial daily precipitation and temperature conditions, at a given warming level. Probability ratio (PR) compares the probability of an extreme in present-day or future climate to the probability of that extreme before industrialization—it is the factor by which the probability of an event has changed, i.e. a PR of 1.5 means an event is 1.5 times more likely to occur.


Observed Changes in Extremes

Note: Observations and projections summarized from IPCC Special Report on Extremes (2012) Table 3-1 and the Third U.S. National Climate Assessment (2014).

Extreme Heat

Since 1950, there have been more record hot days/nights than record cold days/nights, globally. This has also been true in the U.S.—in the 1950’s the ratio of record highs to record lows was roughly 1 to 1 and by the 2000’s the ratio was 2 to 1 (see figure, below). The data also suggest that many regions have experienced more or longer heat waves over this time period.



Heavy Precipitation

There have been large regional variations in heavy precipitation trends, but overall there are more regions with significant increases in heavy precipitation events than decreases.


There has been significant regional variation in drought trends—some regions (e.g. southern Europe and West Africa) have experienced more intense and longer droughts, but there are opposite trends in other places.


There is not much evidence for a change in flooding trends globally, but there have been significant trends in certain regions. For example, the southwestern United States has seen a decrease in flood magnitude, while the Midwest and New England have seen an increase (see Figure 3 (a) in Peterson et al. 2013 for a map of trends in flood magnitude). Globally, there has also been an observed trend toward earlier occurrence of spring peak river flows in snowmelt- and glacier-fed rivers.


Future Extremes

Extreme Heat

As discussed previously, the science indicates that we will experience more unusually warm days and nights as the climate warms. Similarly, we also expect an increase in the length, frequency, and/or intensity of heat waves over most land areas.

Heavy Precipitation

We will see an increase in the frequency of heavy precipitation events or the proportion of total rainfall that comes in the form of these heavy events. This is expected because a warmer atmosphere holds more water vapor when it is saturated, so when it rains in a warmer world more moisture can rain out. This trend is expected to be especially true in the high latitudes and tropics, as well as winter in the northern mid-latitudes.


It is expected that in the future we will experience more impactful ‘hotter droughts’ as a result of warmer temperatures. For example, a recent study in the Southwest and Central Plains of Western North America used 17 different climate models to forecast future soil moisture in those regions and found consistent predictions of severe drying by the end of this century, with an 80% likelihood of a decades-long megadrought between 2050 and 2100 (Cook et al. 2015). For a more in-depth discussion of future drought risk, you may want to re-visit our previous bulletin on global precipitation.

Model projections tend to agree on a future increase in the duration and intensity of drought in certain regions, including southern Europe and the Mediterranean, central Europe, central North America, Central America and Mexico, northeast Brazil, and southern Africa. Currently, poor model agreement makes it difficult to say how drought risk may change in other regions.


A lack of sufficient evidence and model agreement makes it difficult to say what may happen globally in terms of flood magnitude and frequency, but we at least expect that heavy precipitation events will contribute to more rain-generated local flooding, particularly in watersheds with topography that exacerbates the effects of heavy rainfall events (e.g., steep slopes, canyons that rapidly concentration flow).


How do we know an extreme event is related to climate change?

In recent years, there has been a lot of interest in understanding whether specific extreme events, e.g. the 2003 heat wave in Europe, Hurricane Katrina, or the 2012 Midwest heat wave, were caused by climate change. As with any phenomenon that arises from a number of factors, we can’t pin down the cause to a single factor. However, we can begin to understand how a particular factor contributed to the likelihood or probability of the event.

A helpful analogy is a car accident on a wet road. Was the car accident caused by the rainy conditions and wet road? Not exactly—there were other factors at play, including the curviness of the road, the speed of the car, visibility, the driver’s level of fatigue, and others. However, we can look at the statistics of road accidents and determine how much a wet, rainy road increases the chances of an auto accident. In fact, there is an emerging area of climate research that is doing similar work to isolate how much anthropogenic climate change has increased or decreased the likelihood of certain climate extremes, compared to natural background conditions.

This type of research has revealed that climate change very likely contributed to a number of heat waves and heavy precipitation events, such as the 2010 Russian heat wave (Otto et al. 2012), the 2003 European heat wave (Stott et al. 2004), record summer temperatures in Australia in 2013 (Lewis and Karoly 2013), and flooding in England and Wales in 2000 (Pall et al. 2011). Another recent analysis looked at heavy precipitation and hot extremes globally and found a discernable human influence (Fischer and Knutti 2015).


Climate change is changing the odds of certain extreme events—it is loading the dice, so we are more likely to experience certain extremes, such as heat waves, heavy precipitation event, or severe drought.  As more of these events take place, researchers can begin to assess how the probabilities are changing, potentially improving our ability to estimate future risk from climate extremes.

A useful way to think about climate change adaptation in forestry is through the lens of risk management. The same is true for assessing the risk posed by climate extremes. Breaking risk into its component parts (see figure, below) reveals the best strategies for risk reduction.


The first two components of this equation are related to the climate variable, e.g. drought, and the last two are related to the forest, e.g. location, stocking, species mix. Taken together, these four variables determine the overall risk posed by a particular climate extreme.

By leveraging the right management approach, you can reduce your exposure and/or vulnerability to a given climate stressor. Although, you can’t do much to change the first two variables, beyond getting more (and better) information. This is where new data about changing probabilities can play an important role in improving our assessment of risk with regard to climate extremes. Manomet will be closely following emerging research in this area and updating members as the projections become clearer.



Cook, B.I., T.R. Ault, and J.E. Smerdon, 2015: Unprecedented 21st-century drought risk in the American Southwest and Central Plains. Sci. Adv., 1, no. 1, e1400082, doi:10.1126/sciadv.1400082.

Fischer, E.M. and Knutti, R. 2015. Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes. Nature Climate Change. 5(6): 560-564. DOI: 10.1038/NCLIMATE2617

Hansen, J., M. Sato, and R. Ruedy, 2012: Perception of climate change. Proc. Natl. Acad. Sci., 109, 14726-14727, E2415-E2423, doi:10.1073/pnas.1205276109.

IPCC, 2012: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, UK, and New York, NY, USA, 582 pp.

Lewis, S. C. & Karoly, D. J. 2013. Anthropogenic contributions to Australia’s record summer temperatures of 2013. Geophys. Res. Lett. 40, 3705-3709.

Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. doi:10.7930/J0Z31WJ2.

Otto, F. E. L., Massey, N., van Oldenborgh, G. J., Jones, R. G. & Allen, M. R. 2012. Reconciling two approaches to attribution of the 2010 Russian heat wave. Geophys. Res. Lett. 39, L04702.

Pall, P. et al. 2011. Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000. Nature 470, 382-385.

Stott, P. A., Stone, D. A. & Allen, M. R. 2004. Human contribution to the European heatwave of 2003. Nature 432, 610-614.


The Role of Forests in the Paris Climate Conference

By Eric Walberg

(click here to download a printable pdf of this article)


The Paris Climate Conference, officially known as the 21st Conference of the Parties (COP21), will be held November 30 – December 11 in Le Bourget, France.  The goals for the conference, as stated by French Minister of Foreign Affairs Laurent Fabius, include establishing a legally binding international agreement to limit global warming to 2 degrees Celsius. In preparation for the conference, participating countries are submitting Intended Nationally Determined Contributions (INDCs) that state their emission targets and provide high-level outlines of how the targets will be achieved. As of November 18, 2015, 140 INDCs have been submitted representing 167 countries. Forest management is a component of many of the INDCs with approaches including afforestation, reforestation, reduction in forest degradation, and increased stocking explicitly mentioned.

History and Context

The Paris Climate Conference builds on 25 years of international effort to establish an effective and equitable approach to addressing climate change. The United Nations Framework Convention on Climate Change (UNFCCC) was adopted in 1992. The UNFCCC establishes the objective of “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” Commitments established in the UNFCCC include formulating and implementing national programs to reduce and sequester greenhouse gas emissions. The Copenhagen Accord of 2009 established the goal of limiting global warming to 2 degrees Celsius and acknowledged that “deep cuts in global emissions are required according to science”.

Role of Forests in the Draft International Agreement

The draft international agreement to be considered at the Paris conference references forests primarily in the context of two competing approaches to minimizing deforestation in the tropics.  The two approaches explicitly mentioned in the draft agreement are Reducing Emissions from Deforestation and Forest Degradation (REDD) and the Joint Mitigation and Adaptation Mechanism (JMA). REDD is a market-based approach to creating financial value for the carbon stored in forests. JMA is a non-market-based approach to integrating climate change mitigation and adaptation through linking of a range of factors including agriculture and forestry, protection of biodiversity and support of indigenous populations.  REDD has been tested in a variety of tropical nations and has proven to be difficult to implement due to a number of factors including insufficient funding, questions on tenure and ownership of forestland, insufficient or unstable governance structures, and lack of sufficient on-the-ground technical expertise. One possible outcome of the Paris conference is an increase in support of the Green Climate Fund that could be used for direct payments in support of either REDD or JMA.

Role of forests in Individual Nation Commitments

The INDCs submitted prior to the conference include a broad range of commitments across sectors that include energy efficiency, transition to low-carbon energy production, improved transportation efficiency, and use of forests to enhance carbon sequestration and storage. In preparation for the Paris conference, the United Nations Environment Program has released their 2015 Emissions Gap Report. The 2015 report, which is the most recent annual assessment of progress towards meeting the global 2 degree goal, includes an analysis of the potential for greater forest-related mitigation. The following map from that analysis is a depiction of national intention to undertake forest-related mitigation activities through the INDCs and several related previous commitments.


Examples of the types of forest-related commitments included in the INDCs:

  • China has committed to increasing forest stock volume by 4.5 billion cubic meters compared to 2005 levels.
  • India will create an additional carbon sink of 2.5 to 3 billion tons of CO2 equivalent through additional forest and tree cover by 2030.
  • Honduras includes afforestation/reforestation of 1 million hectares of forest by 2030 in their commitments.

It is likely that forest management and carbon market opportunities will also be influenced by efforts to reduce emissions in other sectors. The U. S. Clean Power Plan (CPP), for example, is a key component of U.S. efforts to reduce emissions from existing power generation facilities. Under the CPP, states and regions are allowed to establish emission trading programs as one component of compliance.   This potential expansion of the market for carbon credits could provide incentive for additional afforestation and forest management to maximize carbon sequestration and storage.

Implications for Forest Owners and Managers

While it is not likely that any of the decisions taken at the Paris conference will directly impact the commercial forestry industry, the international agreement that will be negotiated and the INDCs will influence land use decisions and the role of forests in mitigating and adapting to climate change for many years to come. As human population continues to grow and living standards improve, existing tensions between different land uses, including food production, carbon sequestration and storage, protection of biodiversity, and production of wood and fiber will be exacerbated.  How these land use questions are resolved, in conjunction with changing carbon credit markets, will alter the policy and economic contexts that forest owners and managers operate in.

In addition to substantial global reductions in GHG emissions, all of the scenarios under consideration to limit warming to 2 degree C depend on the eventual inclusion of “negative emission technologies”. One such technology that is under consideration is bioenergy combined with carbon capture and storage (BECCS). Theoretically, BECCS contributes to negative emissions through the following steps:

  • Sequestering carbon through the growing of biomass fuels
  • Use of the resultant biomass in energy production to supplant burning of fossil fuels
  • Capturing and storing the carbon that results from the burning of the biomass

BECCS is an unproven technology so it is not possible to know at this point the feasibility of large-scale deployment or the potential role of forests as a fuel source.

Regardless of the specific outcomes of the Paris conference, it is safe to say that forests will play an increasingly important role in meeting multiple goals. Manomet will provide Climate Smart Land Network members with follow-up analysis at the conclusion of the conference.

El Niño Update

October 2015

(click here to download a pdf of this article)

El Niño is the warm phase of an ocean-atmosphere circulation pattern in the equatorial Pacific Ocean, known as the El Niño-Southern Oscillation (ENSO), which is responsible for a large fraction of the year-to-year variability in global climate. In previous CSLN bulletins we have highlighted the role of ENSO in shaping short-term variability in global temperature.

How it works:el-nino-la-nina-figure

The trade winds typically blow east to west around the equator, pushing warm surface ocean water so it piles up in the western Pacific near Indonesia. When these winds strengthen, it causes upwelling of deep, cold ocean water near the west coast of South America and we see below-average surface ocean temperatures in this region, also known as a La Niña. When these winds weaken (every two to seven years), the warm water “sloshes back” toward South America leading to the above-average surface ocean temperatures of an El Niño event (see figure, right).

Why it matters:

These changes alter global atmospheric circulation patterns and consequently have a big influence on temperature and precipitation throughout the world. In particular, ENSO has implications for:

  • Global Temperature – During an El Niño, enough extra heat is transferred from the ocean to the atmosphere that average global temperatures can rise for a period of time, which is why there is a good chance that this year will beat 2014 as the warmest year on record if there is an El Niño.
  • Regional Weather – The domino effect caused by the warmer ocean surface during an El Niño leads to a shift in the jet stream that has implications for weather patterns in the United States, with the effects here predominantly felt in the fall and winter months. 
    The weather in some regions, like the Gulf Coast or southern California, is more directly influenced by El Niño conditions than other regions, such as New England, where we are more likely to experience a change in regional weather if it is a particularly strong El Niño event. Predictions about specific impacts are highly uncertain, but it is generally associated with:

    • Cooler and wetter-than-average conditions along the Gulf Coast (Texas to Florida)*
    • Above-average temperatures and below-average snowfall in New England
    • Warmer temperatures and less snowfall in the upper Midwest to Great Lakes areas
    • Drier-than-normal conditions across the Great Lakes to the Ohio River Valley
    • More severe weather in the southern U.S.
    • More eastern Pacific hurricanes and fewer Atlantic hurricanes
    • Wetter weather in southern Californiawinter_el-nino_pattern_map
  • Forest Productivity & Fire Frequency – Weather changes associated with ENSO affect plant productivity and these changes cascade up through the food web shaping entire ecosystems. For example, El Niño years can flip the Amazon region from being a carbon sink to a carbon source, wetter periods during La Niña episodes have been linked to more successful seedling establishment in some Australian tree species, and ENSO-induced droughts contribute to increased wildfires in some places.

Fire regimes in the western continental U.S. and western boreal forests of North America are particularly sensitive to ENSO-driven climatic change, especially the interaction of ENSO with another climate phenomenon known as the Pacific Decadal Oscillation (PDO). Taken together, these climate patterns explain a notable portion of the fire history in these regions.

ENSO/PDO activity drives fire behavior by influencing fuel moisture. Wildfire frequency, severity, and total area burned are affected by seasonal precipitation, temperature, and atmospheric conditions (specifically the frequency of blocking high pressure systems and extreme fire weather), but the impacts of ENSO are variable and regionally-specific. For example, El Niño typically brings wetter winter-spring conditions to the Southeastern and Southwestern U.S. and the area burned is reduced. In contrast, El Niño events tend to bring warmer and drier winter-spring conditions to the Pacific Northwest and the result is increased wildfire activity during those seasons, but it also typically leads to more precipitation in the summer, coinciding with fewer large fires in the warmer months.

ENSO Status Update:

“There is an approximately 95% chance that El Niño will continue through Northern Hemisphere winter 2015-16, gradually weakening through spring 2016” (NOAA CPC). These estimates are based on several indices generated from monitoring air pressure changes, sea surface temperatures, outgoing longwave radiation, and wind in the equatorial Pacific. Each of these variables capture a component of the ocean and atmospheric dynamics that are part of the ENSO phenomenon.winter_el-nino_outlook_Oct2015

How might climate change influence future ENSO events?

There are a number of ingredients that combine to produce ENSO (atmospheric circulation, ocean convection, cloud cover, etc.) and climate change will affect all or most of these, so it is reasonable to expect that ENSO may change in the future. While some recent research suggests there is a chance for an increase in extreme El Niños, at this time it is difficult to say what the net effect will be on the strength and frequency of El Niño events. One thing that is certain is that climate change may enhance some of the impacts from ENSO events, e.g. warmer average temperatures may turn an El Niño-induced dry period into severe drought, or the biasing of precipitation toward heavy downpours in a warmer atmosphere may exacerbate flash flooding risk in areas where El Niño brings increased precipitation.

Note: For El Niño updates and excellent explanations of emerging and on-going research on the topic of ENSO, check out the NOAA ENSO blog

Forest Pests and Climate Change

(Click to download a pdf of this full article or a one-page synopsis)

By Jennifer Hushaw

Part 1: Overview of Climate-Pest Interactions

Among the many potential impacts of climate change, changes in insect and disease populations rise to the top as the most immediate and possibly significant impact on our forests. This is because of the destructive potential of forest pests and the direct link between climate and pest survival or spread. In particular, climate influences:

  • Frequency and intensity of outbreaks
  • Spatial patterns, size, and geographical range of outbreaks
  • Life cycles, range shifts, range expansions or contractions

Being ectothermic, insects are particularly sensitive to temperature, as it directly influences their metabolic rate, consumption, development, and the timing of life history stages. Water availability is also an important factor determining the interaction between plants and insects. Forest pathogens are similarly sensitive to temperature and moisture conditions. As a result of this sensitivity, these organisms will be directly affected by changing climate, in addition to being indirectly influenced via climate change impacts on other organisms, such as their host species.

Forest insects and pathogens have a number of characteristics that will allow them to rapidly respond to climate change, including: (1) physiological sensitivity to temperature, (2) high mobility, (3) short generation times, and (4) high reproductive potential.

Direct Climate Impacts

A number of climate change-related variables will have direct impacts on the population dynamics of forest insects and pathogens:

  • High overall temperatures, especially milder winters
    • Greater over-winter survival
    • Increased spore production and infectiousness
    • Decreases in insect populations at a certain level of warming, as most insects are susceptible to heat stress between 82 and 90⁰F
  • Longer and warmer growing season
    • Lengthening of reproductive season
    • Accelerated life cycles; increase in number of generations per year
    • Earlier appearance in spring
  • Changing snow pack
    • Affecting overwinter survival
  • Climate variability
    • Affecting performance and survival
Indirect Climate Impacts

Pests will experience the indirect effects of climate change through the following avenues:

  • Host plants
    • Distribution of primary (or alternate) host plants
      • Reduced/shifting habitat suitability may result in the loss of suitable host plants for some pest species within their preferred climate niche
      • Changing habitat suitability may also cause tree stress and increased susceptibility to attack
    • Nutritional quality
      • Elevated CO2 and temperature typically increases the concentration of leaf carbohydrates and decreases nitrogen content, lowering nutritional value
        • This can lead to increased herbivory from “compensatory feeding” – herbivores consume more low-quality food to meet their nutrient needs
      • Plant resistance
        • Little is known about the mechanisms by which increased CO2 and temperature affect plant production of secondary metabolites (defense chemicals), which deter feeding – at this time, we only have observational data that indicates climate-induced changes vary by species and context
        • Conditions that promote increased plant growth may be a double-edged sword because they are often associated with declines in plant production of defense compounds, which is a benefit for certain insects
      • Phenology
        • Accelerating phenology due to warming temperatures can lead to a mismatch between plants and associated organisms, which can be positive or negative for plant life – negative when it leads to a misalignment between plants and their pollinators (this is especially an issue for specialist plant-pollinator interactions) and positive when it reduces the frequency or severity of insect/pathogen attacks
      • Rate of plant development
        • When the rate of plant development shifts relative to insect development, it can amplify or minimize consequences of herbivory
  • Community interactions with:
    • Natural enemies (predators, parasitoids, pathogens), e.g. shifting bird populations may increase predation on arthropods in some areas, increasing the strength of top-down control on those pest populations
    • Competitors
    • Mutualists
  • Abiotic damage that increases tree stress and makes hosts more susceptible to attack*, such as: (1) Storm Damage, (2) Drought, and (3) Extreme heat events.

*Note: Pests can generally be divided into two categories: primary pests that can typically attack healthy trees and secondary pests that tend to attack trees that are already weakened by a predisposing factor like drought, water-logging, or injury (e.g. ambrosia or ips engraver beetles). It is likely that the biggest impacts from abiotic damage will come from these secondary pests.

Part 2: Summary of Anticipated Impacts

As with projections of tree response to climate change, we expect the response of insects and pathogens to be species-specific, or at least to vary depending on host type and feeding guild (e.g. defoliators, gall makers, wood borers, etc.). However, there are some general predictions that can be made:

Asynchrony/Ecological Mismatches

It is likely that projected changes in seasonality will lead to instances where the life cycle or developmental stages of host species and pests are no longer aligned, which could exacerbate or alleviate pest impacts, depending on the species involved. For example, insects that typically feed on young, nutrient rich foliage may be negatively impacted if the growing season begins earlier and causes faster leaf maturation after budbreak, especially if the timing of their spring emergence does not change. Likewise, the timing of spore release by pathogens can be an important determinant of disease incidence and severity, but changing climate conditions are likely to change those interactions. These issues of synchronicity are at the heart of many complex species interactions that may be disrupted by climate change because pests, hosts, and predators may all have different sensitivities to changing climate and varying levels of tolerance and/or plasticity to deal with those changes. Therefore, it is likely that in the future the rate or timing of development may no longer be in alignment among species that have historically been tightly linked.

Range Shifts/Redistribution

In addition to being misaligned in time (as described above), hosts and pests may become misaligned geographically due to climatically-driven changes in species ranges. Generalist pest species tolerate a wide range of climate conditions, are highly mobile, and use a variety of host species, so they will likely fare better as their ranges expand into northern regions. Whereas, more specialized species, such as those commonly found in tropical areas, are likely to see their ranges shift entirely, or contract in instances where migrating populations encounter hard (e.g. continental edges) or soft (e.g. soil moisture) range boundaries. These shifts may be good or bad for forest ecosystems, depending on the species and region in question.

In a general sense, we expect to see the following trends in pest distribution:

  • Expanded northern ranges
  • Invasion of new habitat and forest types
  • Range shifts toward poles and higher elevations (most shifts in temperate regions)
  • Better survival and increased impact from poleward populations

Note: In some cases, poleward populations may be locally adapted to colder temperatures, so there may not be as much of an advantage conferred by warmer temperatures in those regions.

Changing Pest Populations and Outbreak Frequency

Some research suggests that a warmer world doesn’t necessarily mean more forest pest impacts because we may mostly see distribution shifts toward higher latitudes and altitudes, rather than an increase in the overall incidence of pest outbreaks. However, climate change is also likely to amplify abiotic stressors, such as drought, extreme heat, and increased storm strength, which creates conditions that are favorable to more frequent and intense outbreaks.

Herbivorous insects will generally fare well because warmer temperatures will increase winter survival, promote faster development rates, and sometimes allow their populations to grow faster than normal because they can complete more life cycles in a season. That last characteristic (increased voltinism, i.e. number of generations per year) is particularly troubling because it will lead to more herbivory. Multiple generations of herbivores can do significant damage in a single season, especially for long-lived plants (increased likelihood of mortality and impacts on future growth and reproduction) and conifers (they don’t typically releaf and their resin defenses can be over-ridden by large numbers of attackers). In fact, there is evidence that a similar increase in herbivory happened in North America during a past period of global warming known as the Paleocene-Eocene Thermal Maximum approximately 55.8 million years ago.

Increased Pathogen Infectiousness

Evidence suggests that pathogens, including those that affect tree species, are likely to increase their infection rate under increased humidity and temperature conditions associated with climate change. These organisms are also generally able to evolve, adapt, and migrate more quickly than their long-lived hosts, so their role in forest disturbance regimes will probably increase. An increase in pathogen development and survival rates, disease transmission, and host susceptibility will have deleterious effects on forest ecosystems, but there will also be some subset of pathogens that actually decline with warming and lead to improved conditions for their host species.

Increased Vulnerability in Water-Stressed Regions

Some of the most obvious and immediate pest-related impacts will occur in regions with reduced precipitation and more frequent or severe drought conditions. This is particularly true for insect groups and pathogens that typically affect water-stressed hosts – the increased bark beetle activity coincident with drought in the western U.S. is a prime example. Drought conditions create physiological stress for trees that increases their susceptibility to attack and reduces their ability to survive and/or recover. A recent meta-analysis (Jactel et al 2012) of drought effects on damage by forest insects and pathogens found that primary damaging agents (i.e. insect or fungal species that can develop on healthy trees) living in wood caused lower damage than those living on foliage, indicating that the type of feeding substrate was very important for the level of pest damage.

Part 3: Regional Pest Highlights

The following section contains several regional examples of forest pests and pathogens that are likely to be influenced by climate change, including a brief, high-level overview of how climate is anticipated to impact each species.


            Spruce Budworm

Current research on the effects of climate change on eastern spruce budworm (SBW) suggests that there will be an increase in its range at higher latitudes and higher altitudes. This is because SBW is limited primarily by cool summer temperatures, which prevents the eggs from hatching before winter and does not give the larvae sufficient time to find winter shelter. Warmer summer temperatures will allow the pest to move into new territory and that has already been seen at unusually high latitudes on the north shore of the Saint Lawrence River in Quebec (c. 2009).

            Hemlock Woolly Adelgid

The Hemlock Woolly Adelgid (HWA) is vulnerable to low temperatures and its continued spread is primarily limited by minimum winter temperatures, rather than hemlock abundance. In particular, studies have found that an average winter temperature of 23⁰F (-5⁰C) is limiting, which historically meant that about half of the Northeast region was too cold for HWA (including upstate New York and most of Vermont, New Hampshire, and Maine). However, warming temperatures will allow for HWA range expansion – by mid-century half of the area that is currently temperature-limited will become suitable and by the end of the century (under the higher emissions scenario) the entire Northeast will have average winter temperatures above 23⁰F. HWA is an example of a pest that will be directly influenced by climate change, with temperature conditions becoming progressively more suitable for its migration northward and into higher elevation areas. 

Gypsy Moth

Gypsy moth is another example of an insect that is limited by cold in the northern portions of its range. A number of studies indicate that increasing temperatures will lead to an increase in defoliated area and an expansion of gypsy moth populations into new regions. However, precipitation changes are also important, as evidence suggests that a warmer and drier climate will actually decrease defoliated area. Drier conditions can also reduce the buildup of Entomophaga maimaiga, a lethal fungus that thrives during wet spring weather and was introduced from the gypsy moth’s native range in Japan in the early 1900’s as a way to control its population in the U.S.


            Southern Pine Beetle

Model projections and observed changes of Southern Pine Beetle (SPB) populations generally indicate that rising temperatures lead to more outbreaks. Winter minimum temperatures below 6.8⁰F (-14⁰C) cause population declines, but in recent years warming temperatures have allowed these insects to move north into areas where cold was once limiting, including the New Jersey pinelands, Long Island, and, most recently, Connecticut. Although, seasonal changes in temperature are also relevant because increases in winter and spring temperature are projected to increase outbreaks, while increases in fall temperatures will tend to ease outbreaks. The intensity and area of outbreak is also related to precipitation levels, with more precipitation being beneficial for the insects, although it is a less important factor than temperature.

On a positive note, there is evidence that extremely hot summer temperatures are lethal for SPB, leading to increased mortality, reduced activity, and hindered flight. Therefore, it is possible that future increases in extreme heat events in the southeastern U.S. will provide a benefit in terms of SPB population control. Whereas, pine stands in the northern stretches of the beetles’ range will likely see the greatest increase in beetle activity and impact.

            Ips Engraver Beetles

Ips engraver species can be cold-limited in the sense that low temperatures disrupt egg development and synchronized flight activities. These species can also see a reduction in the number of generations per year when cold temperatures persist over a long winter. In this way, climate warming may directly benefit ips by increasing their reproductive rates. In fact, several studies of Ips species in Scandinavia have indicated that higher temperatures will lead to an increase in the frequency and length of late summer swarming events in those regions, as well as an increase in the number of generations per year.

Ips rarely attack healthy trees and instead tend to target trees that are under stress, a condition that is likely to increase in prevalence as various climate stressors interact to increase physiological stress on trees. Drought stress is one example of a condition that increases the risk of ips attack and there is some evidence that decreased precipitation, and the consequent reduction in host tree resistance, contributed to ips outbreaks in the southwest U.S. in the early 2000’s. Projections of increased drought stress under future precipitation patterns may contribute to an increased risk of ips attack.

            Fusiform Rust

A number of climate-related factors influence the extent and severity of fusiform rust infection, including temperature, humidity, and late winter/early spring weather. However, the disease is already widely distributed throughout the host range, so it is likely that climate change will not directly cause an expansion of the affected area. Instead, experts expect this disease to experience indirect climate impacts via changes in the distribution of its host species as a result of rising temperatures, e.g. increased planting of loblolly pine in northern regions or migration of pine (and alternatively, oak) hosts from coastal areas into the Appalachian Mountains.


            Mountain Pine Beetle

As with many other North American forest pests, the latitudinal and elevational limits of the Mountain Pine Beetle (MPB) range are delineated by climatic conditions related to average annual minimum temperature. The beetles generally cannot survive to complete successful brood development in places where this average minimum is less than -40⁰F (-40⁰C). Given that the range of potential hosts is far more extensive, there is significant potential for MPB to expand under the right climate conditions. Warmer winters in British Colombia with an absence of cold snaps sufficient to kill MPB (a week or more of temperatures at or below -31⁰F) have already allowed the insect to have outbreaks in more northerly areas. Studies conducted to date generally predict MPB will continue to expand northward, eastward, and toward higher elevations, with the potential for a reduction at lower elevations in the northwestern region of the U.S. due to future climate-related losses of suitable host species in those areas.

Sudden Oak Death

Sudden Oak Death (SOD) is caused by a fungus-like water mold called Phytophthora ramorum, which produces spores that spread easily in warm, wet conditions, e.g. it is often transmitted when rainwater splashes the spores onto susceptible plants. Extreme weather events contribute to mortality from SOD – heavy rains and extended wet weather create optimal conditions for infection and mortality results when this is followed by extended dry periods, because infected trees are not able to manage water as effectively. Unfortunately, researchers expect climate change, particularly increases in temperature and coastal fog, to exacerbate the effects of this pathogen and shift the at-risk area northward. Alternate hosts include a variety of woody species, especially bay laurel in California. Notably, several studies have highlighted the potential for this pathogen to colonize the southeastern U.S., given the climatic conditions and distribution of potential host species in that region.


            Emerald Ash Borer

Emerald Ash Borer (EAB) have very low supercooling points (the temperature far below freezing that insects can survive through physical and chemical changes in their bodies), but exposure to temperatures at or below 32⁰F (-30⁰C) can cause overwintering mortality and help keep their populations in-check. In this sense, warming temperatures are likely to increase the rate of overwinter survival and potentially allow EAB to colonize previously unsuitable areas.

However, these insect do express some phenotypic plasticity in terms of their cold tolerance – while they can successfully acclimate after being exposed to colder temperatures over several months, they will lose that cold tolerance (i.e. deacclimate) if they experience warm mid-winter temperature fluctuations, and it is not reversible. This means that a mid-winter warm spell may cause EAB to deacclimate and then suffer mortality during the next cold snap because they have lost their cold tolerance. A potential opportunity, in terms of EAB population control, is the projected increase in the likelihood of extreme warm winter events associated with climate change.


There are hundreds of pests and pathogens, both native and introduced, which interact with the forest ecosystems we manage. A challenge is that there are widely varying levels of knowledge about the physiology, life cycle, and climate niche from one organism to the next. The list of species for which researchers have specifically addressed the question of climate impacts is fewer still. The pests and pathogens highlighted in Part 3 are some examples of higher-profile biotic threats for which we have some of this information. Although, there are many others where there is a weak climate link (especially in cases where climate is not the dominant limiting factor) or there is a lack of literature discussing the pest specifically in the context of climate change. Examples include: Asian Longhorned Beetle, Oak Wilt Disease, Dogwood Anthracnose, Pear Thrip, and many others. There is also the practically inevitable reality that new species will continue to be introduced from abroad for which we will have very little initial information.

Given the lack of complete information about climate change impacts on the catalogue of forest pests and diseases, it is useful to take a general, high-level view of pest-climate interactions, such as that presented in Part 1 and 2 of this bulletin. The best strategy is often to identify the life cycle characteristics or physiological limits of a particular pest that are most likely to be impacted by changing climate; for example, a need for synchronicity with budbreak of a particular species, level of cold tolerance, vulnerability to mortality from climate variability and temperature extremes, or a high degree of host specificity. This is where local knowledge and personal experience with a particular pest and forest type becomes really valuable for anticipating how climate, pest, and host may interact in novel ways in the future. Additionally, the importance of monitoring for detecting early changes in pest behavior or abundance cannot be overstated, so it is beneficial to proactively have those monitoring systems in place on your land. However, staying alert to new information is also key, especially in terms of looking beyond your ownership and being aware of pests that may potentially move into your area from other regions as a result of climatic shifts. This bulletin will act as a foundational document on the subject of forest pest-climate interactions and, going forward, the Climate Smart Land Network will continue to monitor and highlight newly documented links between changes in regional climate and important forest pests and pathogens.

So what can land managers do now?

Insects and disease have always been recognized as serious threats to forests and as a result they have received significant research and communication funding. This includes town tree wardens, university, state, and federal funding and support. The whole wood product transport quarantine system is designed to address these issues.

But things are changing rapidly and we need all the eyes and ears we can get.  Foresters and arborists, are ideal data gathers, question askers and teachers in this situation. Stay connected with state insect and disease departments as well as arborist information sources. Watch for new conditions in the woods and report them to these same organizations. If there are any research or education efforts existing, explore becoming part of those efforts. Spread your knowledge to others.



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New Forest Monitoring Information

Implementing simple and cost-effective forest monitoring is an important part of climate-smart management, as it helps practitioners move beyond anecdotal observations to real baseline data and provides a mechanism for feedback over time. A helpful one-page document with a summary of these ideas and some example monitoring protocols can be downloaded here.

Certainty and Uncertainty in Climate Change and Forest Response Part 2: The Forest Response

By Jennifer Hushaw & Si Balch

(Click to download a pdf of this complete article or a one-page synopsis)

Even if we could perfectly predict the details of future climate change, there would still be uncertainty about how ecological systems will respond. Most of the information we have about how forests respond to long-term climatic changes is related to the northward migration of species following the last deglaciation, but this is not necessarily a good proxy for the change we are experiencing today because the current rate of warming is much faster and the landscape is not the barren frontier it was after the glacial retreat – now the movement of species is complicated by other factors, including inter-species competition.

A great deal of work has been done to model how habitat suitability is likely to change in the future for individual tree species and, despite the inherent uncertainty in these models, the results provide some sense for how the species mix may change in various regions – a topic that will be explored in a future bulletin.

The following list includes some key topics in forest response to climate change and describes the aspects that are well-understood, as well as the major areas of uncertainty in each arena:

Response of Insects & Disease

The interaction of changing climate with both native and invasive insects and diseases is likely to be one of the most immediate and drastic climate-related impacts. While these pests are better understood than many of the following topics, there is still considerable uncertainty about life-cycles and population dynamics in new conditions – some pests may benefit from these changing conditions, while others may be held in check.

Warmer Temperatures

Increasing average temperatures and a longer growing season will provide an opportunity for increased forest productivity, especially in northern latitudes. Although, regions with increasing seasonal water stress may see fewer productivity gains than wetter areas.

CO2 Fertilization

Higher ambient CO2 levels can increase plant growth rates. The magnitude of additional carbon storage and whether the growth response will be offset by losses in productivity due to heat stress or other climate-related factors remains uncertain. There is likely to be little if any increase on poor sites where other nutrients are limiting.

CO2 & Water-use Efficiency

Higher ambient CO2 levels can also increase plant water-use efficiency (the ratio of water loss to carbon gain). This effect varies from one species to the next because there is variability in how plants regulate water loss during dry conditions, so the net gains (globally) are still uncertain.


The latest genetics research has revealed that some of an organisms genes can be turned “on” or “off” by external environmental factors and those changes can sometimes be inherited by offspring (a.k.a. transgenerational epigenetics). This is how plants with the same genome can flower at different times in response to different local temperatures. It is also the mechanism whereby environmental stressors experienced by the parent plant affect the next generation.

This has important implications for the adaptability and potential plasticity of different tree species in a novel climate. It suggests that some species may be more resilient to change than we might expect based on their current response to stress, since new environmental conditions can stimulate changes in gene expression that create a different response. For example, some research has shown that a parent plant exposed to repeated drought conditions can produce seedlings that express more drought tolerance. The role of epigenetics in shaping plant population response to climate change is very uncertain and is just beginning to be researched.

Fundamental, Realized, and Tolerance Niche

Each species has a fundamental niche – the full range of conditions and resources under which it can persist indefinitely – but species don’t live there, they live within a smaller realized niche – the places where competition doesn’t preclude them. Some researchers have also introduced the idea of a tolerance niche – this is a marginal zone beyond the fundamental niche in which species can survive, even if they can’t maintain self-sustaining populations (Figure 1, below). However, many of our assumptions about future changes in climate suitability are based only on a species’ current range (its realized niche) rather than the complete range of conditions (its fundamental or tolerance niches), lending uncertainty to habitat shift projections. Considering these broader niches suggests that species may be capable of persisting under a wider set of climate conditions than where they currently grow.


Limits to Knowledge about Plant Physiology

How an individual tree species responds to climate change is likely to be linked to its underlying physiological characteristics, such isohydric vs. anisohydric behavior (plant strategies for stomatal regulation), seedling vs. mature tree response to environmental stressors, heat tolerance, etc., but we only have detailed knowledge about these characteristics in a limited number of model species used in research and some of the more valuable timber species.


These unknowns all contribute to the ‘cascade of uncertainty’ that accumulates as researchers take the outputs of future emissions scenarios, use those in simulations of global climate change, downscale those outputs in a particular region, and then use those projections in forest models. At each step in this chain, there may be:

  • Statistical uncertainty
    • Sampling error, inaccuracy, etc.
  • Scenario uncertainty
    • Plausible changes, which are based on assumptions
  • Uncertainty due to ignorance
    • Known unknowns (uncertainty due to recognized ignorance)
    • Unknown unknowns (uncertainty due total ignorance)

Coping with Uncertainty

Forestry has always required the consideration of future conditions – whether that is anticipating future market demand, timber prices, insect threats, or new invasive species of concern. In the past, however, we could assume that, aside from a few wet or dry years, the average climate conditions would be the same throughout the life of an individual tree from seedling establishment to harvest. We now understand that that assumption may not hold, particularly in regions with a long rotation age. The exact degree and rate of climate change in any given location is uncertain and this may change our ability to achieve desired stand conditions in the future, by hindering our ability to choose the species best-suited for a particular site, for example.

Forest management decisions are influenced by our assumptions about how well we can predict future conditions and our ability to “design” forests well-suited for those conditions. In the context of a changing climate, these assumptions are more relevant than ever, as managers try to determine the most appropriate strategy for developing “resilient” forests that will continue to provide desired products and services. On one end of the spectrum, we have completely deterministic strategies that aim to develop “forests of the future” (where the species mix and stand structure are well-adapted to future conditions) and, at the other end, we have purely indeterministic approaches that aim for more diverse forests (in terms of species mix, structure, age-class, etc) based on the idea that they have more natural capacity to buffer change (Figure 2, below). While the deterministic approach has proven to be successful in the 10- to 15-year planning horizon that is often of most concern for foresters, an indeterministic approach often has the advantage if reality doesn’t meet expectations (in this case, climate predictions). All forest management decisions fall somewhere along this continuum between two valid strategies that are not mutually exclusive, and forest managers will likely use some of each approach.


While these strategies can be employed at any scale, stands are likely best managed from a deterministic basis (the shorter the rotation the more appropriate the deterministic approach), while forests (made up of multiple stands and biologically diverse micro sites) are likely best managed on an indeterministic basis. The latter approach is aimed at developing a maximum flexible forest that has the ability to adapt to any change in the ecological or economic environments, by identifying the most important ecosystem features for allowing high resilience and self-regulating capacity, such as: diverse structural elements, increased species diversity and natural regeneration, secured vitality and fitness of highest possible number of species (including genetic families across all organisms). Ultimately, the ability and desire to implement one or the other of these strategies will depend on management objectives, risk tolerance, and planning horizon.



Linder, M., J.B. Fitzgerald, N.E. Zimmerman, C. Reyer, S. Delzon, E. van der Maaten, M. Schelhaas, P. Lasch, J. Eggers, M. van der Maaten-Theunissen, F. Suckow, A. Psomas, B. Poulter, M. Hanewinkel. 2014. Climate change and European forests: What do we know, what are the uncertainties, and what are the implications for forest management? Journal of Environmental Management. 146:69-83.

Littell, J.S., D. McKenzie, B.K. Kerns, S. Cushman, C.G. Shaw. 2011. Managing uncertainty in climate-driven ecological models to inform adaptation to climate change. Ecosphere 2(9):102. DOI 10.1890/ES11-00114.1

Sax, D.F., R. Early, J. Bellemare. 2013. Niche syndromes, species extinction risks, and management under climate change. Trends in Ecology & Evolution. 28(9):517-523.

Wagner, S., S. Nocentini., F. Huth, M. Hoogstra-Klein. 2014. Forest management approaches for coping with the uncertainty of climate change: trade-offs in service provisioning and adaptability. Ecology and Society 19(1):32.

Yousefpour, R., J.B. Jacobsen, B.J. Thorsen, H. Meilby, M. Manewinkel, K. Oehler. 2012. A review of decision-making approaches to handle uncertainty and risk in adaptive forest management under climate change. Annals of Forest Science 69: 1-15. DOI 10.1007/s13595-011-0153-4



Certainty and Uncertainty in Climate Change and Forest Response Part 1: The Climate System

By Jennifer Hushaw

(Click to download a pdf of this complete article or a one-page synopsis)

While the basic mechanics of climate change are well understood, uncertainties associated with future greenhouse gas emission rates and various climate system feedbacks make it difficult to know the exact rate and extent of warming. Understanding both the degree and the sources of uncertainty is key to effective decision making and, in this bulletin, we will identify aspects of the science that are well established and active areas of research. Part 1 of this bulletin covers certainty and uncertainty associated with the climate system. Next month, Part 2 will cover certainty and uncertainty associated with forest response to climate change.

Even as we identify areas of uncertainty, we recognize that there are situations in which a hazard is not absolutely certain but still poses a risk – the primary reason why we might be interested in flood or fire insurance, for example. Ultimately, all decision-makers, including forest managers, want to understand what is known and unknown, minimize uncertainty wherever possible, and choose the best strategy (based on individual risk tolerance) for dealing with it.

What We Know

The earth’s climate system is immensely complex and, not surprisingly, there is some uncertainty in our understanding of global climate change. However, the uncertainty is primarily in the details – refining the projections of how climate will change in the near-term and on a regional or local scale. The core underlying phenomena have been well-understood for over a century, beginning with Joseph Fourier’s discovery of the greenhouse effect in 1824, John Tyndall’s discovery that CO2 is a greenhouse gas in 1859, and Svante Arrhenius’s initial estimates of how much the earth would warm from human emissions of CO2 in 1896 (history buffs can find a more complete timeline here or here). So, before we delve into the major areas of uncertainty, we’ll recap what we do know:

  • Greenhouse gases (e.g. water vapor, carbon dioxide, methane, surface-level ozone, nitrous oxides and fluorinated gases) are warming the planet
  • Other pollutants (i.e. aerosols, such as sulphur dioxide) are cooling the planet
  • When all climate forcings are totaled (anthropogenic and natural) the total net effect is warming the planet
  • The planet will continue to warm while this imbalance in the energy budget persists
  • Significant regional differences in the rate of warming will continue, with areas near the poles generally warming more rapidly than lower latitudes
  • Drought will be more impactful as temperatures increase
  • Precipitation patterns are changing, with some regions getting wetter and some drier
  • The probability of extreme heat and precipitation is increasing as the planet warms
  • Sea levels will continue to rise for several centuries and beyond


Uncertainty about Future Climate

At the most fundamental level, climate change is about the earth’s energy budget – when there is more energy coming in than going out things must get warmer, and vice versa. While there are many ways to change the temperature in a particular region of the globe, there are only three ways to change the average temperature of an entire planet:definitions_box

  1. Change the amount of energy coming in (i.e. solar activity)
  2. Change the amount of energy reflected back out to space (i.e. albedo/reflectivity)
  3. Change the amount of energy trapped by the atmosphere (i.e. strength of the greenhouse effect)

Accounting for the influence of the sun is fairly straightforward because solar activity follows predictable cycles. Also, changes in solar output are modest compared to these other factors (e.g. the difference between the minimum and maximum of a solar cycle is only 7% as much energy as the amount of additional energy from all human greenhouse gas emissions since pre-industrial times). The major areas of uncertainty about future climate change are related to the last two items – these variables are affected by feedbacks in the climate system and the last is related to the amount of future emissions.


There is uncertainty in our estimates of future global greenhouse gas emissions (hence why researchers typically utilize different emissions ‘scenarios’) because it will depend on how much the world population grows, the nature of future economic development, and the technology we use to meet our energy demands. As of now, global emissions are tracking the highest emissions scenario developed by the Intergovernmental Panel on Climate Change.

Long-lived greenhouse gases, like CO2, are of particular concern because they will stay in the atmosphere for centuries and continue to affect the climate long after we reduce or eliminate human emissions. This long residence time allows concentrations to build and the science has shown that “climate change results from the cumulative buildup of GHGs [greenhouse gases] in the atmosphere over time, not emissions in any particular year” (Baumert et al 2005), highlighting the significant long-term influence of rising greenhouse gas levels.


There are a number of positive and negative feedbacks in the climate system, which amplify or reduce the effect of a given climate forcing. Climate models include these processes, but each model may have slight differences in the relative magnitude of individual feedbacks. This is why there is some uncertainty about the exact amount of warming we will experience from a particular concentration of greenhouse gases.

This question of climate sensitivity has been a central area of research for decades and, as cited in a previous bulletin, the best current estimates suggest that doubling atmospheric CO2 concentrations (to about 550 ppm) will ultimately result in 2.7 to 8.1⁰F of global average warming. We will likely reach those concentrations by the middle of this century, if we continue on the current global emissions trajectory. Forty years of research from independent lines of evidence, including computer models and the study of past climate change, have given us confidence that the answer lies within this range. Although, this range is really a bell curve of possibility (not all values are equally likely) and research has not been able to narrow that range.

Feedbacks also play out on different timescales – from some that occur over the course of several years (e.g. changes in snow/ice cover) to others that take place over millennia (e.g. changes in the carbon cycle or the mass of ice-sheets on land), and beyond.  There is a lot of inertia in the earth’s climate system and this is also why past emissions have already committed us to a certain amount of warming.

Some examples of “fast” feedbacks include:

  • Snow/ice albedo (+)
    • Warmer temperatures melt bright snow/ice cover, revealing darker land and ocean water surfaces that absorb more solar radiation, which increases local warming that leads to more snow/ice melt, and so on.
  • Water vapor (+)
    • A warmer atmosphere can hold more water vapor, which traps more heat, which allows the atmosphere to become even more saturated, which warms things further, and so on. Likewise, cooling causes water vapor to condense and rain out, which reduces temperature, leading to further precipitation, and so on.
    • Water vapor is a very potent greenhouse gas, but it does not contribute significantly to the long-term greenhouse effect because its typical residence time in the atmosphere is only about ten days, unlike CO2 which stays in the atmosphere for centuries.
  • Clouds (+/-)
    • Feedbacks from clouds are complex and they are one of the biggest areas of uncertainty because we don’t know exactly how cloud cover will change under warmer conditions. Whether clouds have a warming or cooling effect depends on cloud formation, persistence, and altitude, for example:
      • Increase cumuliform = decrease % cloud cover = increase temp (+)
      • Increase stratiform = increase % cloud cover = decrease temp (-)

An example of a “slow” feedback would be:

  • Forests (+/-)
    • We spend a lot of time considering how the climate affects forests, but it is not a one-way relationship – forests also interact with the atmosphere and contribute to climate feedbacks. Forests affect the amount of energy absorbed and reflected from the surface (dark forest canopy has lower albedo), the hydrologic cycle (through evapotranspiration), and the carbon cycle (through photosynthesis and carbon sequestration). Through these processes, forests can act as both a negative and positive feedback, and the magnitude of these effects varies depending on forest type (see table below) (Bonan 2008).


Tipping Points

Another important area of uncertainty is related to so-called ‘tipping points’ in the climate system – these are points “beyond which an abrupt or irreversible transition to a different climatic state occurs” (Walsh et al 2014). Tipping points, such as the runaway loss of arctic sea ice, the collapse of some ocean circulation patterns, or large-scale release of carbon from melting permafrost, involve (practically) irreversible impacts that occur when a process crosses a threshold, kicking off feedbacks that will continue to push the climate in one direction, even if we reduce emissions.

There is evidence that these types of tipping points have been reached repeatedly in the past. The challenge is that they are much more difficult to predict than gradual climate changes and they are hard to detect until you’ve already passed them. Despite this uncertainty, the potential for this kind of abrupt change is a big concern because it will be high impact and have major consequences for both human societies and natural systems.


Additional uncertainty comes not from imperfect understanding or modelling of the large-scale climate system, but from the challenge of “downscaling” the results of global climate models. As mentioned in a previous bulletin, the resolution used to simulate global-scale processes does not match the scale of forest management and the use of either statistical or dynamical downscaling methods introduces a new layer of uncertainty in regional climate projections – an important caveat to keep in mind when viewing climate projections for your particular region.



At the simplest level, climate change is about an imbalance in the earth’s energy budget – a stronger greenhouse effect is trapping more energy in the climate system and the planet is getting warmer to radiate an equal amount of energy back out. We know that the average global temperature will continue to increase because of this imbalance, but there is still some uncertainty in the details of exactly how these changes will play out, especially at a regional level. There are also a host of additional side-effects, such as changing precipitation, ecological shifts, changing extremes, and so on. In next month’s bulletin, we will focus on the uncertainty related to forest impacts and discuss the range of strategies for coping with uncertainty in the realm of forest management.



Baumert, Kevin A., Timothy Herzog, and Jonathan Pershing. 2005. “Chapter 6: Cumulative Emissions.” In Navigating the Numbers: Greenhouse Gas Data and International Climate Policy. World Resources Institute.

Bonan, Gordon B. 2008. “Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests.” Science 320 (5882): 1444–49. doi:10.1126/science.1155121.

Le Page, Michael. 2011. “What We Do Know – and What We Don’t.” NewScientist, October 22. 

“Making Sense of Palaeoclimate Sensitivity.” 2012. Nature 491 (7426): 683–91. doi:10.1038/nature11574.

McGuffie, F., and A. Henderson-Sellers. 1997. “1.4 Climate Feedbacks and Sensitivity.” In: Climate Modelling Primer, 2nd ed., 31–39. Chichester, West Sussex, England: John Wiley & Sons.

Seneviratne, S.I., N. Nicholls, D. Easterling, C.M. Goodess, S. Kanae, J. Kossin, Y. Luo, J. Marengo, K. McInnes, M. Rahimi, M. Reichstein, A. Sorteberg, C. Vera, and X. Zhang, 2012: Changes in climate extremes and their impacts on the natural physical environment. In: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, UK, and New York, NY, USA, pp. 109-230.

Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Appendix 4: Frequently Asked Questions. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 790-820. doi:10.7930/J0G15XS3.


Global Precipitation Part 2: Ecosystem & Management Implications

By Jennifer Hushaw

This bulletin is the second in a two part piece on changes in global precipitation. In it, we discuss how water availability shapes forests and recent observations of forest decline linked to drought and heat stress. We then turn our attention to the issue of drought, including the factors that contribute to drought risk and the management options for mitigating it. While there will be a mix of wetter and drier conditions in the future, depending on the region, the background trend of warming temperatures will exacerbate drying – making more frequent and severe drought one of the more obvious climate-related risks.

(click here to download a pdf of this article or a one-page synopsis)

Changes in Global Precipitation: A Recap

Predicting how global climate change will affect future precipitation is one of the most challenging and uncertain areas of climate modeling research. However, there are some consistent patterns that have emerged from model projections, as described in part one of this bulletin. These general rules-of-thumb are summarized here:

  • Increase in total global precipitation
  • Regional differences, i.e. changes will not be uniform – increases in mid and high latitudes; decreases in subtropics
  • General pattern of wet-get-wetter, dry-get-drier
  • Increased contrast between wet and dry seasons
  • Increased winter precipitation in mid and high latitudes
  • Increased frequency of heavy precipitation events
  • A few areas of model agreement in terms of drying (soil moisture), including northeast and southwest South America and southwestern U.S.

Why Water Availability Matters    

Water availability is likely to change in the future, due to the combination of increasing average temperatures and changes in the total, timing, and intensity of precipitation events. Water availability, in terms of soil moisture, is a particularly important metric for forest management, since it can drive changes in forest structure and species composition over time. Water availability affects tree mortality, seedling recruitment, and resource allocation within individual trees (e.g. root-shoot ratio), and these factors ultimately influence competition between species.

Tree Species Migration

Severe or long-term decreases in water availability can predispose forest areas to large-scale die-off that opens the door for colonization of new species. While climate-induced tree species movement is usually a gradual process, it can happen more rapidly when sporadic mortality events eliminate competition from established species – arguably the biggest immediate barrier to species migration. We have seen some examples of this mortality-facilitating-colonization pattern in places like the Green Mountains of Vermont, where there is evidence that the upslope migration of hardwood species was likely accelerated by canopy turnover after red spruce experienced dieback from acid rain in the 1960’s and 70’s (Beckage et al. 2008) .

Tree Resource Allocation

Water availability, including the amount and timing of rainfall, is critical to forest structure because it changes how individual trees allocate their resources between the above and belowground portions of the stem. Under drier conditions, trees will generally respond by decreasing inputs to foliage and aboveground woody biomass while increasing fine roots, which improves their ability to draw on limited soil water resources. This is a helpful adaptation, but it reduces growth in the merchantable part of the tree. Likewise, in wetter areas, trees will put more resources into foliage and increase growth rate, with fewer fine roots. This capitalizes on growth potential and increases competitiveness, but it can also result in a shallower root system that increases risk of blow down and vulnerability to future drought conditions (Farrior et al. 2013; McDowell et al. 2008).

Different species have different amounts of plasticity in the degree and speed with which they can shift resources in response to changing conditions and, as with any adaptation, there are tradeoffs, e.g. it has been shown that this flexibility, while beneficial in terms of adaptability, can be detrimental in stressful environments (Richter et al. 2012).

Water Stress and Forest Mortality

Recently, researchers have documented widespread tree mortality on a global scale that is at least partly attributed to drought and heat stress (see this map from Hartmann et al. 2015, which shows locations of substantial drought- and heat-induced tree mortality around the globe since 1970). The impacts were observed in wet areas, as well as semi-arid regions, which indicates that increasing temperatures may be playing a significant role – by increasing water loss through transpiration, reducing tree vigor, and accelerating insect development and reproduction (Allen et al. 2010; Hartmann et al. 2015).

There are many examples of forest impacts in the U.S. that have been linked to water stress, including aspen decline in the west (Worrall et al. 2013), increased mortality of pine and oak species in the Central Coast and Southern Sierra Ranges of California (USFS 2015), loss of big trees (>2 ft dbh) throughout California (McIntyre et al. 2015), and regional-scale die-off of piñon pine (Breshears et al. 2008). The ultimate consequences of forest die-off driven by drought and heat stress are unclear and researchers have highlighted the importance of investigating these implications (Anderegg, Kane, and Anderegg 2013).


At this time, we don’t have sufficient data to know whether forest mortality is increasing globally. Although, these observations have sparked a huge body of research on the physiological mechanisms that influence how plants avoid, tolerate, and/or recover from drought stress. A better understanding of exactly how and why certain trees succumb to drought will improve predictions of global-scale forest impacts from climate change.

Presently, the understanding is that drought-related mortality happens via three interrelated pathways – carbon starvation, hydraulic failure, and biotic attack (Figure 1). Carbon starvation occurs when photosynthesis is reduced and trees are forced to use up their carbon reserves – a consequence of closing leaf stomata, which reduces water loss but does not allow CO2 uptake. Hydraulic failure occurs when plants dehydrate past the point of no return. Insects and pathogens can amplify or be amplified by both of these processes (McDowell et al. 2008), e.g. carbon starvation will reduce resin production and make it difficult for trees to pitch out an attacking beetle.

Evaluating Drought Risk

Evidence suggests we will experience more frequent and severe drought due to climate change, but this risk is not universal and it varies with site characteristics and forest type.  Determining whether it is a significant risk for your forestland involves considering all the factors that influence intensity, exposure, and vulnerability to drought.


Higher temperatures increase the intensity of individual drought events by water loss through direct evaporation and forest transpiration (collectively known as evapotranspiration). Additionally, as conditions dry, there is a feedback that exaggerates this process – less soil moisture means less cooling from transpiration and temperatures go up even further. This is similar to the way human sweat helps reduce body temperature – if you lose your ability to sweat when you are hot, your body temperature will begin to increase rapidly.

Including evapotranspiration in model simulations (rather than precipitation alone) increases the percentage of global land area that is projected to experience moderate drying by the end of this century (from 12 to 30%). Importantly, researchers found that this effect will even make relatively wet areas more drought prone: “Increased PET [potential evapotranspiration] not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone” (Cook et al. 2014). This interaction with temperature has also been implicated in the severity of the recent California drought, where researchers have found that the occurrence of drought years has increased primarily because of the increased probability of warm-dry conditions, rather than a substantial change in the probability of a precipitation deficit (Diffenbaugh, Swain, and Touma 2015). The bright side is that these drying trends will be beneficial in some areas where conditions have historically been excessively wet – this will help alleviate issues of reduced productivity and limited access in these locations.


Site characteristics, including soil texture, depth to water table, and topography, have a big influence on the amount of drought exposure on a given site (i.e. the likelihood that a given location will experience drought conditions). These factors influence soil water holding capacity, run off, and evaporation rates, which all mediate the direct effects of precipitation change.



Vulnerability is primarily determined by the tree species mix on site, specifically the level of drought tolerance exhibited by each species. The variability in forest drought tolerance from region to region creates some unique advantages and disadvantages in terms of vulnerability. For example, the southeastern U.S. has a species mix with a relatively high drought tolerance compared to the northeast, which reduces the risk of negative drought impacts in that region. However, there is a much higher diversity in terms of the mix of drought tolerant and intolerant species in the Appalachian region and the northeast, which might be beneficial if future conditions are highly variable.

Note: We recommend referencing Russell et al. 2014 for maps of average drought tolerance and diversity of drought tolerance classes among tree species in the eastern U.S. 


Overall Risk

Taken together, these factors tell us that sites that are projected to have large increases in temperature and decreases in precipitation, with low soil water holding capacity, and a drought-intolerant species mix will have the highest levels of drought risk (in terms of intensity, exposure, and vulnerability).  In contrast, an area may have a high likelihood of intense drought in the future, but the risk may be mitigated by a drought tolerant species mix and better site conditions. The regions of greatest concern going forward will be places where all these factors overlap.

Reducing Drought Risk Through Management

From the perspective of an individual forest manager, there is not much that can be done to reduce the intensity of future drought conditions, but the following areas offer opportunities to reduce risk by reducing exposure and/or vulnerability:

  • Land base
    • Focus resources on sites with soil and topographic characteristics that generally retain moisture
  • Species mix
    • Use silvicultural techniques that favor regeneration of drought tolerant species
    • Plant genotypes from warmer and dryer areas of a species range
  • Reduce stocking
    • A number of studies conducted in different forest types throughout the U.S. and Europe (primarily pine-dominated) have highlighted the utility of thinning for moderating drought impacts on growth, increasing drought resistance, and improving the speed of recovery after drought events (D’Amato et al. 2013; Kerhoulas et al. 2013; Kohler et al. 2010; Slodicak, Novak, and Dusek 2011).
    • Although it is worth noting that thinning has been shown to have negligible effects on drought tolerance in sparse forest canopies (B. Law, personal communication, May 6, 2015), such as some dry western forests where self-thinning has naturally taken place.



Allen, Craig D., Alison K. Macalady, Haroun Chenchouni, Dominique Bachelet, Nate McDowell, Michel Vennetier, Thomas Kitzberger, et al. 2010. “A Global Overview of Drought and Heat-Induced Tree Mortality Reveals Emerging Climate Change Risks for Forests.” Forest Ecology and Management 259 (4): 660–84. doi:

Anderegg, William R. L., Jeffrey M. Kane, and Leander D. L. Anderegg. 2013. “Consequences of Widespread Tree Mortality Triggered by Drought and Temperature Stress.” Nature Clim. Change 3 (1): 30–36. doi:10.1038/nclimate1635.

Breshears, David D, Orrin B Myers, Clifton W Meyer, Fairley J Barnes, Chris B Zou, Craig D Allen, Nathan G McDowell, and William T Pockman. 2008. “Tree Die-off in Response to Global Change-Type Drought: Mortality Insights from a Decade of Plant Water Potential Measurements.” Frontiers in Ecology and the Environment 7 (4): 185–89. doi:10.1890/080016.

Cook, BenjaminI., JasonE. Smerdon, Richard Seager, and Sloan Coats. 2014. “Global Warming and 21st Century Drying.” Climate Dynamics 43 (9-10): 2607–27. doi:10.1007/s00382-014-2075-y.

D’Amato, Anthony W., John B. Bradford, Shawn Fraver, and Brian J. Palik. 2013. “Effects of Thinning on Drought Vulnerability and Climate Response in North Temperate Forest Ecosystems.” Ecological Applications 23 (8): 1735–42. doi:10.1890/13-0677.1.

Diffenbaugh, Noah S., Daniel L. Swain, and Danielle Touma. 2015. “Anthropogenic Warming Has Increased Drought Risk in California.” Proceedings of the National Academy of Sciences 112 (13): 3931–36. doi:10.1073/pnas.1422385112.

Farrior, Caroline, E., Ray Dybzinski, Simon A. Levin, and Stephen W. Pacala. 2013. “Competition for Water and Light in Closed-Canopy Forests: A Tractable Model of Carbon Allocation with Implications for Carbon Sinks.” The American Naturalist 181 (3): 314–30.

Hartmann, Henrik, Henry D. Adams, William R. L. Anderegg, Steven Jansen, and Melanie J. B. Zeppel. 2015. “Research Frontiers in Drought-Induced Tree Mortality: Crossing Scales and Disciplines.” New Phytologist 205 (3): 965–69. doi:10.1111/nph.13246.

Kerhoulas, Lucy P., Thomas E. Kolb, Matthew D. Hurteau, and George W. Koch. 2013. “Managing Climate Change Adaptation in Forests: A Case Study from the U.S. Southwest.” Journal of Applied Ecology 50 (6): 1311–20. doi:10.1111/1365-2664.12139.

Kohler, Martin, Julia Sohn, Gregor Nägele, and Jürgen Bauhus. 2010. “Can Drought Tolerance of Norway Spruce (Picea Abies (L.) Karst.) Be Increased through Thinning?” European Journal of Forest Research 129 (6): 1109–18. doi:10.1007/s10342-010-0397-9.

McDowell, Nate, William T. Pockman, Craig D. Allen, David D. Breshears, Neil Cobb, Thomas Kolb, Jennifer Plaut, et al. 2008. “Mechanisms of Plant Survival and Mortality during Drought: Why Do Some Plants Survive While Others Succumb to Drought?” New Phytologist 178 (4): 719–39. doi:10.1111/j.1469-8137.2008.02436.x.

McIntyre, Patrick J., James H. Thorne, Christopher R. Dolanc, Alan L. Flint, Lorraine E. Flint, Maggi Kelly, and David D. Ackerly. 2015. “Twentieth-Century Shifts in Forest Structure in California: Denser Forests, Smaller Trees, and Increased Dominance of Oaks.” Proceedings of the National Academy of Sciences 112 (5): 1458–63. doi:10.1073/pnas.1410186112.

Richter, Sarah, Tabea Kipfer, Thomas Wohlgemuth, Carlos Calderón Guerrero, Jaboury Ghazoul, and Barbara Moser. 2012. “Phenotypic Plasticity Facilitates Resistance to Climate Change in a Highly Variable Environment.” Oecologia 169 (1): 269–79. doi:10.1007/s00442-011-2191-x.

Slodicak, Marian, Jiri Novak, and David Dusek. 2011. “Canopy Reduction as a Possible Measure for Adaptation of Young Scots Pine Stand to Insufficient Precipitation in Central Europe.” Forest Ecology and Management 262 (10): 1913–18. doi:

USDA Forest Service. 2015. Forest Health Protection Survey: Aerial Detection Survey April 15th-17th, 2015. Accessed online at:

Worrall, James J., Gerald E. Rehfeldt, Andreas Hamann, Edward H. Hogg, Suzanne B. Marchetti, Michael Michaelian, and Laura K. Gray. 2013. “Recent Declines of Populus Tremuloides in North America Linked to Climate.” Forest Ecology and Management 299 (0): 35–51. doi:



Global Precipitation Part 1: Trends & Projections

By Jennifer Hushaw

The discussion of global precipitation will be covered in two parts. This bulletin (part one) provides an overview of the precipitation trends observed over the last century, as well as projections of future change. Part two will cover a range of forest ecosystem and management implications of changing precipitation patterns and drought stress.

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Observations of Global Precipitation

Observations suggest that there has been an increase in globally averaged precipitation on land over the last century.  This is based on several global datasets that all show a statistically significant increase since 1900 (Figure 1). However, the datasets do not agree on how much it has increased. This primarily stems from a lack of sufficient data across the globe, especially during the early 1900’s.  The upward trend is also much clearer in the early part of the record than in the years after 1950. As a result, there is only low confidence in the observed global change prior to 1950 and medium confidence in the trends since that time.


The data also reveal notable variability in precipitation trends at different latitudes, but problems with data quality, coverage, and agreement have resulted in low confidence in most regional observations. However, there is at least one area of the world where clear trends have emerged – namely, the mid-latitudes of the Northern Hemisphere (30⁰N to 60⁰N). Here, the data show a likely overall increase in precipitation, with medium confidence prior to 1950 and high confidence afterward (Figure 2).


Future Global Precipitation

There are several metrics to consider when thinking about changes in future precipitation, these include changes in the total annual amount, shifts in timing, and changes in the intensity of individual precipitation events – all of which have important ramifications for soil moisture dynamics and water availability for forests.


On average, models project a gradual increase in global precipitation over the 21st century. This is because warmer temperatures drive more evaporation and the increase in water vapor leads to more precipitation. While we expect an increase in the total global amount, this does not mean every individual region will receive more precipitation. Instead, some areas will actually become much drier because we also expect existing regional precipitation profiles to be amplified – a wet-get-wetter and dry-get-drier pattern. In particular, it is very likely we will see an increase in precipitation at high and some mid-latitudes, whereas in the subtropics an overall decrease is more likely than not (Figure 3).


Note: In the near-term, these projected changes are likely to be fairly small compared to natural variability from circulation patterns like the El Nino-Southern Oscillation, for example. This will also be true for some regions even in the long-term under a low emissions scenario (see extensive hatching in low emissions map, above). This dominance of internal variability at certain scales is an important recurrent theme.


Shifts in the timing and/or amount of precipitation from one season to another can significantly affect ecosystem processes and the hydrologic cycle. Models suggest that we will see these types of shifts and the largest changes will be at high latitudes. In particular, the contrast between the wet and dry season is likely to increase in most places as temperatures warm. Although, there will be some notable regional exceptions along the equator and poleward edges of the subtropical dry zone, where changes in atmospheric circulation will cause precipitation patterns to shift completely. In both mid- and high-latitude regions the projected increase is larger for winter than summer (Figure 4). However, this increase in average winter precipitation will occur at the same time that increasing temperatures contract the length of the snow season.



Warmer temperatures allow the air to hold more water vapor when it is saturated, so when it rains or snows in a warmer world there is simply more water coming down, which will increase the intensity of individual precipitation events. As a result, the probability of heavy precipitation is projected to increase, and the effect will be stronger with increased warming (see Fischer and Knutti 2015, Figure 3 (a), (b), and (c) for maps showing projected change in the probability of heavy precipitation events under different levels of warming across the globe; units are probability ratios, i.e. the ratio of current or future probability to the pre-industrial probability). As a greater fraction of total rainfall comes in the form of these heavy downpours it will also affect soil moisture because more precipitation will be lost as runoff, rather than being absorbed by the soil or going to replenish ground water supplies. For these reasons, intense rainfall events will also increase the likelihood of flash floods.

Uncertainty of Precipitation Projections

We have a high level of confidence in estimates of future temperature because those projections are based on basic physical principles and significant model consensus, but precipitation is much more uncertain. And, as with temperature, the uncertainty grows as we scale down – from estimates of change in the global average to regional projections. There is less agreement among global climate models regarding the direction and magnitude of precipitation change, especially in certain regions.

There are a number of reasons for this uncertainty, including:

  1. The small spatial scales on which precipitation dynamics take place (e.g. cloud microphysics)

The scale of these processes is much smaller than the resolution of global climate models, so those dynamics cannot be simulated from first principles, instead they must be described using parameterization, which introduces some room for error.

  1. A large degree of spatial variability from one location to the next

More data coverage is necessary to accurately capture precipitation trends, in contrast with temperature, which is highly autocorrelated over long spatial scales (i.e. the temperature at one location is closely related to the temperature nearby, so just a few data points can be used to calculate a large area).

  1. The indirect relationship between greenhouse gases and precipitation change

At a very basic level, greenhouse gases influence the climate by changing the amount of heat, which changes atmospheric pressure, which leads to changes in rainfall patterns. Because the influence of greenhouse gases is several steps removed, it is more challenging and complex to simulate how changes in those concentrations will affect precipitation. However, in recent years scientists have improved their understanding of the mechanics underlying projected changes in precipitation and, as a result, we have a much higher degree of confidence in some of the patterns that have emerged from global climate models (e.g. wet-get-wetter, dry-get-drier).


Drought: A Closer Look

Changes in the length and severity of drought are important metrics of precipitation change. Generally, drought implies a moisture deficit relative to some previous norm. More specifically there are three types of drought: meteorological, hydrological, and soil moisture (Figure 5). Of these, soil moisture drought is perhaps the most relevant for forests because it is closely tied to plant water availability.

Soil moisture drought is driven by reduced precipitation and/or increased evapotranspiration. In fact, at seasonal or longer time scales, an increase in evapotranspiration (which is indirectly driven by temperature) can lead to more frequent and intense periods of soil moisture drought. Soil characteristics, rooting depth, vegetation, an on-going lack of precipitation, and previous soil moisture and groundwater conditions also play an important role in determining this type of drought risk.

The 2013 Intergovernmental Panel on Climate Change improved previous work on soil moisture projections by using a consistent soil depth across all climate models in their report. The results yielded high confidence in regions where surface soils are expected to dry, but little-to-no confidence in locations where surface soils are projected to be wetter. The models also consistently projected drying conditions in certain regions, specifically the Mediterranean, northeast and southwest South America, southern Africa, and the southwestern U.S. (Figure 6). Although, they disagreed on the direction of change (wetter or drier) in other large regions, such as central Asia or the high northern latitudes. Consequently, there are only a handful of locations where we can currently be confident about how soil moisture will change in the future and those locations are all projected to become drier.

Part two of the global precipitation bulletin will delve into how and where these projected changes in precipitation, drought, and soil moisture are likely to impact forests. It will also include a discussion of the optimal species characteristics, silvicultural decisions, and infrastructure design for minimizing risk from these climate variables and capitalizing on growth potential.



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