Changing Hurricane Activity & Forest Risk

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

By Jennifer Hushaw Shakun

Hurricanes pose a major risk to infrastructure and human safety, but they are also a significant disturbance agent in our forests, often leaving impacts that persist for decades. The 2017 Atlantic hurricane season included a number of record breaking storms that were linked to above-average ocean temperatures (Murakami et al. 2018) and, since that particularly active hurricane season, there has been increasing discussion about changing hurricane risk in the context of climate change. In this bulletin, we summarize various aspects of hurricane activity and how they are projected to change (or not) in the future, with implications for forest health and timber value.

Recipe for a Hurricane

Hurricanes, which are more generally known as tropical cyclones (see box below), are massive storms fueled by warm, moist air over the tropical ocean waters near the equator, with wind speeds of at least 74 miles per hour. As warm, moist air rises it leads to areas of high and low pressure that ultimately create a spinning storm system of clouds and wind. In the simplest sense, the two most important factors in the development and intensification of hurricanes are (1) warm ocean waters and (2) wind shear (changes in wind direction and/or speed with height). These factors work in opposing ways—warm water provides hurricanes with their energy, while wind shear puts on the brakes by tearing storm systems apart (NOAA GFDL 2018a).

Determining how climate change might affect tropical cyclone activity involves figuring out how these two major factors will change, along with a few additional variables, such as the temperature of the upper atmosphere and relative humidity (NOAA GFDL 2018a). Over 90% of the global warming we have experienced to date has been absorbed by the oceans (Rhein et al. 2013), but warming ocean waters alone will not increase the number of hurricanes each year—it will, however, mean more fuel for them when they do form (Climate Central 2018a; Climate Central 2018b).

How is hurricane activity or risk changing, in terms of…


Detecting changes in hurricane frequency is challenging because records of hurricane activity are less reliable and less complete before the mid-1970’s (after which we have more consistent observations from satellites and other sources). When studies account for these data limitations they find there has been no increase in the global number of hurricanes since the 1800’s (Landsea et al. 2010; NOAA GFDL 2018b).

The number of hurricanes making landfall in the U.S. has not changed significantly either, but there has been an increase in hurricane activity over the entire Atlantic Ocean basin since the 1970’s (NOAA 2012). However, too little time has elapsed to say whether that increase is part of an on-going trend related to human-induced climate change or whether it is within the realm of natural variability (Kossin et al. 2017).

There is still some debate about whether we will have more or fewer tropical cyclones in the future as the climate changes. While there are modeling studies that suggest the frequency will increase (Emanuel 2013), the general consensus is that, globally, the total number of hurricanes will stay about the same or perhaps decrease by up to a third (Knutson et al. 2010; NOAA 2012; Kossin et al. 2017). Although, research suggests we are likely to have an increasing number of the most intense storms, even if overall numbers go down (see section on Intensity, below). In the Atlantic Ocean, in particular, there is currently no consensus about how hurricane frequency will change (NOAA 2012).


The intensity or strength of tropical cyclones is usually measured in terms of wind speed, such as the well-known categories of the Saffir-Simpson Hurricane Wind Scale. The maximum intensity a hurricane can achieve is determined by the temperature of the surface ocean and the thermodynamics of the atmosphere above it (Emanuel 1986; Emanuel 1995; Emanuel 1997). All else being equal, rising ocean surface temperatures will increase the potential intensity of tropical cyclones.

This is borne out by the latest scientific research, which indicates that we will most likely see an overall increase in tropical cyclone intensity, including an increase in wind speed between 1 and 10%, as well as a greater number of category 4 and 5 storms in a warmer world (Kossin et al. 2017; Bender et al. 2010; Knutson et al. 2015; NOAA GFDL 2018b). As one recent publication put it: “We thus expect tropical cyclone intensities to increase with warming, both on average and at the high end of the scale, so that the strongest future storms will exceed the strength of any in the past” (Sobel et al. 2016).

In fact, there is evidence this is already happening. Across the globe, there have been increases in the strongest tropical storms, especially in the North Atlantic where the strongest storms are getting stronger (Kossin et al. 2013; Elsner et al. 2008). There is also at least one study that suggests tropical cyclones are intensifying faster (Kishtawal et al. 2012). In a recent essay, several leading researchers in this field also noted that: “Of these seven [major tropical cyclone] regions, five had the strongest storm on record in the past five years, which would be extremely unlikely just by chance” (Rahmstorf et al. 2017).

There are other measures, beyond wind speed, that give a more holistic assessment of hurricane activity, such as the accumulated cyclone energy (ACE) index or the power dissipation index (PDI). These are functions of wind speed AND storm duration, which are accumulated for a particular region to estimate the overall intensity of the hurricane season (NOAA NWS CPC 2017; Emanuel 2005).

One study found that PDI more than doubled in the Atlantic and increased 75% in the western North Pacific between the mid-1970’s and the early 2000’s. The increase was due to a combination of longer-lasting storms and faster winds (Emanuel 2005). Although, a recent update indicates that annual PDI has actually fallen in the North Atlantic since that study was published (see EPA Climate Change Indicators: Tropical Cyclone Activity, Figure 3). Another recent study (Lin and Chan 2015) found a ~35% drop in PDI from 1992 to 2012 in the western North Pacific. In that case, storm intensity increased, but it was offset by having fewer storms that were not as long-lasting. This is an example of the way indices like PDI can be useful for understanding the interplay between changing intensity, frequency, and duration.


Often times, it is the incredible volume of rain associated with hurricanes that does the most damage. All the resesarch to date points toward increasing rainfall rates in a warmer world—on the order of 10-20% by the end of the century (NOAA GFDL 2018b; Knutson et al. 2010). This is true on a global scale and for the Atlantic basin in particular. Rainfall rates are expected to increase because a warmer atmosphere will hold more water vapor and because these storms may move more slowly, dropping more precipitation in a given location (see section on How Fast They Move, below). As a recent example, the evidence suggests human-induced climate change contributed to the historic rainfall totals from Hurricane Harvey, making the heavy downpours 3 times more likely and 15% more intense (van Oldenborgh et al. 2017; Climate Central 2017).


Sea levels have been rising and will continue to do so for the foreseeable future. This is because ocean water expands slightly as it warms and warming temperatures are releasing large volumes of water that were previously frozen in mountain glaciers and polar regions. All else being equal, rising sea levels will increase the height of hurricane storm surges and the vulnerability of coastal communities to that type of flooding (Knutson et al. 2010; Kossin et al. 2017; NOAA GFDL 2018b).


One measure of tropical cyclone activity that is easier to accurately pin down is when a storm reaches its peak intensity. This is more straightforward than metrics like duration or absolute intensity because you only need to know when a storm is at its relative strongest, without needing to know the absolute wind speed in miles per hour. When researchers looked at global records of tropical cyclone activity for the last 30 years, they found these storms now reach their maximum strength about 200 miles farther from the equator (that is a trend of around 72 (+/- 43) miles per decade). This poleward migration appears to be happening globally, albeit at different rates in different regions, and it is consistent with the expansion of the tropics that has been linked to human-induced climate change (Kossin et al. 2014). If this movement in the location of peak storm intensity continues, it will change the historic patterns of storm risk across different regions.


Recent research indicates that tropical cyclone are moving slower—translation speed (i.e. forward-moving speed, as opposed to rotation speed) has decreased globally by 10% since 1950 (Kossin et al. 2018). The slowdown has been even more dramatic in some regions, with tropical cyclones moving 30% slower over land areas near the western North Pacific and 20% slower over land areas near the North Atlantic. This has big implications for storm-related rainfall because a slower moving tropical cyclone will drop much more water in one region (the historic rainfall totals from Hurricane Harvey were an example of this). Tropical cyclones move within large-scale atmospheric circulation patterns that are affected by climate change, but the observed slowdown cannot be confidently linked to human-induced warming at this time (Kossin et al. 2018).

Forest Impacts from Hurricane Activity

It is estimated that, on average, hurricanes affect almost 3 million acres of forest and cost around $700 million dollars each year in the U.S. (Dale et al. 2001). By that measure, some recent hurricane seasons certainly qualify as above average. The Florida Forest Service puts the timber damage from Hurricane Michael in October of this year at $1.3 billion over 3 million acres of forestland (FDACS 2018). Another estimate of the damage from both Hurricane Michael and Hurricane Florence (which made landfall a month earlier), puts the total loss of timber value around $1.6 billion over 5 million acres across Florida, Georgia, and Alabama (SAF 2018).

This loss of value is also related to loss of aboveground carbon. A study of the impact of Hurricane Katrina on forests in the Gulf Coast estimated there was mortality or damage to ~320 million large trees, which represents a significant portion of the annual U.S. forest tree carbon sink. That same study noted that the predicted increase in storm activity “will reduce forest biomass stocks, increase ecosystem respiration, and may represent an important positive feedback mechanism to elevated atmospheric carbon dioxide” (Chambers et al. 2007). In other words, we expect to have stronger, slower-moving storms in the future that drop more rain, so it is reasonable to expect that the risk to timber resources may change, including loss of value and forest carbon stocks in some places.

In the aftermath of a major storm event, there is an immediate loss of merchantable value due to structural damage or tree mortality, but there are also longer-term impacts to consider—wounded and stressed trees are more vulnerable to attack from insects and pathogens, the increased volume of downed wood can provide additional fuel for wildfires, and there may be infrastructure and access-related issues if forest roads, culverts, etc. have been damaged.

Importantly, surviving trees may also experience growth impacts that can persist for a while after the storm event. One study of coastal forests in Virginia found a decline in radial growth that lasted for up to 4 years after an extreme storm before beginning to recover. They also found that there was a strong correlation between the amount of growth decline and the strength of the storm (as measured by wind speed or storm surge height) (Fernandes et al. 2018). This suggests that the projected increase in hurricane intensity may have corresponding impacts on forest growth in impacted areas.

Things to Do

Of course it is not possible to prevent trees from being severely damaged or uprooted in the strongest winds of category 4 and 5 hurricanes, but there are steps you can take to build windfirmness in forest stands and help them withstand lower intensity storm systems. We recommend re-visiting two of our earlier bulletins on that topic for more detailed information about the factors that increase the risk of wind damage (see Part 1) and the management actions that can maximize resilience to wind-related disturbance (see Part 2).

Other key actions are to maintain forest access and have systems in place for carrying out rapid assessments of forest damage after storm events (with field surveys and/or aerial/satellite imagery). This will help prioritize salvage efforts, which can be time-sensitive if the goal is to limit additional loss of value due to rot, pests, or disease. In some cases, it may also be worthwhile to consider acquiring insurance to hedge against catastrophic timber loss due to extreme wind events.

There are useful resources on this topic that are geared toward urban forestry, but which may have some useful insight, such as this series from the University of Florida. One of the publications in that series (see link under Recommended Resources) outlines a number of lessons learned from 10 hurricanes that hit the Gulf Coast and Puerto Rico between 1995 and 2005, including:

  • The higher the wind speed of the hurricane, the more likely trees will fail.
  • Trees in groups survive winds better than trees growing individually.
  • Some species resist wind better than others.
  • Pines may show no immediate visible damage after hurricanes but may decline over time.
  • Trees that lose all or some of their leaves in hurricanes are not necessarily dead.
  • Native trees survived better in South Florida hurricanes.
  • Older trees are more likely to fail in hurricanes.
  • Unhealthy trees are predisposed to damage.
  • Trees with poor structure or bark inclusions are more vulnerable in the wind.
  • Trees with more rooting space survive better.
  • Damaged root systems make trees vulnerable in the wind.

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Native Pests in Novel Places: The Southern Pine Beetle Example

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

By Jennifer Hushaw Shakun

Climate-driven changes in pests and disease are already causing significant near-term impacts on forest health—a reality that we highlighted in an earlier bulletin on Forest Pests and Climate Change. Notably, several important native pests, including spruce budworm (Gray 2013), mountain pine beetle (Cullingham et al. 2011; de la Giroday et al. 2012), and southern pine beetle (Dodds et al. 2018), have shown signs that their range is expanding with warmer temperatures or is predicted to do so in the near future. They are prime examples of a phenomenon that is likely to become more common in the future, i.e. native pests that are well-known in one region move into new geographies where they were previously unknown.

The novelty creates some immediate challenges for forest managers (Aoki et al. 2018). The good news is that, unlike with most alien invasive species, we have far more information (both research and institutional knowledge) about these organisms. We also have examples of systems and best practices that have worked (or failed) in other regions. With the right networks in place for sharing information across jurisdictions (e.g. the CSLN), the learning curve for managers encountering a particular pest for the first time will not be as steep and the likelihood of an effective and rapid response is increased (Ayres & Lombardero 2018; Morris et al. 2018).

Southern Pine Beetle: An Overview

Southern pine beetle (Dendroctonus frontalis Zimmermann) (SPB) is one of the most destructive pests in the southern U.S. (Anonymous 1989; Coulson & Klepzig 2011), with a loss to timber producers of an estimated $43 million per year (Pye et al. 2011). Its historic range in the U.S. extends from Florida into southern New Jersey and as far west as central Arizona (Hain et al. 2011). Although, “given its wide host range, genetic plasticity, and ability to sustain epidemics in nontraditional species, it appears that the geographic range of the SPB is only constrained by host availability and climatic conditions” (Hain et al. 2011).

The small adults (~ 3 mm long) bore into the tree and create S-shaped galleries in the inner bark where they lay their eggs. The larvae develop in the inner bark and move into the outer bark to pupate before emerging as adults. The entire life cycle can take place in just 30 days under ideal conditions, which allows SPB to have three to seven generations per year (Anonymous 1989).

While SPB “infests and kills all pine species in its range” (Hain et al. 2011), its preferred hosts are southern yellow pines such as loblolly, shortleaf, Virginia, pond, and pitch pines (Anonymous 1989). Pioneer beetles will often infest weakened or stressed trees and begin producing aggregation pheromones that attract additional beetles, which ultimately overwhelms the tree’s defenses. At this stage, the beetles can reproduce within the infested tree and attacks begin on neighboring trees when the new adults emerge, forming the distinctive “spots” of an SPB outbreak (Hain et al. 2011). Pitch tubes (about the size of a piece of popcorn) on the bark and discoloration of foliage are two noticeable indications of an infestation (Anonymous 1989). SPB’s rapid life cycle and its ability to attack and kill healthy trees make it particularly destructive (Dodds et al. 2018).

Over the past several decades, there has been a decline in major SPB outbreaks within its historic range in the southeastern U.S. There are a number of hypotheses for why this reduction has happened, but recent research suggests an “increase among several variables associated with intensive pine silviculture and genetic tree improvement efforts” is a possible explanation (Asaro et al. 2017). Forest management and SPB suppression efforts are known to have had an important influence on the frequency, intensity, and extent of SPB outbreaks (Clarke et al. 2016)—an important insight for managers encountering this pest in new regions.

Southern Pine Beetle Outbreak Risk

As with most forest pests, the risk that an infestation will become a severe outbreak depends on the size and location of the pest population and the susceptibility of the host (Aoki et al. 2018) at the tree, stand, or landscape level. One well-established method for assessing SPB risk relies on one of these two components—using the number of SPB caught per day in traps, along with the ratio of SPB to one of its major predators—to predict infestation trends (Billings & Upton 2010). Alternatively, southern pine beetle hazard maps generated by the US Forest Service use variables related to average diameter, basal area/density, and proximity to the infestation (Krist et al. 2014).

Weakened or stressed trees, e.g. those struck by lightning, are a risk factor (Hain et al. 2011) because they are often the “patient zero” of SPB spots during an outbreak. There are also a variety of stand-level characteristics that affect the vulnerability of host trees by influencing resource availability, density, stand age/size, and the dispersal of beetle pheromones (see Table 1; Aoki et al. 2018). High stand density, in particular, is known to increase the risk of an SPB outbreak (Clarke & Nowak 2009; Guldin 2011), by making it easier for the beetles to attract each other with pheromone plumes and to travel from the infested tree to a new host.

Southern Pine Beetle in the Northern Context

Warmer winter temperatures have facilitated the northern expansion of the SPB range (via improved over-winter survival) and model projections suggest that the climate will become progressively more suitable for SPB in these areas over time (Lesk et al. 2017). But it is more difficult to predict the impact of SPB in this new environment because its population dynamics may be altered and the susceptibility of northern pine species is not completely understood.


Site index and site moisture can be useful predictors of SPB infestation risk in the southern U.S., but they appear to be less important in new areas like the New Jersey Pinelands (Aoki et al. 2018). However, the same study found that “stands with high percentage pine and high pine basal area were more susceptible. Stands composed of smaller, closer together, shorter, and younger trees, with lower percent live crown, were also more susceptible,” i.e. SPB risk seems to vary with stand characteristics in the newly invaded areas in much the same way it did in well-studied systems farther south (Aoki et al. 2018). If that holds true, it may be easier to successfully predict SPB infestations in the Northeast going forward.


SPB primarily attacks hard pines (preferably loblolly and shortleaf) in the Southeast, but it will successfully attack “nontraditional” host species, such as eastern white pine, red spruce, Norway spruce, eastern hemlock, and others, when outbreak populations are large enough (Hain et al. 2011). Pitch pine has proven to be a suitable host in the recent outbreaks in New Jersey and Long Island, while red and Scots pine were identified as hosts in the Connecticut infestation (Dodds et al. 2018). The potential threat to eastern white pine is top of mind for many in the Northeast and, fortunately, successful reproduction of SPB has not be documented in that species so far. Although, both white and jack pine (farther north) have the potential to act as suitable hosts (Hain et al. 2011; Dodds et al. 2018).


Many northeastern pine species may be susceptible to SPB, but it is possible that the composition of northeastern forests (specifically the absence of many pure pine stands and more isolated and disbursed populations of potential hosts) will reduce the risk of severe outbreaks at the landscape level. The risk may be greatest in certain unique, pine-dominated ecosystems, including pitch pine barrens and natural red pine stands. Pitch pine barrens, in particular, are often unmanaged and lack the regular occurrence of fire and management intervention necessary to reduce overstocking and the dense canopy conditions that are conducive to SPB (Dodds et al. 2018). Areas where potential host species occur in relatively pure, high density stands across large areas (such as red spruce) will also be at high risk for SPB infestations.



A new study (Lesk et al. 2017) maps where and when the climate will likely become suitable for southern pine beetle in the northeastern U.S. and southeastern Canada.

Winter minimum temperatures between 7 and -4 ⁰F are sufficient to kill SPB while they overwinter within the inner bark of trees. This limited cold tolerance restricts the northern extent of their geographic range. Bark also has an insulating effect that provides the beetles with some protection and buffers cold extremes by 4-7 ⁰F, which researchers accounted for in this study.

They found that, currently, the northernmost SPB sites are generally at latitudes where the lowest winter temperature in the inner bark is 14 ⁰F. They estimate that an “SPB-suitable” climate will eventually develop in other areas if conditions warm up enough to keep the winter inner bark temperature above 14 ⁰F for ten consecutive years. Using these parameters and the results from two dozen global climate models (which project an increase in annual minimum air temperature of 6-13 ⁰F by mid-century), researchers modeled the emergence of SPB-suitable climate over this century.

On average, they found that by 2090 all of the northeastern U.S. and large areas of southeastern Canada will be hospitable for SPB (see Figure 2a). Although, they also noted that there is considerable uncertainty, with a spread of several decades between the low- (see Figure 2b) and high-end (see Figure 2c) of estimates. The results suggest that several likely northern host species will be vulnerable over large portions of their range by mid- to late-century—by 2050, an estimated 78% of the pitch pine range will be suitable for SPB and by 2080, 71% of red pine and 48% of jack pine range will be suitable.


SPB Management: Things to Do

For managers encountering this pest for the first time, there is extensive literature and practice to draw on for determining the best response. Within its historic range, managers have developed and honed management strategies that control the spread and severity of infestations when they occur and reduce the risk of future outbreaks.

The “things to do” include:

  • prevention (i.e. thinning high hazard areas),
  • landscape prioritization and hazard models (i.e. assessing susceptibility based on stand characteristics to identify priority areas for treatment),
  • detection and monitoring (i.e. aerial surveys and pheromone-baited traps), and
  • evaluation and direct control (i.e. cutting infested trees and a green tree buffer using the cut-and-leave or cut-and-remove method).

(Dodds et al. 2018)

The management tactics used in the southern U.S. appear to have worked equally well in the recent northern outbreaks. They included early monitoring, rapid treatment of initial SPB “spots,” and preventative thinning (Aoki et al. 2018). In fact, thinning (and other methods of reducing density) in pine-dominated stands has proven to be the most effective tactic for reducing susceptibility to SPB. As one source puts it: “The best silvicultural defense against SPB is to manage forest stands so that individual trees are vigorous and stands are not overstocked” (Guldin 2011). Evidence for the effectiveness of this approach can be found within the historic SPB range where there are examples of outbreaks that occurred almost entirely on unthinned stands (Nowak et al. 2015). Although, it is worth noting that sufficient markets and timber harvesting infrastructure must be present to make such preventative treatments feasible and cost-neutral (at best). That may be more of an impediment for northern landowners than in the southeastern U.S. where SPB suppression has been very successful
(Ayres & Lombardero 2018; Morris et al. 2018).




The general management recommendations described above can apply to any pest infestation, not just southern pine beetle—be proactive where possible, watch for change, and plan for control efforts in susceptible areas. As some native pests move into new areas, it will be important to draw on the existing knowledgebase from their historic range, but it is also likely that traditional host species, risk factors, and effective management may be different because of changes in the biology of the pest or differences in host susceptibility. The mountain pine beetle (MPB) in the West provides a recent example, with evidence that MPB enjoyed greater reproductive success in lodgepole pine as it encountered “naïve” host trees in areas where the climate was previously unsuitable—a factor that likely contributed to the unprecedented scale of recent outbreaks in western Canada (Cudmore et al. 2010). This highlights the importance of staying attuned to the expanding range limits of particularly destructive pests, like SPB, and closely monitoring reports from newly affected areas to learn whether there are notable changes in pest behavior or outbreak patterns.



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