By Si Balch, John Hagan and Eric Walberg
Context
On May 6, 2014, the White House released the third U.S. National Climate Assessment Report. The USNCA is the most authoritative and comprehensive source of scientific information to date about climate-change impacts across all U.S. regions and on major sectors of the economy. The Assessment was produced by the U.S. Global Change Research Program, established in 1989 by Congress to assist the Nation and the world to understand, predict, and respond to human-induced and natural processes of global change. The Third Assessment was authored by a group of over 300 scientists and reviewed by the National Academy of Sciences. Climate Smart Land Network members should be aware of this report. The purpose of this Bulletin is to summarize a few of the essential conclusions for you. You can visit the Global Change Program’s web site for more information.
Take Home Messages for Forest Landowners
The central message of the report is that human-caused climate change is not a distant future threat. We are seeing the effects today. Compared to the previous Assessment (2009), the 2014 update finds increasing evidence of sector-specific climate impacts on natural resource-based industries such as forestry and agriculture.
Temperature Trends
U.S. average temperature has increased by 1.5°F – 1.9°F since 1895 with most of the increase occurring since 1970. The most recent decade was the warmest on record and 2012 was the warmest year on record. The recent rapid warming is projected to continue with the rate dependent on greenhouse gas emission rates.¹ Figure 2.7 below shows temperature change over the past 22 years. (Note that all of the figures in the bulletin are from the Assessment and the figure numbers have been maintained to facilitate locating them in the report.)
Lengthening Frost-free Season
The length of the frost-free season (and thus the growing season) has been increasing nationally since the 1980s. As a result, forest productivity has increased in those areas that are not moisture limited. Additional increases of a month or more in the growing season are projected across most of the U.S. as greenhouse gas emission rates continue to rise.¹ Figure 2.10 shows the increase frost-free season length during the period 1991-2012.
Precipitation Trends
Average U.S. precipitation has increased since 1900, but some areas have had increases greater than the national average, and some areas have had decreases. More winter and spring precipitation are projected for the northern U.S., and less for the Southwest over this century.¹ Figure 2.12 shows change in total annual precipitation for 1991 – 2012 as compared to the 1901 – 1960 average.
Increasing Heavy Precipitation
The amount of precipitation falling in the heaviest daily events is increasing for all regions of the U.S. Figure 2.18 shows the increase from 1958 to 2012. This trend is projected to continue for the entire nation.¹
Trends in River Flood Magnitude
The regional trends in river flood magnitude (Figure 2.21) mirror the trends in average annual precipitation. The increase in magnitude of river flooding is smaller than the increase in extreme precipitation due to the fact that the increase in extreme precipitation events have been more concentrated in summer and fall when soil moisture is low and soils can absorb a greater fraction of rainfall.¹ Flash flooding trends in small watersheds with steep slopes that concentrate water flow are more directly linked to regional trends in extreme precipitation.
Detailed Regional Trends and Projections
If you are interested in greater detail on seasonal trends in temperature and precipitation than provided in the Assessment, a set of reports developed by the National Oceanic and Atmospheric Administration in support of the 2014 Assessment are linked on the CSLN website: https://climatesmartnetwork.org/resources-regional-trends-and-projections/. These reports are valuable in that they use consistent methods to identify and depict statistically significant trends for all of the regions in the continental U.S., making it easy to compare trends and projections between regions.
Forests and Climate Change
Chapter 7 of the Assessment covers several topics related to forests and changing climate. The four key messages are:
- “Climate change is increasing the vulnerability of many forests to ecosystem changes and tree mortality through fire, insect infestations, drought, and disease outbreaks.”¹
o Forests in the western U.S. have been severely impacted by a combination of increasing average temperatures and increasing extreme heat that have exacerbated the damage inflicted by naturally occurring drought. The stress placed on western forests by these conditions has coincided with a massive pine beetle epidemic and extensive wild fire resulting in widespread forest mortality. Temperature-related forest stress is projected to worsen in moisture-limited regions as the climate continues to warm.
o Projections for forests in regions with increasing annual precipitation such as the Midwest and Northeast are less dire. As temperatures warm and growing seasons lengthen forest productivity in these regions is increasing. However, stressors such as changing pest and disease pressure, and extreme weather events could blunt opportunities for increased forest productivity.
- U.S. forests absorb and store approximately 16% of the CO2 produced by burning fossil fuels in the U.S.¹
o In regions with ample moisture the combination of warming temperatures and elevated CO2 levels has the potential to increase carbon sequestration and storage rates. However, for U.S. forests in aggregate, the combined impacts of climate change, land use change and forest management practices are projected to result in future reductions in forest CO2 uptake.
- The bioenergy market could play an important role in supporting the restoration of forests killed by drought, insects and fire.¹
o Bioenergy is identified as a possible funding mechanism for salvage from damaged forest. The recent expansion of natural gas production in the U.S. has blunted the development of the wood bioenergy market, but the European market for wood pellets continues to grow.
- Forest management response to climate change will be influenced by a mix of changing ownership patterns, globalization of forestry markets and changing U.S. climate policy.¹
Management Implications
The Assessment underscores the regional differences in climate change impacts. Appropriate management response will be dependent on the region(s) in which your operations are located. Note that future editions of the CSLN Bulletin will cover the management implications of changing climate in greater detail.
Warming Temperatures
Continued warming temperatures will provide a mix of opportunities and increased forest stress.
- Longer growing season and a CO2-rich atmosphere will provide opportunities for enhanced forest productivity in regions that are not moisture limited.
- Opportunities to transition to tree species that thrive in a warming climate will increase.
- Drought is becoming a more potent forest stressor due to increasing evapotranspiration rates and increased incidence of extreme heat.
- Opportunities to access northern forests on frozen ground will continue to diminish.
- The range of pests that are limited by cold conditions will continue to expand.
Changing Precipitation Patterns
- Regions with adequate precipitation will have opportunities for increased forest productivity.
- Stream crossings and culverts may need to be upgraded to accommodate increasing heavy precipitation and increased flooding, particularly in those regions that are experiencing the sharpest upward trends.
- The continued trend towards precipitation coming in heavy downpours will increase forest stress.
¹ U.S. Global Change Research Program. Climate change impacts in the United States: U.S. national climate assessment. (2014). at http://purl.fdlp.gov/GPO/gpo48682