Reidmiller et al. (eds.) 2018
RESEARCH BRIEF 27
In future decades, wildfire risk is expected to increase in many U.S. regions due to increased temperatures and drought.
Recurring prescribed fire may mitigate climate change related wildfire risks.
Land management practices can increase forest tolerance to heat, drought, and wildfire, and may lessen impacts related to air quality.
The 1500+ page Fourth National Climate Assessment (NCA4) describes the physical impacts on earth systems in the U.S. due to climate change. The report broadly covers projected impacts and risks for society, human health, and the environment for 10 U.S. regions and18 topic areas. NCA4 does not address wildland fire as a stand-alone chapter topic, therefore this brief synthesizes information contained throughout that relates to forests, wildfire, and prescribed fire. This summary is focused on three NCA4 regions (Midwest, Southeast, and Southern Great Plains) that intersect the eight states included in the Oak Woodlands & Forests Fire Consortium region (Arkansas, Illinois, Indiana, Kentucky, Missouri, Oklahoma, Tennessee, and Texas). We have organized the information in terms of climate change impacts on temperature, current and future forest conditions, atmospheric carbon and air quality, as well as by region, followed by adaptation strategies.
A primary indicator used to monitor the changing climate is temperature, and since 1900, average annual global temperatures have risen 1.8°F. Since 1986, global annual average temperatures have risen faster than any 30-year period in at least the last 1700 years. Numerous record breaking weather extremes have occurred in recent years. For example, 16 of the last 17 years have been the warmest recorded. Projected scenarios suggest that average annual temperatures in the U.S. may increase by about 2.5°F within a few decades and perhaps by 3°F to 12°F by the end of this century. Additional climate change indicators reported by NCA4 scientists include: precipitation, snow cover, glaciers, growing season timing, sea ice, extreme weather events, lake levels, drought, and wildfires.
Current & Future Forest Conditions
Modern forest practices have resulted in reduced structural diversity and decreased biodiversity. Fire suppression has encouraged growth of more mesic tree species, such as maples, while decreasing populations of drought-tolerant species such as oak, hickory, and pine. It has also led to denser forests with high fuel loads. Under these conditions, forests are more at risk as the likelihood of drought increases.
An increase in the frequency and intensity of severe weather events is expected, which will increase forest disturbances, leading to rapid and persistent forest changes. At a slower pace, gradual climate change is also likely to affect forest health and tree distributions. In the eastern U.S., changing precipitation trends have already spurred species migration. These changes are expected to further impact tree species distributions, particularly through drought-altered fire regimes.
Since 1979, the number of days per year when conditions were conducive to fire has increased, and wildfire size, frequency, and severity are increasing in the U.S. Higher temperatures, reduced summer precipitation, and earlier snowmelt have extended the wildfire season in the western U.S., and the area burned there doubled between 1984 and 2015. Some of the larger, more intense fires have thwarted suppression, putting property, residents, and firefighters at risk, particularly at the wildland-urban interface. In 2017, when some areas experienced extreme drought, the federal government spent about $2.9 billion to suppress wildfires, almost triple the annual average over the previous three decades. In the future, wildfire-related expenditures are projected to further rise, including costs such as property losses, firefighting expenses, impacts on health, and preventative measures such as fuel management.
With frequent and longer droughts, warmer springs, and drier soils, the frequency and size of wildfires will continue to increase. By mid-21st century, the area burned in the U.S. every year could be 2-6 times greater than at present. These fires and their associated smoke, along with other climate-related factors, are expected to reduce outdoor activities and recreational use in U.S. forests and reduce forest economic productivity. However, not all fires will have a negative impact, particularly in fire adapted forests. Previously burned areas will provide fuel breaks that lower the severity, extent, and vegetation mortality for future fires. As incidents of wildfire increase, the factors that will affect the resilience of ecosystems in fire-prone forests are: the presence or absence of fire-adapted species; the intensity and frequency of fires; and societal responses to fires.
Atmospheric carbon & air quality
Trees are vital to carbon sequestration, though their ability to store or release carbon varies greatly regionally and can be affected by heat waves, insects, drought, and wildfires. A substantive transfer of carbon into the atmosphere is expected as tree mortality from fire and other causes prompts conversions to shrubs and grass. However, aerosols and particulate matter released by fire are reflective, which may partially offset warming. Effects of fire on atmospheric carbon dioxide depend on fire frequency, severity, and extent. High severity wildfires emit large quantities of carbon, and following such fires, forest regrowth will increase carbon storage, but short term gains may be small compared to prefire conditions.
An increase in wildfires will impact air quality. Wildfire smoke can travel hundreds of miles; it impairs visibility, disrupts outdoor activities, and contains ozone forming gases and particulate matter. Inhalation of particulates is associated with most air pollution-related health problems in the U.S., and can aggravate conditions including asthma, bronchitis, pneumonia, and respiratory illness. In 2011, wildland fires were responsible for about 40% of particulates directly emitted into U.S. air, and wildfires are expected to drive future summertime particulate concentrations. However, complex weather variables, especially precipitation, which removes particulates from the air, cause difficulty in projecting future climate-driven concentrations of particulate matter.
Forests in the southeastern U.S. comprise almost 27% of the national total; many are privately owned, with high crop value. Forest coverage is 50-80% in these states, and forest areas are interspersed with population centers. Fire has played an important historical role in creating and maintaining biological diversity, and some species are dependent on fire. In the southeastern U.S. exist some of the largest landscapes managed with prescribed fire, and the number of wildfires is also highest, though more acres are burned by wildfires in the western U.S.
Studies suggest that in the near future the southeastern U.S. can expect a longer wildfire season, larger and more frequent fires, and increased effects from severe droughts and rising temperatures. This will negatively impact local economies and air quality, and people living in forested areas will be particularly vulnerable. While a reduction of human-ignited wildfire is expected, the area burned by lightning fires is projected to increase 30% by 2060. Fire-adapted ecosystems are expected to be resilient in the face of periodic fire, but humans living there are not as well-adapted, and thus increased losses of life and property are likely. Although high intensity, stand-replacing fires are infrequent, they do occur in the region during times of extended drought and high winds, such as the destructive and deadly fire in the area of Gatlinburg, Tennessee, in the fall of 2016.
In fire-dependent forests in the Southeast, wildfire risk can be reduced by thinning tree stands, using recurrent prescribed fire, and allowing certain wildfires to burn if manageable. Low intensity prescribed fire is considered an effective approach to reduce risk of high severity wildfire. Doubling prescribed fires at the landscape scale has been shown to reduce incidents of wildfire by a multiple of four. A 30-year study at Ft. Benning, Georgia, showed that wildfires decrease when prescribed fires increase, and that restoration of native longleaf pine enhances drought resistance in forests. However, urbanization near forests could reduce the ability to manage with prescribed fire.
While NCA4’s Midwest assessment did not directly address fire frequency or fire impacts, it did project that higher temperatures will have a pervasive influence on the region’s forests in coming decades. Here, the frost-free season may increase by 10 days over the next three decades and may increase by about a month by the end of the 21st century. By mid-century, 5-day average high temperatures could be as much as 13°F warmer than similar periods in the late 1900s. In southern Missouri, the average annual 5-day maximum temperature historically has been 97°F; climate models project that this will increase to 102-103° F by mid-century.
These higher growing season temperatures, changing precipitation patterns, and drier air are expected to stress and kill young, drought-sensitive trees, especially in areas where soil moisture is low, such as dense stands with high competition and where forests transition to grassland. Such stress will make trees more susceptible to diseases and insects, which are expected to survive better and expand ranges as winters warm. Suitable habitat for any given species is generally expected to move north, and Midwest tree species would typically need to move about 90 miles north to reach an area that is 1.8°F cooler. Thus, population numbers are expected to drop for species at the edge of their range. Tree mortality and changes in forest composition by century’s end would likely cause substantial losses of timber value. Forest managers can increase resilience by selecting tree species adapted to the anticipated future conditions.
Southern Great Plains
This region’s grassland, rangeland, and forest ecosystems are adapted to fire. Grazing land for livestock represents more than half of the conterminous U.S. land area, and is of great cultural and economic importance in the southern Great Plains. However, agricultural productivity is expected to decline as livestock forage is reduced due to droughts and fires. Wildfire risk and the length of fire seasons are expected to increase as summer temperatures rise. NCA4 links a number of recent extreme wildfires to the ongoing impact of climate change, citing several fires since 2011 in Texas, Oklahoma, and Kansas that burned vast acreage, killing people and livestock, and destroying more than a thousand houses.
Land management & Adaptation
Land management will play a critical role in mitigating potential wildfire severity. Some practices already in use have been shown to increase tolerance within forest communities to drought, high temperature, and disturbance. Existing forest management strategies and practices that may lower wildfire risk and negative effects include: reducing stand densities, prescribed burning, reducing surface fuel, managing invasive species, curtailing accidental ignitions, restoring aquatic habitats, and changing fire suppression practices.
Research is needed to determine best local management applications. In particular, reducing fuels on large (e.g., landscape) scales has the added advantage of providing firebreaks to help fight wildfires and manage prescribed fires, especially near communities. However, land managers face high implementation costs, legal barriers, and tradeoffs in protecting other natural resources. Human actions (and inactions) affect the frequency and severity of wildfires, and land management decisions such as these may outweigh climate change effects.
Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.), 2018: Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II
FOR FURTHER READING
Addington, R.N., S.J. Hudson, K.J. Hiers,M.D. Hurteau, T.F. Hutcherson, G. Matusick, J.M. Parker. (2015). Relation
ships among wildfire, prescribed fire, and drought in a fire-prone landscape in the south-eastern United States. International NCA4, Figure ,6.1. Journal of Wildland Fire 24, 778-783.
The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, land owners and the public about the use, application, and effects of fire in the region. www.oakfirescience.com
This research brief was funded by The Joint Fire Science Program. www.firescience.gov