Since the 1980s, wildfire activity in the Western United States has increased abruptly. A 2020 study found that between 1985 and 2017, the annual area burned by high severity fires in western forests increased eight-fold. The West’s record-setting 2020 fire season resulted in more than 6 million acres burned, over 10,000 structures damaged or destroyed, and immeasurable harm to human health including thousands of smoke-related deaths. That year, more than 3.7 million acres were burned in California, where the state experienced five of the six largest wildfires in state history.
A 2016 study on the increasing western U.S. forest wildfire activity examines how a variety of climate-driven factors collectively influence the timing, extent, and severity of wildfires. Across the West, hotter temperatures in the spring and summer months are drying out fire fuels, defined by the Department of Interior as “all kinds of plant material, including grasses, shrubs, trees, dead leaves, and fallen pine needles.” The western states have experienced a 2.1oF increase in summer temperatures in the past 45 years. Since 1970, the years with the hottest spring and summer temperatures were typically also the years with the highest number of large fires on National Forest land. According to the National Oceanic and Atmospheric Administration (NOAA), summer 2021 was the hottest on record for five states––California, Idaho, Nevada, Oregon, and Utah––all of which happen to be in the western region of the country.
Another climate-related factor that influences wildfire behavior is changes in mountain snowpack. In recent years, more winter precipitation has occurred in the form of rain rather than snow in the West. This leads to less water stored in mountain snowpacks that play a vital role in keeping fuels moist during the hotter, drier wildfire season. Additionally, warmer spring and summer temperatures result in an earlier melting of the snowpack. Similar to the data on warmer temperatures, research shows that years with higher wildfire frequency follow the same trend as years with low snowpack.
To make matters worse, warmer, drier conditions also contribute to the spread of the mountain pine beetle and other insects that can weaken or kill trees, building up the fuels in a forest. Since 2000, the mountain pine beetle has killed roughly 100,000 square miles of trees across western North America. These beetles burrow inside living trees to lay eggs, and their offspring then feed on the tree from the inside, destroying the tree’s vital systems. This process takes only a year and is occurring more frequently as climate change has eliminated the cold spells that would normally kill off the beetles and made the trees easier targets that are stressed from drought conditions.
One key difference between historic wildfire trends and the current climate-induced wildfires that have been devastating the Western states is the number of high severity fires. The U.S. Forest Service (USFS) defines fire severity as “the amount of fire-induced change to physical ecosystem components such as vegetation and soil.” Fires that are classified as high severity are the wildfires that cause the most devastation to forests and lead to major ecological impacts and long recovery periods. According to the USFS, from 1985 to 2017, there has been a significant increase in the annual area burned in high severity fires in most regions across the country, with the most drastic increase occurring in western forests.
Several factors, including the amount of live fuel, topography, climate, and fire weather, influence the probability of high-severity wildfire events. A 2018 study determined that live fuel was the most important factor driving high-severity fire among forests in the Western United States. An increase in hot dry weather in these areas because of climate change results in more dead or dry vegetation, which acts as live fuel, and more days with extreme temperatures that are classified as “fire weather.”
These destructive high-severity fires not only kill most trees in their paths but also increase carbon emissions and biomass consumption, degrade water quality through erosion, and alter post-fire vegetation recovery. High-severity fire's threat to forests is also a threat to one of mankind's most important carbon sinks that are needed to mitigate greenhouse gas (GHG) emissions. Trees naturally capture carbon from the atmosphere and store it throughout their lifetime. Across the United States, forests sequester 866 million tons of carbon annually, which counters 16 percent of the country’s annual GHG emissions. Unfortunately, when biomass burns and fire-killed trees decompose, forests shift from carbon sinks to sources for years or decades. High-severity fires kill massive amounts of trees in a short period of time, releasing massive amounts of carbon into the atmosphere at an abnormally fast rate. A forest’s ability to transition back to a sink is dependent on post-fire succession, which in some cases can result in non-forested areas. This is especially common in areas that underwent multiple wildfires in a short period of time.
Wildfires have a unique relationship with climate change because they are driven by climate change and they also help drive it. Wildfires are an example of a positive climate feedback loop meaning they accelerate climate change through a cyclical chain reaction. Understanding this climate feedback loop is crucial for understanding the extent of the issue at hand. At the end of 2020, the California Air Resources Board determined that the 2020 wildfires surmounted to an estimated 112 million metric tons of carbon dioxide (CO2), which is equivalent to the emissions of 24.2 million passenger cars annually. Wildfires also emit aerosols, or tiny particles of organic matter, into the atmosphere. These aerosols can come in the form of black or brown carbon, both of which have a warming effect on the earth. Black carbon, commonly referred to as soot or ash, is produced when fires are extremely hot and has a very intense warming effect. Compared to CO2, black carbon’s warming impact on climate is 460 to 1,500 times stronger per unit of mass. This vicious cycle of a warming climate and worsening wildfires is set to continue and will require serious forest management changes and climate adaptation strategies.
Two important management strategies should be incorporated into forest planning in order to slow the cycle of climate change and wildfires. First and foremost, forest carbon sinks need to be maintained and enhanced. In a 2020 study on carbon sequestration and climate adaptation, researchers synthesized broad forest carbon management concepts to create a list of actionable tactics that forest managers can use to ensure greater forest carbon resources. The study recommends that managers increase the extent of forest ecosystems by avoiding forest conversion to non-forest land uses, reforesting lands that have been deforested and afforesting suitable lands, and increasing the extent of forest cover within urban and agricultural areas. Additionally, it suggests that managers sustain fundamental ecological functions through improved soil and nutrient cycling, hydrology, pest resistance, and protection of native over invasive species. It further concludes that managers should enhance forest recovery following disturbances through techniques such as promptly revegetating sites after disturbance, restoring disturbed sites with a diversity of species that are adapted to future conditions, and guiding species composition at early stages of development to meet expected future conditions.
The second strategy that forest managers should prioritize is the use of prescribed burns to assist in wildfire management. Periodic wildfires have always been an integral part of healthy forest ecosystems. According to Jennifer Fawcett, a forestry expert from NC State University, prescribed burns can mimic the effect of the low-intensity wildfires that would naturally occur in forests if humans did not suppress them. These controlled fires produce similar benefits by helping maintain wildlife habitat, prevent forest overcrowding, and lessen the severity and frequency of destructive wildfires.
The above forest management strategies, along with many others, can be used to assist the United States in meeting its Paris Agreement goals. In April 2021, the United States published its revised Nationally Determined Contribution (NDC) that announced the nation’s most up-to-date climate action goals, most importantly being its plan to reduce net emissions by 50 to 52 percent below 2005 levels by 2030. The NDC outlines the wide range of strategies necessary to meet this goal, including a plan to “reduce emissions from forests. . . and enhance carbon sinks through a range of programs and measures.” With the worsening effects of climate change looming over our shoulders and another devastating fire season right around the corner, taking steps toward climate-resilient forest management has never been more urgent.
