Understanding the climatic conditions that influence wildfire patterns can improve our ability to predict the occurrence and severity of future wildfires, and ultimately support the development of effective adaptation strategies for the 21^st century.

Research on the relationship between wildfire and other ecological disturbances such as drought is invaluable to resource managers working to plan for and adapt to the evolving threat that fire poses to humans, infrastructure, and ecosystems. Changes in climate conditions and disturbance events like drought will likely alter fire regimes, affecting people and ecosystems in new ways. Understanding how landscapes will respond to these shifts is key to adaptive management. In response to this need, Jeremy Littell, a USGS research ecologist at the Alaska CASC, has been working to contribute to our existing body of wildfire knowledge. The results of some of this work were published in two papers in 2018.

First, Littell examines wildfire response to drought in a review of recent and relevant literature focusing on this relationship. The presence of drought does not automatically predict wildfire, though both phenomena share obvious indicators such as lack of precipitation or decreased water availability caused by climate-driven changes to the atmosphere. The nuances involved in the causal factors of both phenomena are considered in Littell’s review, beginning with drought type.  Global-change-type droughts, for example, are droughts influenced by steadily increasing temperatures and precipitation deficits over long periods of time. Alternatively, flash droughts are the rapidly-occurring result of temperatures reaching ultimate highs, as seen in the Midwest and Northwest, or precipitation dropping to severe lows in a given region, such as the Great Plains. This knowledge has helped scientists pinpoint the causes of a few extreme wildfire incidents. For instance, historical droughts in Washington were linked to long-term precipitation shortages, but the cause of several wildfires during the summer of 2015 was one uncharacteristically warm winter which occurred despite the state’s near normal winter precipitation that same year.

This anomaly is one example of the extent to which drought can influence both the spatial and temporal conditions of wildfires. Long-term droughts can illicit changes in regional vegetation characteristics over the course of many years.  Vegetation, or fuel for fires, increases during wetter periods, and can decrease during drier periods. Consequently, the amount and arrangement of regional vegetation available dictates the expanse of a potential fire. In contrast, the short-term, even seasonal, conditions associated with a fire or group of fires within a region in a given year is called fire weather. An increase in these conditions, including low precipitation, low humidity, rising temperatures, wind, and increased vegetation flammability, is caused by changes in and persistence of atmospheric pressure patterns, partially controlled by ocean surface temperatures. This type of information, coupled with a greater understanding of how people affect and respond to wildfires in the long-term, could ultimately result in predictions of the timing and size of future wildfires which regional managers could use to plan accordingly.

Second, Littell and colleagues developed climate-fire models to quantify expected rates of change in ecosystems due to fire. They analyzed the resulting projections of future wildfire potential in the western United States and discussed their outcomes in a second publication. The relationship between climate and wildfire also greatly depends on the type of ecosystem. Different ecosystems contain different types of vegetation, or fuels, and fuel type determines the sensitivity to climate and ultimately the projected area burned. The proliferation of fuels in different ecosystems is controlled by regional climate. For example, drier ecosystems burn more easily, but only with a sufficient amount of fuel. Vegetation growth in dry ecosystems may be stimulated by an abnormally wet year, creating more fuel and the potential for larger wildfires and a greater percentage of regional area burned.

In this additional study, Littell and colleagues further investigate how the percentage of burned areas of different ecosystem types in the West may be affected by future changes in climate. The project team characterized 70 different western ecosystems and discovered that some are more vulnerable to future wildfire than others. Based on the projections from these models, Littell and colleagues expect that western land burned by wildfire will increase in forests and in some areas with a mix of forests and non-forest vegetation, though these projections do not represent long-term forecasts. The goal was to deliver actionable results, meaning the projections from these models are fine-tuned to each ecosystem analyzed in the study and are relevant to the management of each individual landscape found in the western United States.