This wildfire resource is from Climate Signals:
Climate change has increased the risk of wildfires through warmer temperatures and drier conditions that lengthen wildfire season, increase the chances of a fire starting, and help a burning fire spread. Warmer and 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.
Scientists have observed a significant increasing trend in the number of large fires and the total area burned per year in the United States. In the West, anthropogenic climate change has been directly linked to drier conditions and increases in forest fire activity.
Climate science at a glance
- Higher temperatures, drier conditions, increased fuel availability, and lengthening warm seasons—all linked to climate change—are increasing wildfire risk.
- A warmer world has drier fuels, and drier fuels make it easier for fires to start and spread.
- Climate change lengthens the window of time each year conducive to forest fires.
- Earlier snowmelt, temperature changes, and drought associated with climate change are important contributors to the increase in large and long fires.
The factors that influence wildfire frequency
The frequency of large wildfires is influenced by a complex combination of natural and human factors. Temperature, soil moisture, relative humidity, wind speed, and vegetation (fuel density) are important aspects of the relationship between fire frequency and ecosystems. Forest management and fire suppression practices can also alter this relationship from what it was in the preindustrial era.
The role of fire management practices
In addition to climate change, historic fire suppression has played a role in wildfire activity.
Past fire suppression has led to changes in fuels, fire frequency, and fire intensity in some southwestern ponderosa pine and Sierran forests but has had relatively little impact on fire activity in portions of the Rocky Mountains and in the low-lying grasses of southern California.
Changes in firefighting practices over time—such as more frequent use of intentional burning to clear fuels as a fire suppression tactic—may have had impacts on the boundaries of burn areas, but generally, the effects of human development vary regionally, in some cases increasing fire activity and in others decreasing it.
Regardless of changes in the landscape due to forest management, hotter and dryer conditions due to human-caused climate change make it easier for fires to spread. Observations show that climate change has already had a hand in shaping fire seasons, especially in California and the western United States.
US wildfire trends and climate change
- Recent decades have seen a profound increase in forest fire activity over the western United States.
- Hotter and drier weather and earlier snowmelt mean that wildfires in the West start earlier in the spring, last later into the fall, and burn more acreage.
- Two climate factors affect fire in the western United States: increased fuel flammability driven by warmer, drier conditions and increased fuel availability driven by prior moisture.
- There is a clear link between increased drought and increased fire risk.
- More than half the US Western states have experienced their largest wildfire on record since 2000.
- From 1980 to 2010, there was a fourfold increase in the number of large and long-duration forest fires in the American West; the length of the fire season expanded by 2.5 months; and the size of wildfires increased severalfold.
- Warmer winter temperatures affect the prevalence and distribution of pine beetles by allowing the beetles to breed more frequently and successfully.
- The southwestern United States has already begun a long-predicted shift into a decidedly drier climate.
- Higher temperatures, reduced snow pack, increased drought risk, and longer warm seasons are all linked to climate change, and in recent decades, these have increased wildfire risk, contributing to the frequency and severity of wildfires.
- In California, 14 of the state’s 20 largest wildfires on record have all burned since 2000.
- In southern California, weather conditions, such as unusually hot local temperatures, are the primary driver of the size of spring and summer fires.
- A 2006 study found a statistically significant relationship between warming in the North Pacific and all the major wildfire events in the northwestern US from 1980-2002.
- A 2017 study found that the total area burned by large wildfires in the Great Plains rose 400 percent over a three-decade study period (1984-2014). The study also found that the average number of large wildfires in the biome increased from about 33 per year from 1985 to 1994 to about 117 wildfires per year from 2005 to 2014.
- The southeastern US (including Texas and Oklahoma) leads the nation in number of wildfires, averaging 45,000 fires per year, and this number continues to increase.
- Increasing temperatures contribute to increased fire frequency, intensity, and size.
- Lightning is a frequent initiator of wildfires, and the Southeast currently has the greatest frequency of lightning strikes of any region of the country. Increasing temperatures and changing atmospheric patterns may affect the number of lightning strikes in the Southeast, which could influence air quality, direct injury, and wildfires.
- Recent decades have seen a profound increase in forest fire activity in Alaska.
- Total area burned and the number of large fires (those with area greater than 1,000 square km or 386 square miles) in Alaska have increased since 1959.
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