Study: Increasingly frequent wildfires linked to human-caused climate change

NASA astronauts could see smoke from a Northern California wildfire in 2019 from the International Space Station. Credit: NASA
Researchers from Lawrence Livermore National Laboratory and UCLA have confirmed that climate change is the primary reason for the increasing amount of land that has been burned by large wildfires in the west U.S. over the past 20 years.

Rong Fu, an UCLA professor of atmospheric-oceanic sciences and study's coauthor, stated that the trend will only get worse in the coming years. "I fear that record-breaking fire seasons in recent decades are just the beginning of what is to come due to climate change. Our society is not ready for the rapid rise in weather conditions contributing to wildfires throughout the American West.

U.S. Geological Survey data shows that wildfires have caused a dramatic increase in damage. The average annual burned area in the 11 western states over the period 1984-2000 was 1.69million acres. Through 2018, the average annual burned area was 3.35 million acres. According to a report by the National Interagency Coordination Center, wildfires burned 8.8 million acres in the West in 2020. This is more than the entire state of Maryland.

The question is: What percentage of this trend can be attributed to human-induced climate changes? How much could natural variability, changing weather patterns, forest management, early springtime snowmelt, and reduced summer rainfall explain the large increase in precipitation?

The study was published in Proceedings of the National Academy of Sciences on Nov. 9, 2009. It used artificial intelligence to analyze fire and climate data to determine the role of climate change and other factors in wildfire risk.

The amount of moisture that the air can retain when saturated is called the vapor pressure deficit. It's less than the amount of water in the air. The VPD (vapor pressure deficit) is a measure of how much air can absorb moisture from soil or plants. It is higher than the normal. Large areas that have been burned by wildfires, particularly those located far from urban areas, are more likely to experience high vapor pressure deficits. These conditions are often associated with dry, warm air.

According to the study, human-caused global heating was responsible for 68% of the increase in the vapor pressure deficit in the western U.S. from 1979 to 2020. According to the authors, the remaining 32 percent of the change was likely due to naturally occurring changes in weather patterns.

These findings indicate that climate change caused by humans is responsible for the increase in fire weather in the west United States.

Fu, who is the director of UCLA's Joint Institute for Regional Earth System Science and Engineering in collaboration with NASA's Jet Propulsion Laboratory, stated that "And our estimates regarding the human-induced effect on the increase fire weather risk are likely not to be conservative."

Researchers analyzed the 2020 August Complex wildfire, which blazed more than 1 million acres in Northern California. The researchers concluded that 50 percent of the unimaginably high VPD observed in the area during the month when the fire started was likely due to human-induced heating.

Fu stated that wildfires will continue to be more intense and frequent in the west, even though it could be more difficult to find wetter or cooler weather. Even though the vapor pressure deficit is increasing, wildfires are unlikely to increase in areas that have lost large swathes of vegetation to fires, droughts, heatwaves, and the construction of roads.

Fu stated that "our results suggest that the west United States appears have passed a critical thresholdthat human induced warming is now more accountable for the increase in vapor pressure deficit than natural variations of atmospheric circulation." Fu said that the analysis showed that this change occurred much earlier than expected.

Explore more Dryer and warmer night air makes some Western wildfires more active at nights

Information: Yizhou Zhuang and colleagues, Quantifying the contributions of natural variability to increased fire risk in the western United States, Proceedings of the National Academy of Sciences (2021). Information from the Journal: Proceedings of the National Academy of Sciences Yizhou Zhuang and colleagues, Quantifying the contributions of natural variability as well as anthropogenic forcings on an increased fire risk in the western United States. (2021). DOI: 10.1073/pnas.2111875118