Action urged to combat wildfire threats

Human-caused climate change is the main driving factor behind more frequent and intense wildfires in the western United States, according to a new study that calls for urgent action to mitigate the trend toward increasing fire weather risk.

The western part of the US has experienced a rapid increase in fire weather in recent decades, especially during the warm season from May to September. Anthropogenic warming, which means temperature increases caused or influenced by people, has contributed at least twice as much as natural climate variability to the rapid rise, according to the study published on the Proceedings of the National Academy of Sciences website on Monday.

Researchers at the University of California, Los Angeles, and the Lawrence Livermore National Laboratory used vapor pressure deficit, or VPD, as an indicator for fire weather. Considered as an important factor in predicting fire risk, VPD measures the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated.

By observing the VPD trend from 1979 to 2020, the researchers estimate that natural variability is only 32 percent responsible for the weather fueling the increased wildfires, while anthropogenic warming accounts for 68 percent.

Climate models also suggest that anthropogenic warming contributes 88 percent of the VPD trend. Both estimates suggest that anthropogenic warming is the main cause for increasing fire weather.

Raging blazes

Wildfires razed an average 13,400 square kilometers of land per year in the western US from 2001 to 2018－nearly double the area burned when compared with the period from 1984 to 2000. Last year was a record-breaking fire season in the history of the western US, especially in the coastal states of California, Oregon and Washington.

The August Complex, California's largest wildfire on record, occurred in the northern part of the state in August 2020. The single fire covered about 4,000 sq km. Researchers estimated that anthropogenic warming contributed 50 percent to this "gigafire".

This year, California reported its hottest summer on record and its driest water year in nearly a century. Wildfires have burned about 10,000 sq km in the state this year.

The study noted that its analogue-based estimate of human influence on the increase in fire weather risk is conservative because anthropogenic forcing can affect atmospheric variability, which in turn influences the VPD trend.

The researchers also found that the effects of human-caused climate change are arriving more rapidly than predicted. Rong Fu, one of the authors of the study, told the Los Angeles Times that they had not expected human-caused warming to outpace natural climate variability as the main contributor to fire weather until much later this century, around 2080.

"The trend toward increasing risk will likely continue over the western US. This change in risk requires urgent and effective societal adaptation and mitigation responses," the study concluded.