TORONTO -- A new study strengthens the case that climate change has been the main cause of the growing amount of land destroyed by wildfires over the past two decades in the western U.S., and one researcher says the trend is likely to worsen in the years to come.
"I am afraid that the record fire seasons in recent years are only the beginning of what will come due to climate change, and our society is not prepared for the rapid increase of weather contributing to wildfires in the American West," Rong Fu, study co-author and University of California, Los Angeles, professor of atmospheric and oceanic sciences, said in a .
The increasing destructiveness of wildfires is shown by U.S. Geological Survey data. Between 1984 to 2000, the average burned area across 11 western states was nearly seven thousand square kilometres per year. For the next 17 years, through 2018, the average burned area was approximately 13.5 thousand square kilometres per year.
In 2020, according to a U.S. National Interagency Coordination Center report, the amount of land burned by wildfires in the American West reached 35.6 thousand square kilometres -- an area larger than the state of Maryland.
By comparison, British Columbia had its in history in 2018, during which about 13.5 thousand square kilometres of land was burned. That broke the previous record set a year earlier, when approximately 12.2 thousand square kilometres of land was scorched.
The next two years were well below average, but during a challenging season in 2021, between , wildfires in British Columbia burned about 8.7 thousand square kilometres of land.
, published in the peer-reviewed journal Proceedings of the National Academy of Sciences, tackles the question of what has caused this massive increase in destruction, and whether it is indeed climate change or simply changing weather patterns.
The researchers applied artificial intelligence to climate and fire data in order to determine the impact of climate change, among other factors, on vapour pressure deficit (VPD), which is the key climate variable of wildfire risk.
VPD is the difference between the amount of moisture in the air and how much moisture the air can hold when saturated. When the VPD is higher than the present amount of moisture, the air can draw additional moisture from soil and plants. Large wildfire-burned areas tend to have high VPD levels.
The study found that the 68 per cent of the increase in VPD across the western U.S. between 1979 and 2020 was likely due to climate change. The rest was likely caused by naturally occurring changes in weather patterns.
"And our estimates of the human-induced influence on the increase in fire weather risk are likely to be conservative," Fu said.
Fu said she expects wildfires to become more intense and more frequent in the western U.S. as time goes on.
"Our results suggest that the western United States appears to have passed a critical threshold, that human-induced warming is now more responsible for the increase of vapour pressure deficit than natural variations in atmospheric circulation," Fu said. "Our analysis shows this change has occurred since the beginning of the 21st century, much earlier than we anticipated."
