In a finding that scientists thought was decades away from becoming a reality, California researchers say climate change is now the overwhelming cause of conditions driving extreme wildfire behavior in the western United States.
When world leaders met in Scotland this week to discuss plans to reduce greenhouse gas emissions, a study released Monday said global warming was essentially two-thirds to 88% responsible for the atmospheric conditions that fuel the increasingly destructive wildfires. fueled.
And that’s a conservative estimate, said study author Rong Fu, a climate researcher at UCLA.
“It happened so much faster than we expected before,” she added.
The study, published this week in the Proceedings of the National Academy of Sciences, looks at what’s known as the vapor pressure deficit, which basically describes how thirsty the atmosphere is, Fu said.
The researchers found that this is the most important meteorological variable that determines how much land in the western US is burned during a given burning season. The higher the deficiency, the more moisture the atmosphere takes from the soil and plants, preparing the landscape to burn.
Previous studies have shown that the atmosphere in the western US has become thirstier over the past 40 years. Experts have theorized that this is due to natural fluctuations in weather and because carbon dioxide emissions have allowed the planet to warm and warmer air to hold more moisture.
This team of researchers, including scientists from UCLA and the Lawrence Livermore National Laboratory, tried to explain exactly how much of each of those factors caused the increase. They used a machine learning approach to compare recent vapor pressure deficit readings with those in the past during similar weather patterns.
“Before 2000, we can pretty much explain this firefighting just using the weather patterns,” Fu said. “But now we can only explain 30% of what we see with the firefighting.”
After excluding other influences, such as changes in vegetation and cloud cover, the researchers concluded that the remaining 70% is due to greenhouse gases warming the planet, she said, a figure that could be as high as 88% according to some climate modeling.
The findings have important implications for firefighting. If natural weather cycles caused the fire hazard to increase, it could decrease again at some point. But the planet is expected to continue to warm, meaning the risk is likely only going to increase, Fu said.
“This new research, combined with what we have known before, means we can be confident that ongoing global warming will continue to intensify the conditions that are creating this record- or near-record fuel drought in the landscape,” said Noah. Diffenbaugh. , a climate scientist at Stanford University who was not involved in the study. “And we know from recent years what that means for our fire preparation and response system. It means a higher risk of more severe fire conditions, simultaneously, in several parts of the region.”
That’s consistent with what fire officials have seen on the ground, said Jon Heggie, a battalion chief with the California Department of Forestry and Fire Protection.
“We’re looking at temperatures and fire indices and days when we have a high fire risk,” he said. “And they’ve definitely increased — there’s no denying that.”
Over the past seven years, vegetation has become more combustible as summers have become longer, hotter and drier, resulting in busier, more dangerous fire seasons characterized by larger, more intense and faster-moving fires, Heggie said. It’s no longer uncommon for a fire to burn 20,000 acres in one day, a feat that in the past would normally have lasted two weeks, he said.
“What was previously unprecedented is now normal,” he said. “And we see it wasn’t a one-off for a few years. It happens every year.”
In this changed environment, fire services had to switch to new ways to fight and prevent fires. They include everything from adopting new technologies such as drones and night-flying helicopters to doubling efforts to focus on fuel management and community outreach and education.
“We are at the dawn of this new era of firefighting and we are still looking for opportunities to address this growing problem,” Heggie said.
Wildfires have burned nearly 2.5 million acres in California this year, suffocating the region with poor air quality that has sometimes reached the entire continent. This year’s total is second to last year when more than 4.1 million hectares were burned.
Nearly a quarter of that area was scorched by the fire of the Augustus Complex, which was caused by a dry lightning siege on August 16, 2020, and later became the largest fire in the state’s history. The vapor pressure deficit over much of the western US that day was the highest recorded in 40 years, Fu said.
When researchers compared that to similar past weather patterns, they could explain only about 60% of the above-normal reading, she said.
Conditions were equally extreme during last year’s Creek fire, which burned in the Sierra National Forest, and researchers were only able to explain about 40% of the deviation from normal, Fu said.
“So if we look at the weather pattern that occurred in those days, we can only explain about half of the above-normal vapor pressure deficit value,” she said. They attribute the other half to climate change, she added.
The researchers’ estimate of the effects of climate change is conservative because they failed to account for its role in shifting weather patterns themselves, Fu said. For example, some research has suggested that global warming changes the strength and frequency of certain high-pressure ridging patterns that result in critical firefighting, but these researchers assumed that all changes in weather patterns were natural, she said.
They also found that the effects of climate change are occurring faster than some had predicted. The researchers initially expected that human-caused warming would not surpass natural climate variability in causing fires until later in the 21st century, for example, after 2080, Fu said.
“But now, even in the first 20 years of the 21st century, we see that greenhouse gas firefighting has already surpassed natural variability,” she said. “This is a lot faster than we expected, at least in the western US for fire.”
While other researchers said this wasn’t much different from their earlier projections, they agreed that the effects of climate change are accelerating faster than society can handle.
“What’s very clear is that the gap between what’s happening and what we’re prepared for is much wider than the gap between what’s happening and what was predicted,” Diffenbaugh said.
California has been front and center in this rapid shift. This year, the state reported its hottest summer on record, its driest water year in nearly a century, and its second-largest wildfire: the Dixie Fire, which scorched more than 960,000 acres in Northern California.
“We’ve certainly seen some really record-breaking extreme events in recent years,” said Deepti Singh, an assistant professor in Washington State University’s School of the Environment, who was not involved in the study. “And while they are surprising when they do occur, they are fairly consistent with our knowledge and understanding of how the climate responds to elevated greenhouse gases.”
Still, she said the knowledge provided by the study is powerful because it indicates that human activities play a key role in determining future fire risk, and that the risk could one day be reduced if the world changes orbits to rely less on fossil fuels. to be trusted.
“The fire risk in this region will be proportional to the extent to which the earth warms,” she said. “And how much the Earth warms depends on how society decides to make progress on emissions.”