Two wildfires erupted on the outskirts of cities near Los Angeles, forcing more than 100,000 people to evacuate their homes Monday as powerful Santa Ana winds swept the flames through dry grasses and brush. With strong winds and extremely low humidity, large parts of California were under red flag warnings.
High fire risk days have been common this year as the 2020 wildfire season shatters records across the West.
More than 4 million acres have burned in California – 4% of the state’s land area and more than double the previous annual record. Five of the state’s six largest historical fires happened in 2020. In Colorado, the Pine Gulch fire that started in June broke the record for size, only to be topped in October by the Cameron Peak and East Troublesome fires. Oregon saw one of the most destructive fire seasons in its recorded history.
What caused the 2020 fire season to become so extreme?
Fires thrive on three elements: heat, dryness and wind. The 2020 season was dry, but the Western U.S. has seen worse droughts in the recent decade. It had several record-breaking heat waves, but the fires did not necessarily follow the locations with the highest temperatures.
What 2020 did have was heat and dryness hitting simultaneously. When even a moderate drought and heat wave hit a region at the same time, along with wind to fan the flames, it becomes a powerful force that can fuel megafires.
That’s what we’ve been seeing in California, Colorado and Oregon this year. Research shows it’s happening more often with higher intensity, and affecting ever-increasing areas.
Climate change intensified dry-hot extremes
We are scientists and engineers who study climate extremes, including wildfires. Our research shows that the probability of a drought and heat wave occurring at the same time in the U.S. has increased significantly over the past century.
The kind of dry and hot conditions that would have been expected to occur only once every 25 years on average have occurred five to 10 times in several regions of the U.S. over the past quarter-century. Even more alarming, we found that extreme dry-hot conditions that would have been expected only once every 75 years have occurred three to six times in many areas over the same period.
We also found that what triggers these simultaneous extremes appears to be changing.
During the Dust Bowl of the 1930s, the lack of rainfall allowed the air to become hotter, and that process fueled simultaneous dry and hot conditions. Today, excess heat is a larger driver of dry-hot conditions than lack of rain.
This has important implications for the future of dry-hot extremes.
Warmer air can hold more moisture, so as global temperatures rise, evaporation can suck more water from plants and soil, leading to drier conditions. Higher temperatures and drier conditions mean vegetation is more combustible. A study in 2016 calculated that the excess heat from human-caused climate change was responsible for nearly doubling the amount of Western U.S. forest that burned between 1979 and 2015.
When soil moisture is low, more solar radiation will turn into sensible heat – heat you can feel. That heat evaporates more water and further dries the environment. This cycle continues until a large-scale weather pattern breaks it. The heat can also trigger the same feedback loop in a neighboring region, extending the dry-hot conditions and raising the probability of dry-hot extremes across broad stretches of the country.
All of this translates into higher wildfire risk for the Western U.S.
In Southern California, for example, we found that the number of dry-hot-windy days has increased at a greater rate than dry, hot or windy days individually over the past four decades, tripling the number of megafire danger days in the region.
- ^ 100,000 people (apnews.com)
- ^ Santa Ana winds (www.washingtonpost.com)
- ^ More than 4 million acres (www.fire.ca.gov)
- ^ Five of the state’s six largest historical fires happened in 2020 (www.fire.ca.gov)
- ^ broke the record (coloradosun.com)
- ^ Cameron Peak and East Troublesome fires (www.coloradoan.com)
- ^ most destructive fire seasons in its recorded history (www.bbc.com)
- ^ scientists (scholar.google.com)
- ^ and (scholar.google.com)
- ^ engineers (scholar.google.ca)
- ^ Our research (doi.org)
- ^ excess heat is a larger driver (doi.org)
- ^ global temperatures rise, (www.climate.gov)
- ^ nearly doubling (doi.org)
- ^ spread downwind (doi.org)
- ^ tripling the number of megafire danger days (doi.org)
- ^ The U.S. Drought Monitor is jointly produced by the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. Map courtesy of NDMC. (droughtmonitor.unl.edu)
- ^ CC BY (creativecommons.org)
- ^ snowstorm stopped its advance (www.cpr.org)
- ^ Grand County Sheriff Brett Schroetlin said of the East Troublesome fire on Oct. 22 (www.npr.org)
- ^ Alizadeh, et al, Science Advances 2020 (advances.sciencemag.org)
- ^ Sign up for The Conversation’s newsletter (theconversation.com)
- ^ built in the wildland-urban interface (www.pnas.org)
Authors: Mojtaba Sadegh, Assistant Professor of Civil Engineering, Boise State University