How climate change is fueling California’s record wildfires

Yosemite under a smoky haze from wildfires

California’s 2018 wildfire season has been the deadliest in the state’s history.

By Dana Nuccitelli, CCL Science Policy Network Team

According to the latest Yale climate public opinion survey, only about half of Americans think global warming is already harming people in the U.S. now, or will within 10 years.

Yale Program on Climate Change Communication

As a result, only 38% of Americans viewed global warming as a “very important” factor in deciding their 2018 midterm election votes, making it just the 15th most important issue.  Although Americans are becoming a bit more worried about climate change, most still view it as a problem distant in time and space – one that will hurt other people, in the future.

But in reality, climate scientists have found that global warming is impacting us now. It’s making extreme weather like droughts, floods, heat waves, and wildfires worse. It’s important to be able to communicate the science to people to help them understand that climate change is a much higher priority. The scientific evidence connecting climate change to California’s horrific wildfires presents one such opportunity (see also the CCL laser talk on wildfires). 2017 was the state’s costliest and most destructive fire season on record. The Mendocino wildfire in July 2018 was California’s largest-ever by a whopping 60%. And the Camp Fire in November 2018 has been its most destructive and deadliest ever.

There are four primary ways climate change worsens these fires:

  • Higher temperatures dry out vegetation and soil, creating more wildfire fuel.
  • Climate change is shortening the California rainy season, thus extending the fire season.
  • Climate change is also shifting the Santa Ana winds that fan particularly dangerous wildfires in Southern California.
  • The warming atmosphere is slowing the jet stream, leading to more California heat waves and high-pressure ridges in the Pacific. Those ridges deflect from the state some storms that would otherwise bring much-needed moisture to slow the spread of fires.

Hot and dry – a deadly combination

When conditions are hot and dry, it creates more wildfire fuel. Global warming obviously makes it hotter, and the past five years have been California’s five hottest on record. High temperatures increase “evapotranspiration – the combination of evaporation and transpiration that transfers more moisture from land and water surfaces and plants to the atmosphere. Essentially, global warming causes plants and soil to dry out as the atmosphere holds more water vapor.

On top of this direct drying effect, climate change is causing a shift in rain patterns. Northern California has received only one inch of rain this season, which is about one-fifth of normal. A 2018 paper published in Nature Climate Change, led by UCLA’s Daniel Swain, found that as a result of global warming, California’s rainy season will become increasingly concentrated in the winter months between December and February. April, May, September, October, and November will become increasingly dry, meaning that the state’s wildfire season will start earlier and end later.

Over the past two decades, California has been increasingly both hot and dry. With these hotter, drier conditions extending late into the year, wildfires have become larger, and they spread faster, cause more damage, and are more difficult to contain.

SoCal’s Santa Ana winds

In Southern California, the worst wildfires are often fanned by Santa Ana winds. When winds are particularly strong, it enables wildfires to spread faster and jump over large roads and other potential obstructions, making them more dangerous and deadlier.  

In a 2006 paper published in Geophysical Research letters, Berkeley scientists Norman Miller and Nicole Schlegel predicted that global warming would push the Southern California fire season associated with Santa Ana winds into the winter months. Those Santa Ana fires are especially costly because of the speed at which they spread due to the winds and because of their proximity to urban areas. The November 2018 Woolsey fire around Malibu and Thousand Oaks, California, is a tragic example.

Connections to the Arctic and jet stream

To paraphrase the cliché, what happens in the Arctic doesn’t stay in the Arctic. The Northern Hemisphere jet stream is a result of the temperature difference between the cold Arctic and warmer lower latitudes in regions like North America and Europe. But the Arctic is the fastest-warming region on Earth, largely because as reflective sea ice disappears, the Arctic surface is increasingly covered by dark oceans that absorb more sunlight. The rapidly warming Arctic is shrinking the temperature difference between that region and the lower latitudes, which in turn weakens the jet stream. As a result, rather than a fast-moving flow of air, the jet stream increasingly is taking a slow, meandering path across the Northern Hemisphere.

Weather patterns tend to get stuck between those jet stream waves and “stall” in place. This can make a storm or hurricane dump a lot of rain in one spot; or a heat wave or cold polar air persist in a given region; or in California’s case, high-pressure ridges tend to stall off the coast in the Pacific Ocean.  

These high-pressure systems tend to divert storm systems to the north of California, exacerbating dry conditions. This often happened during California’s 2012–2016 drought – the state’s worst in over a millennium – and another such ridge has been sitting of California’s coast in 2018:

jet stream wildfires

A high-pressure ridge off the West Coast in November 2018. (Image credit:

In a 2017 paper in Nature Communications, researchers led by Ivana Cvijanovic and Ben Santer found still more evidence of a connection between disappearing Arctic sea ice and these high-pressure ridges in the Pacific. And in an October 2018 paper in Science Advances, scientists Michael Mann and Stefan Rahmstorf and colleagues found that depending on how human fossil fuel pollution changes in the coming years, the frequency of wavy jet stream events could triple by the end of the century.

Clear evidence of wildfire-climate connection

Climate scientists have identified numerous ways in which human-caused global warming is exacerbating California wildfires. A 2015 special report in the Bulletin of the American Meteorological Society found that “An increase in fire risk in California is attributable to human-induced climate change.” And a 2016 study in the Proceedings of the National Academy of Sciences found that human-caused global warming doubled the area burned by wildfires in the western U.S. over just the past 30 years.

The reality is that the more global warming humanity causes, the worse California’s wildfires will become.

Dana Nuccitelli is an Environmental Scientist and writes about climate change for The Guardian and Skeptical Science.

The Science Policy Network is a team of CCL leaders and supporters with a diverse background in the field of climate science. These network contributors write regular guest posts, offering thorough insight into topics that fall within their expertise. This post and other resources are available in the form of white papers on CCL Community