California has been ravaged by record wildfires in recent years. 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 percent.

Even though California’s wildfire season has traditionally ended in October, the Camp Fire raging in November 2018 is the state’s most destructive on record.

The data tell the story: Six of California’s ten most destructive wildfires on record have now struck in just the past three years.

President Trump’s tweets suggesting forest mismanagement is to blame for California’s wildfire woes, and threatening to withhold federal funding, have prompted widespread rebukes for their insensitivity as thousands of citizens flee the fires – some, tragically, unsuccessfully – and as an affront to thousands of weary firefighters.

The reality is that about 57 percent of the state’s forests are owned and managed by the federal government, and another 40 percent by families, companies, and Native American tribes. Forest management does play some role in creating wildfire fuel, but some wildfires aren’t even located in forests. Moreover, scientific evidence clearly shows that climate change is exacerbating California’s wildfires in different ways:

• 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.

The Golden State’s hotter, drier conditions

Global warming causes higher temperatures, and 2014 through 2018 have been California’s five hottest years on record. This pattern leads to an increase in evapotranspiration – the combination of evaporation and transpiration transferring 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. As Swain noted in an informative Twitter thread about California’s November 2018 wildfires,

If Northern California had received anywhere near the typical amount of autumn precipitation this year (around 4-5 in. of rain near #CampFire point of origin), explosive fire behavior & stunning tragedy in #Paradise would almost certainly not have occurred.

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.

In Southern California, Santa Ana winds coming later in year

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.

Researchers of a 2015 study published in Environmental Research Letters, led by Yufang Jin at UC Davis, forecast that the area burned by Southern California wildfires will increase by about 70 percent by mid-century as a result of the drier, hotter, windier conditions caused by global warming. And these Southern California wildfires often occur outside of forests, according to the president of the Pasadena Fire Association.

Connections to the Arctic and jet stream

Rutgers climate scientist Jennifer Francis over the past decade has been researching the connection between changes in the Arctic and extreme weather patterns throughout the Northern Hemisphere. In recent years a growing number of climate scientists have found evidence supporting her groundbreaking research.

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.

In between these jet stream waves, weather patterns have a tendency to stall in place. These patterns include heat waves, the polar vortex, hurricanes and associated record rainfall, and high-pressure ridges, and the latter has tended to occur off the California coast. During the drought of 2012 to 2016 – California’s worst in over a millennium – a high-pressure ridge in the Pacific was so persistent, that Swain coined it the “Ridiculously Resilient Ridge.” A similar system has formed off the coast in late 2018:

These high-pressure ridges divert storm systems around California, leading to the lack of rainfall in the state during these dry spells. 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.

That’s an inconvenient conclusion for those who want to maintain a “business as usual” approach to the use of fossil fuels. And also to those favoring the President’s suggestion that California’s wildfire troubles can largely be solved just by adjusting forest management, or his recent 60 Minutes TV view that “it’ll change back again.”

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

Editor’s note: We welcome this article as the first of a monthly series of reports by new Yale Climate Connections regular contributor Dana Nuccitelli.