See how a warmer world primed California for large fires

The state is just hotter and drier than it used to be, and that's driving a trend toward larger fires.

Most recent two decades in records

1999-2018

Earliest two-decade period

1895-1914

Hotter summers

Average daily temperature

2014

2003

2017

2001

2015

2018

2012

70 degrees Fahrenheit

2006

2008

2016

2009

2013

69

2002

2007

2004

2000

2005

68

1999

2011

2010

67

66

65

6 inches

2

4

Drier summers

Total precipitation

KENNEDY ELLIOTT, NG STAFF.

SOURCE: NOAA

2014

The hottest and driest

summers have all

occurred in the

last 20 years.

2003

2017

2001

Hotter summers

2015

2018

Average daily

temperature

2012

2006

2008

2016

2009

2013

69 degrees Fahrenheit

2002

2007

2004

2000

2005

1999

68

2011

2010

67

Most recent two decades

in records

1999-2018

Earliest two-decade period

1895-1914

Drier summers

Total precipitation

4

6 inches

KENNEDY ELLIOTT, NG STAFF.

SOURCE: NOAA

2014

Hotter summers

2003

The hottest and driest

summers have all

occurred in the

last 20 years.

2017

2001

2015

Average daily temperature

2018

2012

70 degrees Fahrenheit

2006

2008

2016

2009

2013

Most recent two decades in records

1999-2018

2002

2007

2004

2000

Earliest two-decade period

1895-1914

2005

1999

2011

2010

67

65

Drier summers

Total precipitation

4

6 inches

KENNEDY ELLIOTT, NG STAFF.

SOURCE: NOAA

2014

Hotter summers

2003

The hottest and driest

summers have all occurred in the last 20 years.

Average daily temperature

2017

2001

2015

2018

Most recent two decades in records

1999-2018

2012

Earliest two-decade period

1895-1914

2006

2008

2016

2009

2013

2002

69 degrees Fahrenheit

2007

2004

2000

2005

1999

2011

2010

67

65

Drier summers

Total precipitation

6 inches

4

2

KENNEDY ELLIOTT, NG STAFF.

SOURCE: NOAA

Fires are natural in California: Many of its ecosystems, from the chaparral of Southern California to the northern pine forests, evolved to burn frequently. But since the 1980s, the size and ferocity of the fires that sweep across the state have trended upward. Fifteen of the 20 largest fires in California history have occurred since 2000.

The graphic above shows why: Most of the state’s hottest and driest years have occurred during the last two decades as well.

Over the past century, California has warmed by about three degrees Fahrenheit. That extra-warmed air sucks water out of plants and soils, leaving the trees, shrubs, and rolling grasslands of the state dry and primed to burn.

That vegetation-drying effect compounds with every degree of warming, explains Daniel Swain, a climate scientist at the University of California, Los Angeles, meaning that plants lose their water more efficiently today than they did before climate change ratcheted up California's temperatures.

Climate 101: Wildfires

Because of this effect of climate change, wildfires are increasing in size, both in California and across the western U.S., says Park Williams, a fire expert at Columbia University. Since the 1980’s, he and a colleague reported in 2016, climate change contributed to an extra 10 million acres of burning in western forests— an area about the size of Massachusetts and Connecticut combined.

Changes in precipitation are another factor. California's summer dry season has also been lengthening. Each extra day lets plants dry out more, increasing their susceptibility to burning.

“Usually—or, I don't want to even say usually anymore because things are changing so fast—we get some rains around Halloween that wet things down,” says Faith Kearns, a scientist at University of California Institute for Water Resources in Oakland. But in the past few years, those rains haven't come until much later in the autumn—November, or even December.

That may seem like a minor issue, but it has big effects. In the fall, California is often buffeted by whipping winds. So if a fire gets sparked, it can spread fast and hard. That's what happened this year, as well as in last year's Thomas fire.

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“We've been lengthening fire season by shortening the precipitation season, and we're warming throughout,” says Swain. “That's essentially what’s enabled these recent fires to be so destructive, at times of the year when you wouldn't really expect them.”

The total number of wildfires in California hasn’t increased; in fact the numbers were a lot higher in the 1980s and 1990s than in the past decade. The total acreage burned fluctuates considerably from year to year, depending on many factors, including luck: Rain dampens things down early, or fires start in places where they are easier to contain.

But climate change is driving a clear trend: When wildfires happen in California, they have a better chance of growing large and destructive.

“These same fires today are occurring in a world roughly three degrees Fahrenheit warmer than it would have been without warming,” says Williams. “Which means that the current fires are probably harder to fight than they would have been in a cooler world.”