How Global Warming Stokes Wildfires

This post is by James Wang, Ph.D., a climate scientist at Environmental Defense.

The wildfires in California prompted the largest mass evacuation in California history, destroying half a million acres and over 1500 homes. People are saying that global warming may have played a part in the severity of the blazes. But where is the connection? And how can global warming cause both floods and droughts?

Satellite photo of California wildfires. Source: NASA.

Global warming intensifies the "hydrological cycle" – water evaporates into the air, forms clouds, and then rains back down on the Earth. Higher temperatures cause evaporation to occur more quickly. This can cause very dry conditions on land, even drought. While it’s impossible to say for sure that a particular event is caused by global warming, one of the main factors behind the wildfires’ severity was that the 2006-2007 rainy season was the driest on record in Southern California, leaving vegetation extremely dry and flammable.

The greater amount of water vapor rising from the Earth also causes more clouds to form, so when it does rain, the rainfall can be unusually heavy, sometimes enough to cause flooding. Thus some seasons are very wet while others are very dry. In fact, the 2004-2005 rainy season was the second wettest on record for Southern California. During times of extreme wetness, vegetation grows rapidly. Then during times of extreme dryness, all that vegetation becomes fuel for raging wildfires, as happened this year.

Excess fuel comes from another source, as well. Southern California is home to a type of vegetation called chaparral. This shrub land is highly prone to fire, and in fact fire is part of its natural cycle. Fire revitalizes the ecosystem by clearing out small bushes and debris to prevent crowding and restore nutrients to the soil. But people have built homes in this area, so fires are quickly extinguished. When natural wildfires are suppressed, the debris that normally burns away builds up. Eventually a fire breaks out that can’t be immediately controlled because there’s so much fuel.

Unfortunately, uncontrolled climate change and increasing urban sprawl will likely conspire to create worsening wildfire disasters in California in the future.

And Southern California isn’t the only place experiencing worsening wildfires. Wildfires are increasing throughout the Western U.S., both in severity and frequency. So says a 2006 cover story in Science, which collected and analyzed western forest fire data from 1970 to 2003. Since the mid-1980s, the incidence of large wildfires has increased, and the length of the fire season has increased.

Source: Science, August 2006. Parts A and B show that wildfires (Part A, red bars) are more frequent when temperatures (Part A, black line) are high and snow melts early (Part B, black line). Part C shows the length of the fire season (the height of the red and black areas).

Some of this is due to land use and fire suppression, but climate may still be the "primary driver". The high altitudes of the Rockies, where wildfires used to be rare, are now seeing large wildfires with increasing frequency. Warming causes an earlier snowmelt, and "[a]n earlier snowmelt can lead to an earlier, longer dry season, providing greater opportunities for large fires due both to the longer period in which ignitions could potentially occur and to the greater drying of soils and vegetation."

The authors conclude by warning that if western forests burn more frequently, they will release carbon rather than act as a carbon sink, intensifying global warming and leading to even more fires and other harmful consequences.

There is another way that climate change can shift rainfall patterns in a region, and that’s through changes in global wind patterns. I’ll address this in a future article, when I talk about the drought in the Southeastern U.S.