U.S. wildfire damage to increase 50% due to global warming

The wildfire season to date has not been dire: Nearly 3.3 million acres burned in the U.S., roughly 11% less than the 10-year average to date. (Though it's still bigger than Delaware and Rhode Island combined.) Then again, the last 10 years have seen some wicked fire seasons -- and forecasters expect this and subsequent seasons to be similarly wicked.

A forecast just released by the Forest Service says to expect that the 15 fires currently burning 330,000 acres is only a hint of what is to come. Hot, dry weather should produce an above-average fire season, according to press reports.

And the coming years will only see the fire season get worse. That's nothing new: Scientists have said that global warming has already caused the fire season to lengthen and intensify across the West, and that it would continue to do so in the coming decades.

Gov. Arnold Schwarzenegger famously declared the end to a single fire "season," now that a persistent drought and an altered climate have made the California landscape fire-prone year-round. Forest management and the suburbanization of forested and fire-prone areas also are key contributors to risk of fires and fire damage, but scientists have separated out the various risk factors and determined that climate change is alone a driving force, independent of the other factors.

A new study published in the Journal of Geophysical Research quantifies the risk: The area burned by wildfires will increase 50% by 2050, with the Rocky Mountains (175% increase) and the Pacific Northwest (78% increase) shouldering the brunt of the burn. And the authors of the study call that prediction "conservative," given that temperatures may well increase more than the 1.6 C they assume.

As temperatures rise forests become dryer and this leads to larger and more serious fires. The bigger fires will also contribute to a 40% increase in soot in the air, a serious pollution problem that affects human health and visibility at national landmarks like the Grand Canyon.

How global warming increases wildfire risk

How does global warming increase wildfire risk? By creating conditions on the ground that increase the chances of a new fire forming: Global warming results in more hot, dry days ripe for a fire, dries out vegetation to make it combustible and can even provide the spark. Here's how The Daily Green explains it:

1. More heat equals earlier snow melt.
A buffer to the Western fire season is high mountain snow and ice that accumulates through the winter and melts through the spring, running off and irrigating the valleys below. Global warming is creating conditions that lead to less snow and, more consistently, more evaporation and a faster, earlier runoff. That leaves the valleys below more prone to drought earlier in the year, and more prone to extreme drought throughout the year. Drought dries out trees, shrubs and grasses, making them combustible.

The figure at right shows projected future changes in the timing of runoff in snow-dominated areas of the western United States. The timing of runoff shifts earlier in almost all areas as greenhouse gas concentrations increase. These snow-dominated areas currently act as natural reservoirs, with melting in the spring and summer providing critical fresh water throughout the western United States.
Credit: Purdue University/Diffenbaugh Laboratory

2. More heat equals more evaporation.
Similarly, whatever moisture does flow into the valleys is more likely to evaporate before it sinks in and nourishes plants, leading to drier vegetation.

3. More heat equals new tree pests.
As the West has warmed, pine bark beetles have spread throughout millions of square miles of forest from New Mexico up into Canada. As the bugs do their work, trees are weakened or killed, creating yet more wild firewood.

4. More heat equals stronger storms.
Scientists have predicted -- and observed -- the formation of more frequent, intense storms as a result of a warmer climate. That means two things: One, more frequent thunderstorms and lightning strikes that can naturally ignite wildfires; and two, flashier rainstorms that result in bursts of rain running off in streams, rather than soaking into the ground and substantially irrigating the landscape.

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