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Saturday, May 9, 2009

Four ways global warming increases wildfire risk

As yet another destructive wildfire sweeps down the California coast, it’s time to revisit the global scientific link between global warming and wildfires.

At least 20 homes — described by Reuters as multimillion dollar mansions — have burned as the out-of-control wildfire burns toward Santa Barbara.

This fire was caused — as all fire are — by local weather conditions coinciding with a spark. In this case, it was a string of hot days when temperature reached over 100 degrees and gusty hot winds of up to 50 mph, along with suspected arson.

The first thing any responsible report about global warming and the weather should say is: There is no way for anyone to say that a single event was caused by global warming. What we can say is that scientists have not only predicted that global warming will increase fire risk, particularly in the Western U.S., but that it has already produced more fires. In fact, increased wildfire risk is among the predictions scientists are most confident in making, when it comes to global warming effects. Even California Gov. Arnold Schwarzenegger has warned that global warming has effectively eliminated the concept of a “fire season,” since weather conditions now result in a year-round 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:

predicted water runoff map

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.

The caveat: A new study predicts that changes in the distribution of vegetation under a changing climate will have more to do with fire risk than the changing climate itself: So if the right plants colonize the West (eventually), they could offset the increased fire risk created by global warming.

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