Wildfire and Climate Change

I didn’t have a chance to do a story for the newspaper on the new Science paper by Westerling et al.[1] on wildfires and climate change, so I just got around to reading it this evening.

In a sense, it seems like a bit of a no-brainer. Common sense would suggest that when temperatures are warmer, stuff burns up more easily. But the fire story in the western United States is hugely complicated by the ecosystem effects of more than a century of fire suppression and land use changes (grazing especially), so common sense is a poor guide here, and data seems better. In this case, the data sheds some important light on the sort of question Roger Pielke Sr. keeps harping on as an important goal of relevant climate research: the effects of climate change and variability on “regional and local societal and environmental resources of importance.”

Westerling and Tom Swetnam (one of the co-authors of the new Science paper) have taken a crack at this question before, musing in a paper in EOS a few years back about the difficulties posed by the confounding variables:

(M)odern wildfire regimes (and recordkeeping) are highly influenced by ongoing changes in management personnel and policy. Even without such changes, increasing fuel loads from decades of fire suppression are apparently decreasing the effectiveness of fire suppression efforts in some ecosystems. Consequently, even with a record spanning most of the last century, it is difficult to discern the effects of natural variability—including climatic variations—from those of land use and management on wildfire.

In the EOS paper, Westerling and Swetnam found nice strong links between fire and large-scale ocean controls on continental climate (ENSO and the PDO). And there have been previous studies pointing tentatively to a link between wildfire and warming . I wrote about one of them a couple of years ago, a study by Donald McKenzie that drew a tentative link:[3]

Increased temperature in the future will likely extend fire seasons throughout the western United States, with more fires occurring earlier and later than is currently typical, and will increase the total area burned in some regions.

But the McKenzie paper had an important shortcoming: it wasn’t published in Science, and therefore did not make the NBC Nightly News.

All snark aside, in addition to hitting the Science jackpot, Westerling and his colleagues have marshalled a nice data set of 1,166 forest wildfires from 1970 to 2003 on federally owned land (In the western U.S., the government owns most of the woods.) and shown statistically significant links between fire frequency and both warmer temperatures and earlier spring snowmelt:

Interannual variability in wildfire frequency is strongly associated with regional spring and summer temperature
(Spearman’s correlation of 0.76, p<0.001, n=34). A second-order polynomial fit to the regional temperature signal alone
explains 66% of variance in the annual incidence of these fires, with many more wildfires burning in hotter than in cooler years.


Overall, 56% of wildfires and 72% of area burned in wildfires occurred in Early (i.e. lower tercile CT1) snowmelt
years, while just 11% of wildfires and 4% of area burned occurred in Late (i.e. upper tercile CT1) snowmelt years.

The new paper points at climate effects that are most significant in the Northern Rockies, but the main reason we here in the southwestn don’t show up much in their data is that we don’t have as much woods (and less of a spring effect):

Southwest forests, where fire exclusion has had the greatest effect on fire risks, have also experienced increased numbers of large wildfires, but the relatively small forest area there limits the impact on the regional total, and the trend appears to be less affected by changes in the timing of Spring.

It’s worth noting that Westerling et al. do not explicitly blame Al Gore-style “global warming” for what’s happened so far. But whatever the cause up ’til now, they note that there’s good reason to expect it to heat up in the future:

Whether the changes observed in western hydro-climate and wildfire are the result of greenhouse gasinduced
global warming, or only an unusual natural fluctuation, is presently unclear. Regardless of past trends, virtually all climate model projections indicate that warmer springs and summers will occur over the region in coming decades. These trends will reinforce the tendency toward early spring snowmelt and longer fire seasons. This will accentuate conditions favorable to the occurrence of large wildfires, amplifying the vulnerability the region has experienced since the mid-1980s.

[1] Warming and Earlier Spring Increases Western U.S. Forest Wildfire Activity, Westerling et al., Published Online July 6, 2006, Science DOI: 10.1126/science.1128834
[2] Westerling and Swetnam, EOS, Vol. 84 No. 49, 9 Dec. 2003
[3] Climatic Change, Wildfire, and Conservation, McKenzie et al., Conservation Biology, Volume 18 Page 890 – August 2004

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