July 7, 2006

The Fire This Time: More Perspective Needed

Filed under: Droughts

Some prominent scientists are becoming increasingly restive about the shrill portrayal of global warming science in popular media. The latest round concerned a paper by A. L. Westerling (Scripps Institute of Oceanography) relating an dramatic increase in western forest fires to regional warming and changes in the onset of snowmelt.

Coloroado University’s Roger Pielke Jr., one of the nation’s preeminent scholars about how science and society interact, called it “a useful paper that adds to our knowledge and hopefully will stimulate further research on the integrated effects of climate-society-policy.” But then, he warned that “At the same time I can envisage the paper being used simply as a caricature in the global warming debate—Global Warming Causes Forest Fires!—but that would be a shame because fire policy is more complex than that.”

Well, of course, what he feared would happen, did happen. And the resultant headlines are another sad commentary on how cursory reporting on global warming has become, and how little attention is paid to the facts as they stand. Nowhere, for example, did we read Westerling’s words: “Whether the changes observed in western hydro-climate and wildfire are the result of greenhouse gas-induced global warming or only an unusual natural fluctuation, is presently unclear.”

Why so unclear? In large part, because the science isn’t straightforward, and three decades is a very short period of climate time.

Snowmelt and temperature are thought to be the driving factors for western wildfires. The authors note that wildfire frequency since 1986 is four times the average from 1970 through 1986, and that “it is not surprising that the incidence of wildfires is strongly associated with snowmelt timing.” But, as is obvious from their own data, there is no difference whatsoever in the average date of snowmelt initiation between these two periods.

Figure 1. Timing of spring snowmelt (the more negative the value, the earlier in the year the spring snowmelt occurred) There is no statistically significant trend in this time history. (Source: Westerling et al., 2006)

However, spring and summer regional temperatures have gone up slightly, about one degree Celsius, which serves to increase evaporation. Consequently, there should be an increase in flammability.

It’s certainly not unprecedented. Rather than limiting the perspective to a mere 34 years, how about looking at the last 1200? Two years ago, Edward Cook and several colleagues reconstructed the West’s drought history back to 800 A.D. They wrote that “Compared to earlier megadroughts that are reconstructed to have occurred around AD 936, 1034, 1150, and 1253, however, the current drought does not stand out as an extreme event, because it has not yet lasted nearly as long.”

Figure 2. Reconstructed drought history in the western United States (source: Cook et al., 2004).

In fact, their study shows a general decline in western drought over the last millennium, with the recent era looking pretty much like the long-term average. In other words, what the West is “naturally” used to is more drought than has been experienced in its very recent colonization. It is also noteworthy that the western population boom pretty much began in the wettest era of the last 1200 years, which was the early 20th century (Woodhouse et al., 2005).

What is perhaps more interesting is the changes in overall national moisture status that have accompanied the warming of the U.S. in 20th century. “Drought” is a combination of lack of rain and increasing evaporation. The latter is obviously dependent upon temperature, as more water evaporates into a warmer atmosphere.

But, as the country warmed, precipitation also went up. In fact it went up far more than did evaporation. So, everything else being equal, the U.S. as a whole is a wetter place than it was before the planet’s surface temperature began to rise.

This doesn’t mitigate the fact that the West is a dry, fire-prone region, and it will continue to be so, if history is any guide. But relating human-induced global warming to western drought is more complicated.

There are many indices of drought severity, all of which attempt to balance rainfall, evaporation, streamflow and other factors. Perhaps the most oft-used on is called the Palmer Drought Severity Index (PDSI). It has been around for over half of a century. The National Climatic Data Center maintains PDSI records for different regions of the country.

Figure 3. Annual Palmer Drought Severity Index (PDSI) for the western United States from 1895-2005. The more negative the PDSI, the greater the drought. There is no trend in this data, but there are clearly wet and dry periods that correspond well to the fire frequency described by Westerling et al. (2006). (Source: National Climatic Data Center)

Most scientists think people have had something to do with a planetary warming that commenced in the mid-1970s. There was another warming of similar magnitude in the early 20th century which was “natural.” So, what is the relationship between drought in the western U.S. and global warming?

There isn’t any. Statistically speaking, the correlation zero, which means, as humans have warmed the planet, they haven’t influenced western drought. This lack of a relationship holds whether one starts at the beginning of the Palmer record, which is 1895, or the starting year for Westerling’s study, which is 1970.

Figure 4. Scatterplot of global average temperature anomaly vs. western PDSI for the period 1895-2005. There is no relationship here, indicating that “global warming” is not affecting drought in the western United States.

Seen as we have had about a hundred years of global warming, about half of which is “natural” and the other half caused by people. The fact that there is no relationship between global temperature and western drought should be reassuring, especially because the relationship between drought and fire is quite real, even if it is much more complicated than the “caricature” that Dr. Pielke feared.


Cook, E.R., et al., 2004. Long-term Aridity Changes in the Western Unites States. Science, 206, 1015-1018.

Westerling, A.L., et al., 2006. Warming and Earlier Spring Increases Western U.S. Forest Wildfire Activity. Sciencexpress, July 6, 2006.

Woodhouse, C.A., et al., 2005. The 20th century pluvial in the western United Sates. Geophysical Research Letters, 32, L07701, doi:10.1029/2005GL022413.

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