At least that’s what lots of news stories said in response to a July 6 Sciencexpress paper by A.L. Westerling of the Scripps Institute of Oceanography and three co-authors.
Like most scientific issues, though, this one is more complicated than the headlines suggest.
Westerling examined wildfire data between 1970 and 2003 and found that major fires have occurred four times as frequently, on average, since 1986 than they did from 1979 through 1985. But he had one key conclusion: “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 years with high wildfire frequency tend to be those in which snow begins melting earlier than normal, which the authors of the paper said was “not surprising.” But examination of Westerling’s own data shows no significant difference in snowmelt timing between the periods of 1970 to 1985 and 1986 to 2003.
However, spring and summer regional temperatures have gone up slightly — about 1 degree Celsius — over the same period, which serves to increase evaporation and, thus, flammability.
Changes of this sort are certainly not unprecedented. Rather than limiting the perspective to 34 years, why not look at the last 1,200? Two years ago, Columbia University scientist 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 A.D. 936, 1034, 1150, and 1253 … the current drought does not stand out as an extreme event, because it has not yet lasted nearly as long.”
In fact, Cook’s 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, the West is naturally accustomed to more drought than it has experienced since it was colonized by immigrants. It is also worth noting that the Western population boom began in the early 20th century, the wettest era of the last 1,200 years.
What are perhaps more interesting are the changes in overall moisture 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 surface moisture evaporates into a warmer atmosphere.
But as the country warmed, precipitation also went up. In fact, it went up far more than evaporation did. So, all 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 will continue to be, if history is any guide. But relating human-induced global warming to Western drought is more difficult.
There are many indices of drought severity, all of which attempt to balance rainfall, evaporation, streamflow and other factors. Perhaps the most often used is the Palmer Drought Severity Index. It has been around for more than half a century, and the National Climatic Data Center, in Asheville, N.C., records it for different regions of the country.
Most scientists think humans are behind the planetary warming that began in the mid-1970s. (Another warming of similar magnitude occurred in the early 20th century, but was entirely natural in origin). But what is the relationship between global warming and drought in the Western U.S.?
There isn’t any. Statistically speaking, the correlation is zero, which means that as humans have warmed the planet, they haven’t influenced Western drought. This holds whether one starts at the beginning of the Palmer record, in 1895, or the first year of Westerling’s study, 1970.
Seeing as we have had about a hundred years of global warming, the lack of a clear relationship between earth temperature and Western drought is reassuring, because the relationship between drought and fire is real, even if it is more complicated than people are led to believe.
