Climate change means worse droughts for American Southwest, Australia
Part one presented the synopsis of the remarkable new U.S. Climate Change Science Program (a.k.a. the Bush Administration) report, Weather and Climate Extremes in a Changing Climate. One central point in the synopsis is
Droughts are becoming more severe in some regions, though there are no clear trends for North America as a whole … Substantial areas of North America are likely to have more frequent droughts of greater severity.
Seems pretty clear, no? Dry areas will see more evaporation, hence less soil moisture (defined as precipitation minus evaporation), hence more drought. Further, many dry areas will see less precipitation under climate change (due to the expansion of the Hadley Cell and subtropics, see “Australia faces the ‘permanent dry,’ as do we“).
Simply put, dry areas will get drier. The Bush report even summarizes a study I have written a lot about (see “The Century of Drought“):
For example, extreme drought increases from 1 percent of present day land area (by definition) to 30 percent by the end of the century in the Hadley Centre AOGCM’s A2 scenario.
[Note: The A2 scenario leads to atmospheric concentrations of CO2 of about 850 ppm by century’s end. On our current path, we are headed beyond 1000 ppm (see here).]
On the other hand, climate change science projects also more overall precipitation because the atmosphere will contain more water vapor (see “Still, waters run deep“). Simply put, wet areas will get wetter.
Obviously, a country like the United States will see some areas getting wetter and some areas getting drier, so we would expect to see no clear drought trend for the country as a whole, but much worse weather extremes in different places. Bad news. At least, to some.
But suppose you are a climate change
delayer enabler like, oh, I don’t know, Roger Pielke, Jr. How would you summarize the report? Well, you would list a bunch of “remarkable conclusions” that “somehow did not seem to make it into the official press release,” including (remarkably):
Yes, you read that correctly. From p. 5:
Averaged over the continental U.S. and southern Canada the most severe droughts occurred in the 1930s and there is no indication of an overall trend in the observational record …
Yes, you read that correctly. Pielke intentionally misleads his readers by cutting short the full quote from the report. Why do I say “intentionally”? Because the very next sentence explicitly explains why the nationwide trend isn’t meaningful:
However, it is more meaningful to consider drought at a regional scale, because as one area of the continent is dry, often another is wet. In Mexico and the U.S. Southwest, the 1950s were the driest period, though droughts in the past 10 years now rival the 1950s drought. There are also recent regional tendencies toward more severe droughts in parts of Canada and Alaska.
If that wasn’t enough, page 5 also has a sidebar that reads,
A contributing factor to droughts becoming more frequent and severe is higher air temperatures increasing evaporation when water is available.
Nobody who is truly serious about avoiding catastrophic climate outcomes, which include extreme drought over large parts of the planet, including the American Southwest, would want to leave the impression that the report’s first sentence represented its central message on droughts.
So how bad could droughts get post-2050 in this country if we don’t very aggressively pursue national and global efforts to stabilize below 450 ppm? According to the report, probably the best U.S. drought modeling is done by Richard Seager (et al) of the Lamont Doherty Earth Observatory. The report notes that only Seager’s “model simulates the 1950s drought over North America.” A 2007 Science paper by Seager et al, “Model Projections of an Imminent Transition to a More Arid Climate in Southwestern North America” ($ub. req’d) concludes:
Here we show that there is a broad consensus among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the 1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.
[Note: Seager et al model the A1B emissions scenario, which leads to only 720 ppm by century’s end.]
In short, more extreme weather ultimately morphs into climate change, as drought-prone areas become deserts.
The time to act is yesterday.