Patrick Michaels had a piece this week on Tech Central Station that made a good point about global warming and drought in western North America. His point is that we should be cautious in asserting a connection. But the math he uses to get there is, umm, unusual.
(climate wonks, click through for more)
The backstory: last fall, Ed Cook and his colleagues published a a paper in Science (paid sub. req. - more here) quantifying the extent of drought in Western North America over the last 1,200 years. The time of most widespread drought, they concluded, was "in AD 900 to 1300, an interval broadly consistent with the Medieval Warm Period."
If elevated aridity in the western United States is a natural response to climate warming, then any trend toward warmer temperatures in the future could lead to a serious long-term increase in aridity over western North America.
He does it by returning to a climate reconstruction published earlier this year in Nature by Moberg et al.. In his TCS piece, Michaels suggests that periods that in Moberg's data were "cool," such as the interval from 800 to 1000 AD, were also dry. He finds several centennial-scale periods when it was, by his definition, "cool" but also dry.
It's not clear how Michaels defines "cool" and "warm," but it seems to me a reasonable definition might involved temperatures higher and lower than the long-term mean. By that measure (Moberg's data is on line), the period from 800 to 1000 was about a tenth of a degree warmer than the mean of Moberg's data set. Similarly, 1200-1400, which Michaels calls "cool," was just a hair above average. In fact, if you look at the periods Michaels has chosen and actually calculate average temperature and area of drought (Cook's Drought Area Index data is also on the web) you'll find that in every case Michaels chose in his example, above average temperature coincides with above average drought, and below average temperature coincides with below average drought.
I don't know whether Michaels simply eyeballed Moberg's graph, or what. But the numbers don't support what he himself acknowledges are "rough comparisons."
I'm not saying this proves a global warming-drought link. Quite the contrary. I agree with Michaels' underlying point: caution is in order in drawing a connection between greenhouse warming and western drought. Drought is a complicated phenomenon. There is, however, some interesting evidence worth examining, including the Cook et al. paper and another recent effort by Mann, Cane, Zebiak and Clement that looks at an underlying mechanism that could explain a warm climate-western drought link.
Folks who live out west need to be paying attention to this. The variability of the drought cycle is going to be there, global warming link or not, and it deserves westerners' attention regardless. Population growth is likely to continue to be the dominant variable in the drought equation, swamping any trend toward increased drought that might come from global warming. In practical terms, after all, drought is best defined as happening at the intersection of two curves - water supply vs. demand. The demand side is exploding far faster than any supply side deviation that might be caused by global warming. And given our apparent commitment to global warming whatever we might do about greenhouse gas emissions now, we here in the west are sorta stuck with whatever might happen for the foreseeable future.
But folks shouldn't just accept Michaels' suggestion that this particular variable, marginal though it may be, can be safely ignored.
Posted by John Fleck at April 16, 2005 09:31 AM