I find myself scratching my head over the latest debate between the RealClimate crew and Roger Pielke Sr. regarding the definition and use of “chaos” (non-linear sensitive dependence on initial conditions) in weather and climate modelling.

As often is the case in debates like this, both sides are correct in the point they’re making, differing not on the technical issue but the choice of emphasis. (I can’t help but note the irony here that, in arguing over “chaos,” the disputants are engaging in exactly the sort of “proxy argument” that Roger Pielke The Younger complains about so articulately.)

William and James at RealClimate argue that weather is chaotic and climate (as defined as weather averaged over time) is not. Roger argues (in the RC comments and also his own blog) that such a definition of climate fails to capture an important characteristic: “Observations show chaotic behavior of the climate system on all time scales, including sudden regime transitions.”

I don’t disagree with either group here. Given their definition, which is useful for some purposes, James and William are correct. It just so happens that for me, as a consumer of climate science, Roger’s definition is more useful. I recognize the GCMs’ globally averaged long-term rising trends under greenhouse emissions as important, but the sort of abrupt transitions that the models don’t yet capture are far more important to me, because (see drought posts below), it’s variability that kills ya, not long term trends. From Roger in the comments at RC:

In terms of how we are altering the climate, it is these sudden transitions that we need to understand, rather than focus so much of our resources on assessments of the global averaged temperature trend.

While we’re on the subject of droughts, Ed Cook and his colleagues and Lamont-Doherty have a new review paper (preprint, 10 meg pdf) of North American drought that makes some relevant points about the differences between what happens here and the current African drought/famine I mentioned below.

Point one is the distinction between the effects of the Dust Bowl of the ’30s and the Southwest drought of the 1950s:

The 11-year 1950’s Southwest drought was likewise extreme, with it being centered primarily over Texas and New Mexico. The environmental impact of the Southwest drought was severe, but it had less socioeconomic impact than the Dust Bowl drought because of irrigation, improved agricultural practices, better governmental support, and a much stronger underlying economy.

Point two relates to my comment about a society’s resilience on all relevant time scales. In this case, “relevant” could mean on much longer time scales than we’re currently prepared for:

For example, the 16th century megadrought identified by Stahle et al. (2000) is now seen to have affected large areas of North America, especially in the West and northern Mexico, and was much more prolonged than any of the 20th century droughts. The drought reconstructions also provide clear evidence for a much drier climate across the West and Great Plains during Medieval times, a drought that lasted with few interruptions for a few hundred years and which greatly taxed both hunter-gatherer and agriculturalist populations (Jones et al., 1999). Such “no analog” megadroughts are scary because the modern-day agricultural and hydrologic systems that depend upon adequate water supplies to produce and function may not have the resilience to survive much beyond the observed “worst case scenario” droughts of the past 100-150 years, e.g., the Dust Bowl drought.

Michael Wines had a terrific story in Wednesday’s New York Times on famine in Africa that evokes a point Mickey Glantz makes in his book Climate Affairs. First Glantz:

Closer scrutiny of most famine situations uncovers a multistressed political environment, of which drought was but one factor.

Now Wines:

War wrecked Mozambique’s economy; socialism and plunging copper prices reduced Zambia to penury; Zimbabwe’s economy collapsed after the government seized its richest farms, which were owned by whites.

In Malawi, 20 years of shifting political rule and economic policies have turned an already poor nation into a basket case. The AIDS pandemic – the rate of infection is about 15 percent among all adults, but perhaps 25 percent in Chikwawa – has cut down family breadwinners and left 900,000 children without one or both parents.

Most Malawians survive on plots of a couple of acres, often lacking even oxen for plowing. Irrigation is unheard of, leaving them dependent on good rains for survival.

Lately, rains have been spotty. There were severe hunger crises in 2002 and 2003, and this year’s disaster was brought on when good rains in late 2004 dried up in 2005, just as the corn crop was ripening.

The underlying climate message here is that meteorological “drought” is a normal phenomenon, and the bad shit comes down when it’s overlayed on societies that don’t have the resiliency, for whatever reason, to cope with the bottom quartile of their normal variability – on all time scales.

Here’s a fun little bit of business to warm a weather wonk’s heart:

That’s from John Wesley Powell’s 1878-79 Report on the lands of the arid region of the United States, and is the earliest climatic data from Albuquerque that I have ever run across. The data was collected by Charles Schott of the Smithsonian, who had been publishing national precipitation tables. He hurried up his new set of data, processing the western states first, so they could be included in Powell’s report. When I have some more time, I’ll blog more about the report. It’s a remarkable document in its insight into the problems of living in an arid land.

Compared to the current data, it’s a strikingly reasonable number for annual precipitation, given that it’s based on just a dozen years’ record. The seasonal distribution’s a bit out of whack, so the match for annual precipitation no doubt is just dumb luck.

I’ve read elsewhere that there was weather data collected here beginning in 1850. I’ll have to see if I can track it down.