A thoughtful reader points out, in regard to last night’s post about climate variability in the Colorado River basin, that I didn’t make what I will call the Pielke Jr. point: “the problem is not the climate change but the societal changes!” (That’s quoting my thoughtful reader (TR), not Pielke.)

TR is of course right. It’s not climate variability per se that is the problem here. The problem is twofold. First, it’s a problem that the folks who set up the system for allocation of Colorado River water didn’t understand the extremes of variability that could affect the river’s supplies. The second is that population and associated human systems have grown up around the Colorado River’s water with no clear understanding of the variability in water availability that is inevitable in the long run. We are, as a result, extremely vulnerable to the sort of severe, sustained drought that tree ring records suggest has happend in the not-so-distant past. We’re not vulnerable in the same way as the Anasazi. We son’t starve. But there could be significant societal dislocations.

One thing worth doing in this context is is to examine the difference between the hunter-gatherers who inhabited the Four Corners region before the puebloans, as compared to the Anasazi. Hunter-gatherers survived droughts worse than the ones that coincide with the end of the Anasazi culture. The short answer for why is that the hunter-gatherers had apparently read Pielke, and therefore did not max out the resources, leaving them the flexibility to respond to drought when it happened. In other words, they reduced their societal vulnerability to climate variability.

The Anasazi did not read Pielke, and died.

This is Delph Carpenter, speaking Nov. 12, 1922, ad the twelfth meeting of the Colorado River Commission:

The hydrographers and experts advise me that a twenty-year record on a river is adequate in its completeness and includes enough years to warrant an assumption that the average there deduced would be the average flow of the river in the future.

How wrong he was.

The green dots represent the Colorado River’s flow at Lee’s Ferry (adjusted to represent the flow if the river would have been undammed). The black line is the amount the commission assumed based on the available river data – about 20 years’ worth. The blue line is the actual long-term average since 1896. And the red jiggly line is the rolling 20-year average, positioned at the end of the 20 year interval. You can see it’s highest in the early 1920s – at about the time the commission was meeting in Santa Fe. (data from the Upper Colorado River Commission)

Carpenter and the other commissioners thought that a sufficiently long period – perhaps ten years, but certainly twenty – should be sufficient to average out the ups and downs of long-term climate cycles in the west. You can see they were quite wrong.

So the Sahel, where people really die when drought happens, could be wetter in a globally warmed world, according to new research by a team led by Reindert J. Haarsma of the Royal Netherlands Meteorological Institute:

We hypothesize that through this mechanism greenhouse warming will cause an increase in Sahel rainfall, because the warming is expected to be more prominent over the summer continents than over the oceans. This has been confirmed using an ensemble of 62 coupled model runs forced with a business as usual scenario. The ensemble mean increase in Sahel rainfall between 1980 and 2080 is about 1?2 mm day−1 (25?50 percent) during July?September, thereby strongly reducing the probability of prolonged droughts.

Published in GRL Sept. 10.