Climate Change and Southwestern Drought

If we can all avert our eyes for a moment from the CRU emails, the inexorable momentum of climate science hurtles down the track with a new paper in today’s Science using paleo records to suggest (among many interesting things) that a warming world is, for the southwestern US, a drier world.

Mike Mann and colleagues (yeah, I know, get your mind off the emails, dammit!) have used a network of proxies in an attempt to reconstruct spatial variability for the Medieval Warm Period and Little Ice Age. For those of us in the Southwest, the reconstruction of sea surface temperatures in the equatorial Pacific is of particular import. The paper supports an argument Mann and others have previously made that the MWP was characterized by persistent La Nina-like conditions, a cooling along the equator that, among other things, leads to generally drier conditions here in the Southwest.

Mann and others (see Volcanic and Solar Forcing of the Tropical Pacific over the Past 1000 Years, Journal of Climate, Vol. 18, 447-456) have argued in the past for a sort of “thermostat” mechanism, where generally warm conditions create a cool anomaly in the Pacific, a “thermostat effect”. From the new paper:

The paleoclimate reconstructions presented here hold important implications for future climate change. For example, if the tropical Pacific thermostat response suggested by our analyses of past changes applies to anthropogenic climate change, this holds profound implications for regional climate change effects such as future drought patterns.


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  2. I think Grant Meyer of UNM sees a similar pattern in his fire-related sedimentation data from the Rockies: increase in fires in the Medieval Warm Period. But the Medieval Warm Period had a different regional pattern of temperatures than the current warming does, doesn’t it? If it’s the La Nina/cooler Pacific surface that causes the drier conditions, then the affect of future climate change in the SW would depend on the affect of climate change on the Pacific. (I don’t think a simplistic geologist’s view of “the past is the key to the future” helps much in this kind of situation. That’s why the modeling matters, too.)

  3. Strange that the drier conditions in the Southwest supported a large native American population in the Sedona AZ area. Are you aware of anyone who has studied the growth and timing of these settlements with respect to the MWP?

  4. Sam –

    Great question.

    Jeff Dean at the University of Arizona has done the most detailed look at this question that I’m aware of. His argument is that the real distinction that matters is not so much wet/dry but stable/unstable. Dean’s data suggests that the period of greatest population expansion in the Four Corners area happens during the first half of the MWP here, with Chaco and Mesa Verde collapsing during the second half. During the time of population growth, things tended to be drier than they are today, but the year-to-year variability was not great, so the societies that developed could count on the climate from year to year. By the mid-1100s, though, that started to change, with both increased year-to-year variability and increased spatial variability. The year-to-year variability means the dry years were drier. Dean argues that the spatial variability is also critical. The Chacoan culture was spread across multiple climate zones. When drought happened in one part of the region, the other would be wet enough to grow crops, and vice versa. But somewhere around the mid-1100s, according to Dean, things broke down, and everyone was having drought at once. The tree ring record shows widespread drought around the 1150s throughout the west. It’s around that time that Chaco was abandoned. Similar big pulse of abandonment happens in the late 1200s, when a similar event occurs.

    In other words, as long as the climate was warm and dry but stable, they tended to do OK, but they took advantage of that stability to expand their population. It was the instability that caused the problems.

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