Interesting work presented at AGU by a University of Alabama/NASA/etc. group suggesting the the possibility that Mayan deforestation may have brought on the drought conditions that some blame for the collapse of classic Maya culture.

I have thought that the Maya might make a good case study for a society done in by drought, expanding in wet times and therefore losing the resilience necessary to cope with drought. Smart archaeologists disagree on the drought hypothesis, with some arguing that “collapse” was not synchronous across the Maya region and therefore drought is not a reasonable explanation. But it’s at the very least an idea worth entertaining.

This new work suggests an intriguing wild card. What if cutting down the rainforest reduced the amount of atmsopheric moisture available via evapotranspiration, thereby reducing rainfall? The group’s modelling run suggests it’s a possibility:

Domain averaged dry season rainfall in the Maya lowlands decreases to 128.9mm for the deforested scenario, a decrease in 9.7 percet over current conditions…. Continued deforestation that would be representative of those prior to the collapse of the Maya civilization in the region can be expected to lead to additional decreases in dry season precipitation throughout the entire region by about 10mm to 100mm. Improper land use management in this region could lead to futures catastrophes for the modern humans.

(Hat tip Roger Pielke Sr., who is one of the coauthors and who pointed this out to me.)

Jeff Bingaman, fresh from Montreal, talks about U.S. greenhouse gas policies:

Sen. Jeff Bingaman, D-N.M., thinks now is the time for the United States to act on climate change.

But Bingaman, who has emerged as a congressional leader on the issue, acknowledges he has his work cut out for him in persuading the Bush administration.

That was the reality Bingaman faced last week as he stepped to the lectern as the keynote speaker at an international gathering of legislators in Montreal.

Individual state and local governments, along with many in U.S. industry, are pushing for U.S. leadership in curbing greenhouse gas emissions, Bingaman told the audience, according to a text of his talk. But the Bush administration does not agree.

The Bush administration opposes participation in the international community’s mandatory emissions caps, saying it would harm the U.S. economy while exempting developing nations like China and India.

“The truth is,” Bingaman told the gathering, “there is a diverging view within the United States that I do not believe is accurately represented by the president and the current administration.”

Johannes J. Feddema and colleagues have a paper in Science today(Feddema et al. 2005) on the importance of land use in climate change. (And the suddenly ubiquitous Roger Pielke Sr. has an accompanying Perspective piece(Pielke Sr. 2005), and Roger also blogs it here.)

What Feddema and company did was run a global climate model with greenhouse gas changes alone and with land use changes as well. No surprise – the land use changes had a significant effect on the results – especially in terms of regional spatial variability.

The details of the changes here are less important, I think, than the underlying message – that land use changes matter a lot, especially at the regional scale, which is, after all, where we all live.

The details are, however, pretty interesting here in the southwest. Their simulation suggests a weakened Hadley circulation because of deforestation in the Amazon, which changes the circulation that contributes moisture to the North American Monsoon system, intensifying the summer monsoons that bring parts of New Mexico up to 40 percent of its annual precipitation.

I don’t think you should view this as a prediction, so much as a clear indication of the scope and significance of land use changes in climate change at the regional scale, and of the importance of seeing this question explored further.

Feddema, J.J., K.W. Oleson, G.B. Bonan, L.O. Mearns, L.E. Buja, G.A. Meehl, and W.M. Washington. 2005. “The Importance of Land-Cover Change in Simulating Future Climates.” Science 310(5754):1674-1678.
Pielke Sr., R.A. 2005. “ATMOSPHERIC SCIENCE: Land Use and Climate Change.” Science 310(5754):1625-1626.