There’s a good good post on Prometheus about a new paper by anthropologist Myanna Lahsen on the culture of climate modelers. The bottom line is the way modelers tend to believe their own models – to substitute them in their own mind for reality. It reminded me of a paper published last year in which B.G. Hunt and T.I. Elliott model climatic conditions in the Yucatan and use them to discuss drought and its implications for the Maya.

Based on proxy records, some researchers have suggested that drought played a role in the collapse of the Classic Maya (it’s debated), and Hunt and Elliott wanted to model the thing.

It’s an interesting paper, with careful discussion of the model’s shortcomings – like, for example, “its inability to reproduce the observed range of SST (sea surface temperature) anomalies associated with El Niño/Southern Oscillation (ENSO) events.” And then they offer this howler. A set of model-generated figures show rainfall anomalies that seem to match up nicely with the proxy records:

These figures confirm that major rainfall deficiences are a systematic, but irregular, characteristic of the Yucatan region.

No, they don’t. They show that the model exhibits systematic but irregular rainfall deficiencies. What they tell us about the rainfall itself is a bit more epistemologically tricky.

This is a bit of a cheap shot, because much of their discussion is generally more subtle, treating the model as a useful tool in exploring the physical system, rather than a direct representation of it. But in the context of Lahsen’s paper, it’s an interesting slip.

My story on Johannes Feddema’s paper on the global effects of land use changes finally ran in this morning’s paper. Forgive the headline. I tried to play the bait and switch game in the story and I guess I didn’t get away with it – at least with the copy editor who wrote the headline. The bait was the model’s tantalizing suggestion that deforestation in the Amazon could make New Mexico’s summer’s wetter. The switch was the caution from Feddema that the study was less a prediction than a sensitivity analysis, suggesting big effects from land use changes without being too specific yet about what they might be.

You probably shouldn’t buy another umbrella right away. The research is preliminary, said Johannes Feddema of the University of Kansas, who led the study.

But the complex daisy chain of continental-scale atmospheric changes in his simulation is a clear indication that large-scale land use changes have the potential to change climate around the globe, Feddema said.

In the comments over at RealClimate, Bradfield Lyon posted links to a couple of excellent global drought monitoring resources:

• Global Weighted Anomaly Standardized Precipitation indexes, and
• Global Standardized Precipitation Indexes

Rasmus had discussed the potential for global warming to have been implicated in the current drought in the Amazon, and I had asked the underlying question of how much of a precipitation shortfall there had been. It’s a key question in understanding drought, because much of what we think of today as “drought” is in fact a relatively modest shortfall in precipitation, combined with substantial population growth and therefore huge increased water demands.

You can see that, by both indices cited by Lyon, the shortfall over the past year in the Amazon is substantial.

(FYI, SPI for the United States is available on the Western Regional Climate Center web site, sliced up a number of different useful ways.)