Sigma Xi Talk: Tropical Glaciers Are Weird

Tuesday night was the annual Sigma Xi induction banquet on campus (I’m currently the president of the local chapter, and have been scrambling to organize the whole thing in between all my other responsibilities these past few weeks). Sigma Xi, for those not familiar with it, is the scientific research honor society– like Phi Beta Kappa for science nerds. We had thirty-odd students nominated for membership based on research they have done as undergraduates, and had a little banquet and induction ceremony to celebrate their accomplishments.

Tradition for this sort of thing calls for an after-dinner talk about something science-related. My fallback plan was to cop out and give a version of the talk I did at the Perimeter Institute last fall, but happily for everyone involved, my first choice came through. Professor Don Rodbell of the Geology department gave a talk about the work he’s done studying lakes in the Andes.

The basic message can be summarized as: tropical glaciers are weird. Don and a long series of students and collaborators have looked at the behavior of glaciers as recorded in high mountain lakes and moraines, going back a few tens of thousands of years, and what they find doesn’t really track with what people see in Northern Hemisphere glaciation.

Specifically, the core samples they have from a bunch of these lakes shows something kind of peculiar. The Andean glaciers appear to have expanded significantly a bit more than 20,000 years ago, but right around the point where the ice sheets in North America and Europe hit their maximum extent, the glaciers in the Andes practically disappeared. They rebounded sharply a few thousand years later, right around the time that the ice started to retreat in the Northern Hemisphere.

They see the same basic pattern in a bunch of different climate proxies, over a wide range of places in South America. This suggests that the climate in the tropics, or at least that part of the tropics, is driven by some different mechanisms than the things that drive climate in the polar regions.

This is emphatically not a denial of the evidence of modern warming, which is extremely solid, and not really open to debate. Rather, it’s a reminder that climate is a really complicated thing, and that the consequences of changes in climate can go off in unexpected directions. He pointed out that this may have particularly dire implications for the people who live on the high plains of Peru, who are almost entirely dependent on glacial melt water. If those glaciers disappear (as they are doing at a rapid rate), the whole region might become uninhabitable.

The work in question is apparently not without controversy– there’s apparently some argument about the dating of their samples, which would affect the timing of the glacial retreat. He makes a pretty solid case for it, though, at least to this non-geologist. Climate science and geology types with actual knowledge to bring to bear are more than welcome to expand on this in the comments.