Nature Climate Change | Climate Feedback

AGU Chapman: It’s all about the bumps

A scant 21,000 years ago, Columbus, Ohio, was blanketed by the Laurentide ice sheet. Today it is home to the Byrd Polar Research Centre at Ohio State University, where this morning I sat in a glacially air-conditioned lecture hall watching an animation of that sheet flickering rapidly back and forth across Columbus and the rest of the northern parts of the continent. Such strobe-light climate change from the Earth’s past is the focus of the AGU Chapman Conference on Abrupt Climate Change, being held here this week.

Though it’s a fairly small gathering of 150 experts, it doesn’t have the annual reunion feeling of some meetings; many of the people here seem never to have met before. We’ve got palaeoscientists of various persuasions: they reconstruct climatic history via models, ice, sediment, or – as geochemist Henry Pollack described his work on borehole temperature records to me over hors d’oerves – by “taking the Earth’s temperature through its rectum”. (Jokes about giant thermometers ensued.)

The common thread is figuring out what caused abrupt changes in the past – and what that implies about the prospects for their return. Says Richard Alley, a Penn State glaciologist and IPCC author, “The IPCC reports are the most optimistic thing we can put forward, because the projections are smooth. If you look at any palaeo record, there are bumps. This meeting is all about the bumps.”

One innovative study on show today applied the models behind those smooth future projections to the bumpy past record. Bette Otto-Bliesner of NCAR says her group’s research is the first to feed palaeo data into an IPCC-style coupled global climate model and run it continuously for several thousand years during the last deglaciation – rather than just taking snapshots in time, as was done previously. Having given the model instructions about what the greenhouse gas levels, sea ice extent, and meltwater flows should be, they found that it beautifully reproduced the bumpy North Atlantic temperature record.

The model’s success allows a look at the mechanisms at play – the group concludes that meltwater from the Northern Hemisphere was affecting Atlantic circulation and driving the temperatures – and it also raises confidence that this type of simulation is faithful to the real Earth system. “It says we do get it about right,” says Otto-Bliesner.

Another talk announced a previously unappreciated bump during the Eemian, the last warm interglacial period. The Eemian is often invoked as a possible analogue for our future, since it’s a de-iced period believed to have been a few degrees warmer than the present. (Cue slide of hippos lounging in London’s Trafalgar Square.) Not that simple, says Henning Bauch of the Leibniz Institute of Marine Sciences in Germany: his previous work with foraminifera fossils in ocean sediment cores suggested that the Norwegian Sea saw a cooler Eemian, and a new high-resolution core shows more specifically that in this region the warmest stretch of the Eemian was divided in half by a dramatic cooling spike. More records that span the Eemian are badly needed, he says, but this one indicates we may not be able to take the period as a window on centuries to come.

Anna Barnett


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