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The year the climate changed

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An elegant Reuters headline from the paleoclimate world:

Climate chill came exactly 12,679 years ago: study

I got a kick out of this story, having read Gavin Schmidt and Elisabeth Moyer’s NRCC op-ed last week on the chilly gap between paleoclimatologists and climate modellers (and the perhaps-even-chillier one between climate scientists and economists). Schmidt and Moyer point out that while the paleo crowd may “assume that modellers have a myopic view of climate history”, modellers “may assume that palaeo-science is too anecdotal, qualitative and localized to be of use for quantitative modelling.”

Per that headline, however, paleo researchers occasionally make delightfully precise statements – when they get their hands on a sample that yields data at the right timescale. But even then, connecting snapshots of the past with future climate scenarios is not straightforward.


In this case, Achim Brauer of the German Research Centre for Geosciences and co-workers are reporting in a new Nature Geoscience paper (subscription required) that they’ve got a very useful sediment core from the bottom of a German lake. It has a separate layer of mud for every year over the 230-year period that ended about 12,500 years ago.

That happens to be when an abrupt cooling period called the Younger Dryas hit the Northern Hemisphere. The Younger Dryas was the last ice age’s last hurrah, and it caused broad environmental shifts – it’s named after an alpine flower that had a pollen-producing bonanza during the cold spell. How abrupt was it? The new paper says that winds over the lake, which left their mark on the sediment layers, started blowing much stronger in just one year.

Samples from one lake can say only so much about shifts that occurred through at least half the world – but there’s more. Another new paper, from Steffensen et al. in Science (subscription required), analyzes Greenland ice cores and likewise concludes the YD kicked off within 1-3 years.

“It happened in the past so could happen again” is the generic warning to conclude a paleoclimate story, and Reuters doesn’t fail to sound it. The Younger Dryas freeze is usually tracked back to a spillover of freshwater into the ocean, in part from melting glaciers, which altered North Atlantic circulation patterns. (The new paper adds details about how westerly winds could have carried climate changes from North America and Greenland to Europe.) Worst-case climate change worriers have pictured a similar sequence in future (more on that here).

But “It happened in the past so could happen again” (which I’ll admit to using myself) is a crude argument when it comes to learning from past climate change, as paleoclimatologist Eric Steig argued recently in NRCC. “We still don’t understand abrupt climate changes well enough to be confident in making such a statement,” he says.

More nuanced connections may come out of paleo and modelling collaborations – if the two fields settle their differences.

Anna Barnett

Image: A shift in the lake cores, from Brauer et al.

UPDATE: Vote or comment on the importance of the new paper in the Journal Club at Nature Reports Climate Change.

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