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A Disconcerting Cold Snap

About 6250 BC, there was a drop in temperature, followed by recovery to what was normal for the time, over perhaps 200 years. Cooling was as much as –5°C at the summit of the Greenland Ice Sheet, and signs of a cooler, or in some places a drier or a dustier, atmosphere can be seen across much of the Northern Hemisphere.

Palaeoclimatologists call this cold snap “the 8.2 ka” (ka being thousands of years before the present, present being defined as 1950 AD). It is a much lesser phenomenon than the 15,000-year course of deglaciation, and lesser even than some of the other short-term anomalies we can see during deglaciation, but the 8.2 ka is nevertheless a clearly-defined, short, sharp blip in the record.

Why did the 8.2 ka start, and why, having started, did it stop? We now think we know the answer to the first of these questions. The simplest explanation is one which also explains, in a rather satisfying way, some of the other cold spells during deglaciation. It rests on how the bulk of the meltwater from the Laurentide Ice Sheet, covering northern North America, found its way to the ocean, and what it did when it got there. The four largest outlets for Laurentide meltwater were the Mississippi, the Mackenzie, Hudson Strait (draining all of the region now occupied by Hudson Bay) and the St Lawrence. As the ice sheet shrank, the volume of meltwater varied but so too did the path it followed to reach the Atlantic.

We can connect evidence from earlier times during deglaciation quite confidently with switches from the Mississippi to the St Lawrence as the main Laurentide meltwater outlet, and from the St Lawrence to the Mackenzie.

Both of these switches were followed by hemisphere-wide cold spells, but the 8.2 ka has a more dramatic precursor than either. It began with, or at least followed, the final dismemberment of the ice sheet into two parts, one east and one west of Hudson Bay. The dismemberment was due to a colossal flood. At the time, meltwater was ponded between the ice margin and the higher ground to the south, as the long-vanished but very large Lake Agassiz-Ojibway. Apparently the lake water was able to force open a subglacial channel beneath the dwindling neck of the ice sheet, draining in one fell swoop (or possibly two) towards Hudson Strait over a time believed to be a year or less. The level of the world ocean would have risen by something like 100–200 millimetres in each swoop, but more significantly the catchment delivering fresh water to the Atlantic via Hudson Strait would thereafter have been close to its present-day extent.

Why should it matter how the meltwater gets where it is going? The key to this question is in the adjective “fresh”. In oceanography, fresh means not salty, and not salty means less dense. Make the surface layers of the north Atlantic less dense and you make them less likely to sink, which is bad news for the meridional overturning circulation or MOC. If you think the Laurentide Ice Sheet was big, you should check out the MOC, which is a major player on the global climatic playing field.

The 8.2 ka is one signal from the past for which I can’t think of an immediate near-future angle. There are plenty of worrying ice-dammed lakes in the modern world, but there is no chance at all of a repeat of the 8.2 ka in modern times because there are no stores of ice-dammed water anywhere near the size of Lake Agassiz-Ojibway.

However, we don’t know the answer to the second question: why did the 8.2 ka stop? Evidently it wasn’t big enough to switch off the MOC, and according to the most authoritative recent assessment such an event is “very unlikely” in the foreseeable future. But it would be nice if we could be more confident about such assessments. It would help a lot if we could find out whether the 8.2 ka was a near miss or just a mildly interesting blip in the climatic record.

Posted by Graham Cogley on August 24, 2009 3:00 PM | Permalink