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Chris Mooney @chriscmooney
, 16 tweets, 3 min read Read on Twitter
1. The “Younger Dryas” was an exceedingly weird climatic era beginning around 13,000 years ago. And it could have something really important to tell us about the future.…
2. It’s weird because it’s an ice age throwback at the end of an ice age. The world was getting warmer, ice was melting – and then suddenly it wasn’t any more (in the Northern hemisphere, that is, and especially Europe).
3. One theory has been that this has something to do with the vulnerable ocean "conveyor belt," or overturning circulation, in the Atlantic Ocean. If it shut down, it could have caused a cooling. But what shut it down?
4. The search has been on for geological evidence that would definitively answer that question.
5. The thought has been that somehow, as the ice age dwindled and the vast ice sheet atop the North American continent melted and retreated northward, a lake of meltwater burst some type of barrier and flooded the ocean, freshening it and blocking the circulation.
6. There was a lake big enough to do this – glacial Lake Agassiz, which formed from glacial melt as the ice sheet backed away northward.
7. Assuming Lake Agassiz was the cause, another question has been where the floodwater entered the ocean. That’s also key, because the ocean circulation is most vulnerable to freshwater – which stops the sinking or “overturning” at high latitudes -- at key locations.
8. So there’s been a debate over whether the water came out to the east, at the Saint Lawrence River (near present day Montreal), or the far northwest, the Mackenzie River, in today’s Northwest Territories. These are a vast distance apart.
9. Well, as I reported yesterday, new research has picked up evidence of a glacial flood at this time, embedded in the shells of deceased marine organisms in the Arctic. The findings were in the Beaufort Sea. The Mackenzie flows into the Beaufort.…
10. This doesn’t close the debate by any means but it’s certainly a big new finding that is going to have to be either rejected or explained.
11. How is this relevant to the present? In some ways the Earth is walking a similar path right now – although in other ways, it is not.
12. Similar: More warming and ice melt is happening, and there is evidence that the Atlantic circulation is weakening. These three things seem to go together, in the present just as they did in the past.…
13. Dissimilar: There’s no ice sheet atop North America. And the big glacial water source that matters right now, Greenland, is no Lake Agassiz. It’s an ice sheet on a vast island, not an ocean-sized lake in the middle of a continent.
14. However, Greenland does contain enough ice/water to seriously interfere with the circulation. The real question is the rate of loss and when there becomes enough fresh water to matter. It’s not clear where that threshold is.
15. So we are not doomed to repeat the past and we will not follow an exactly parallel path.
16. But there’s certainly plenty of evidence that as the Earth warms and ice melts, hiccups in the ocean circulation – or more than hiccups – can happen. And they can happen fast. /end
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