Previous research had found that, around the same time, a northern ice dam burst, releasing the contents of a vast glacial lake into the Labrador Sea, between Canada and Denmark (see map).
Normally a warm ocean current called the Gulf Stream runs up the east coast of North America, helping to keep the region balmier than it should be, considering how far north it is.
But the entire glacial lake drained within less than a year, injecting a huge pulse of freshwater into the North Atlantic Ocean.
Daley and colleagues think the lake water diluted the salty ocean current and slowed the Gulf Stream, which in turn led to rapid cooling in North America.
"As a result, Canadian summer temperatures would have been similar to those currently experienced in autumn or spring," said team member Neil Loader, also of Swansea University.
Climate records from Greenland and Europe also show a sudden cooling during the same time period, but this is the first clear evidence for a North American chill.
Less Dramatic Drop
The moss data show that current climate models "significantly underestimate the impact and duration of the climate perturbation resulting from the megaflood," said Swansea team member Alayne Perrott.
This means these same models might not be accurately predicting what might happen in the future if Greenland's ice sheet continues to melt.
However, some scientists say that the data showing a prehistoric North American cool down may only indicate a coastal phenomenon.
"The study site is very close to the North Atlantic Ocean, and it is very likely that the climate change is primarily an oceanic signal," said Hans Renssen, a climate researcher at Vrije University in Amsterdam, who was not involved in the study.
As for whether today's melt in Greenland could trigger another round of cooling, Renssen thinks it's possible, but he doesn't believe the change would be as dramatic as last time.
In fact, he said, any future cooling is likely to be overwhelmed by human-caused warming, "resulting in no cooling in North America at all, only less warming than without the event."
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