for National Geographic News
Analysis of ancient moss from Newfoundland, Canada, links an injection of freshwater from a burst glacial lake to a rapid drop in air temperatures by a few degrees Celsius along North America's East Coast.
This event created a colder year-round climate with a much shorter growing season for about 150 years, from northern Canada to what is now Cape Hatteras, North Carolina.
The results suggest that North America's climate is highly sensitive to meltwater flowing into the ocean, said lead study author Tim Daley of Swansea University in the U.K.
The work also means that history could repeat itself: Currently Greenland's ice sheet is melting at a rapid clip, releasing freshwater into the North Atlantic.
In a worst-case scenario, the authors say, a sudden melt could trigger another regional cooling event—although other experts say today's extreme, human-driven warming might cancel out any strong cooling effect.
Daley and colleagues studied mosses dating back more than 8,700 years that were preserved in a Newfoundland peat bog.
The ratios of two different types of oxygen in the mosses allowed the team to trace changes in atmospheric temperature over time.
When air temperatures are lower, the mosses contain less oxygen-18, a heavier version of the more common type, oxygen-16.
About 8,350 years ago, the amount of oxygen-18 relative to oxygen-16 suddenly dropped, the team reports in the September issue of the journal Geology.
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