From: Kate Ravilious, National Geographic
Published September 16, 2009 06:16 AM

Global Warming Could Cool N. America

Global warming could actually chill down North America within just a few decades, according to a new study that says a sudden cooling event gripped the region about 8,300 years ago.

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.

Cooling Record

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.

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.

(See recent pictures showing a massive Greenland glacier disintegrating.)

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."