For most of our current geologic period, the Holocene, temperatures seem to have fluctuated within a narrow range, indicating a fairly stable climate. But the end of the glacial period that preceded the Holocene saw a series of rapid temperature changes, with swings of up to 10°C in as little as a few centuries; the world would seem to be just about out of the glacial period, only to be plunged back into the cold for centuries. In the last few weeks, however, a couple of papers have indicated that these cold snaps may have had a single underlying cause: the sudden draining of massive glacial lakes left behind by the ice.

The idea that the melting that produced glacial lakes could ultimately have an enormous cooling effect on the climate may seem counterintuitive, but it's based on fairly straightforward geography. Large areas of North America drain northwards, through the St. Lawrence River basin or into Hudson Bay. During the last glacial period, these outlets were trapped under a massive ice sheet, leaving the water nowhere to go. So it built up into enormous glacial lakes, such as Lake Agassiz (shown below), until the lake surface reached a level where drainage would take place to the south.

But, as the ice sheets that kept these lakes in place melted, the remaining, weakened glaciers came under increased pressure from the enormous reservoirs they held in check. When the ice failed, staggering amounts of fresh water flowed into the ocean in a short time period.

Geologists think that the massive volume of fresh water dumped into the North Atlantic would be sufficient to interfere with the normal circulation that brings warm water north from the tropics. This in turn would give the Northern Hemisphere a chill, allowing the ice sheets to re-form; the expanded ice would reflect more sunlight back into space, enhancing the cooling. The net result, the hypothesis goes, is an extended cold snap that would persist until the general warming trend overwhelmed it.

That may sound like a tidy explanation, but it hasn't been universally accepted. A number of researchers think they've found evidence that the impact of a comet or meteor coincided with the biggest of the chills. Others, however, contest those findings. So what we seem to have is a genuine scientific controversy, and it sets the stage for the recent papers.

The first paper, in today's issue of Science, takes a look at a relatively minor cold snap occurred approximately 9300 years ago—minor being a drop of 2°C, about twice the magnitude of what we call the Little Ice Age. The authors use sediment records from a number of sites near Lake Superior to determine when various small bodies of water were connected to Superior itself. Based on the elevation of these sites, they can make inferences about the altitude of Lake Superior's surface.

Putting the records together, the authors conclude that the lake experienced a sudden and significant drop in surface levels. Lake Superior seems to have lost about 45m worth of water in a single event that coincides with the temperature drop that occurred 9300 years ago. (The authors found evidence that the water bypassed Lakes Erie and Ontario, and took a more direct route to the sea across southern Canada.) So, that seems to support the proposal that dumping a lot of fresh water into the Atlantic can drive a significant chill.



Image: Areas that, at different times, were covered by Lake Agassiz.Image: North Dakota

Draining Agassiz

Taking a bit off the top of Lake Superior may be a pretty big deal, but it was probably nothing on the event that may have set off the Younger Dryas, a chill that lasted over 1,000 years. That is thought to have been triggered by the emptying of Lake Agassiz. At various times, Agassiz covered portions of two Canadian provinces, and extended south into the US. The one problem here: nobody had conclusively identified the path the waters took on their way to the North Atlantic.

A paper from a recent edition of Nature provides a good explanation for that: they didn't exit into the North Atlantic. Instead, its authors argue that they've found physical evidence that the flood dumped the water into the Arctic near the Canada-Alaska border, along the course of the Mackenzie River. According to their calculations of land and ice sheet elevations, this route allowed the water to skirt the edge of the ice sheet for most of its trip to the sea.

If they're right, that's good news for the general idea that draining glacial lakes can have a huge impact on the climate. But it would seem to throw a bit of a monkey wrench into matters, since the fresh water would end up in the Arctic Ocean, where it's unlikely to have an immediate, direct effect on the Atlantic currents. The authors cite earlier work that had suggested, given the lower ocean levels and positions of other ice sheets, the only place for the fresh water to exit the Arctic was past Greenland and into the North Atlantic. So, the end result would have been the same, even if the route was different.

Combined, the two papers make the case that freshwater entry into the Atlantic can have a major impact on the hemisphere's climate and, with strong enough feedback effects, alter the global climate.

Science, 2010. DOI: 10.1126/science.1187860

Nature, 2010. DOI: 10.1038/nature08954 (About DOIs).

Listing image by NASA