Scientists seeking the cause of a big freeze that hit shortly after the last ice age have eliminated one prime suspect: a gigantic lake spilling into the Atlantic Ocean and disrupting warm water currents. Experts say the new research provides valuable lessons for scientists attempting to predict the consequences of today's warming trend.
About 8200 years ago, the paleoclimate record suggests, temperatures across the Northern Hemisphere plummeted. Within a century or so, that half of the globe appeared ready to plunge into another ice age, although temperatures eventually warmed again. For years, scientists have been trying to figure out what caused the cooling. One popular possibility has been the draining of Lake Agassiz. The lake, which spread over much of central Canada and the northern United States and at its maximum covered more area than all of the Great Lakes combined, emerged as the glaciers melted. Eventually, Lake Agassiz's water poured out through Hudson Bay and into the northwestern Atlantic Ocean off the coast of Labrador. Researchers suspected that by dumping a huge amount of fresh cold water into the ocean within a few hundred years, the lake might have disrupted the currents that transport warm water from the tropics to eastern North America, Greenland, and Western Europe.
Now, that scenario seems much less likely. A Canadian team has examined a large sample of drill cores taken from the ocean bottom off the Labrador coast. On 2 August in Geophysical Research Letters, the researchers report that the sediment samples within the cores show no connection between the known timetable of Lake Agassiz's draining and significant changes in the ocean's circulation pattern. Instead, the team says, the big chill probably resulted from a combination of influences--perhaps including the 90-meter rise in sea level caused by glacial melting at the end of the ice age--that probably managed to scramble ocean currents more than did the water from Lake Agassiz.
That's a "plausible" explanation, says oceanographer Carl Wunsch of the Massachusetts Institute of Technology in Cambridge. "Whatever happened around [that time] is far more complicated than the simplistic stories of cause and effect now appearing in the literature," he says. The research also presents an object lesson for present climate predictions, Wunsch adds. Injecting a large volume of fresh water into the Atlantic, such as might occur if the melting of the Greenland ice cap accelerates, would activate a complex series of processes "that even the most sophisticated of climate models cannot calculate," he says. Those processes, such as changes in land and sea ice cover, in Earth's reflectivity, and in evaporation and rainfall patterns, can overwhelm the ability of present-day computer simulations, Wunsch explains.