The work is the first to recreate the event via computer modeling, and the first time that model results have been confirmed by comparison to the climate record, which includes data from sources such as ice cores and tree rings.

"We only have one example of how the climate reacts to changes, the past," said Gavin A. Schmidt, at NASA's Goddard Institute for Space Studies. "If we're going to accurately simulate the Earth's future, we need to be able to replicate past events. This was a real test of the model's skill."

Allegra LeGrande, a graduate student in the department of Earth and Environmental Sciences at Columbia University, led the research, which appeared in the January issue of the Proceedings of the National Academy of Sciences.

The group used an atmosphere-ocean coupled climate computer model known as GISS Model E-R to simulate the climate impact of a massive freshwater flood into the North Atlantic that happened about 8,200 years ago, after the end of the last Ice Age. Retreating glaciers opened a route for two ancient meltwater lakes, known as Agassiz and Ojibway, to drain suddenly and catastrophically from the middle of the North American continent.

At approximately the same time, climate records show Earth experienced its last abrupt climate shift. Scientists suspect the massive freshwater pulse interfered with the ocean's overturning circulation, which distributes heat around the globe. According to data known as climate proxies, average air temperatures apparently fell as much as several degrees in some areas of the northern hemisphere.

Climate researchers use these proxies - chemical signals locked in minerals and ice bubbles as well as pollen and other biological indicators - as indirect measures of temperature and precipitation patterns in the distant past. Because GISS Model E-R incorporates the response of these proxies in its output, the authors of the PNAS study were able to compare their results directly to the historical record.

The researchers prodded their model with a freshwater flow equaling between 25 and 50 times the flow of the Amazon River in 12 model runs that took more than a year to complete. Although the simulations largely agreed with records from North Atlantic sediment cores and Greenland ice cores, the team's results showed the flood had much milder effects around the globe than many people thought.

According to the model, temperatures in the North Atlantic and Greenland showed the largest decrease, with slightly less cooling over parts of North America and Europe. The rest of the northern hemisphere, however, showed very little effect, and temperatures in the southern hemisphere remained largely unchanged. Moreover, ocean circulation, which initially dropped by half after simulated flood, appeared to rebound within 50 to 150 years.

"The flood we looked at was even larger than anything that could happen today," LeGrande said. "Still, it's important for us to study because the real thing occurred during a period when conditions were not that much different from the present day."

The GISS climate model is also being used for the latest simulations by the United Nations Intergovernmental Panel on Climate Change to simulate Earth's present and future climate. "Hopefully, successful simulations of the past such as this will increase confidence in the validity of model projections," Schmidt said.

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