Sea-Level Rise Gives Clue to Big Chill

December 6, 2004

Many scientists believe that about 8,200 years ago a glacial lake more than twice the size of the Caspian Sea poured into the North Atlantic and triggered a precipitous drop in temperature just as Earth was exiting the last ice age.

Climate modelers are keenly interested in accurately re-creating the conditions that drove this big chill, known as the 8.2 ka event, so they can predict if and when a similar scenario might occur in the future.

"One thing they would like to get a better constrain on is exactly how much freshwater drained into the North Atlantic," said Torbjörn Törnqvist, an assistant professor of earth and environmental sciences at the University of Illinois at Chicago.

Reporting in the December 11 online issue of the science journal Geophysical Research Letters, Törnqvist and his colleagues present evidence that the influx of water rose sea level no more than 4 feet (1.2 meters).

By converting the sea-level rise to a measurable amount of freshwater, the researchers will get an idea by how much water was involved in the 8.2 ka event. This finding indicates the amount of water was less than several previous studies suggested.

"What meltwater does is it totally messes up ocean circulation," Törnqvist said. "Freshwater suddenly released into the North Atlantic basically reduces the salinity of the ocean water [and] salinity plays an important role in driving ocean circulation."

The North Atlantic climate is driven by a giant ocean conveyor belt known as the thermohaline circulation. It carries warm, salty surface water from southern latitudes to the north where it cools, which makes it denser. The dense, cool water sinks to the deep ocean and is exported south.

The addition of freshwater to the system in the North Atlantic lowers the density of the water, counteracting the cooling process that increases density. A weakening of the circulation can lead to a drastic cooling of northern regions because less warm water is brought north, according to Törnqvist.

Scientists believe a sudden influx of freshwater from North America's ancient Lake Agassiz to the North Atlantic weakened the thermohaline circulation 8,200 years ago and triggered a cooling of the region by up to 10 degrees Fahrenheit (about 6 degrees Celsius).

Hans Renssen, a climate modeler at the Free University in Amsterdam, The Netherlands, said "a crucial thing that we don't know very well is how sensitive the thermohaline circulation is to freshwater perturbations, as different climate models give different responses."

He and his colleagues use the 8.2 ka event as a test case of the circulation's sensitivity. "For such a test case, it is important to constrain the amount of freshwater as precisely as possible."

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