The ice constituting mountain glaciers and continental ice sheets is derived from ocean water. During the Pleistocene epoch, the sea level dropped 100 meters, supplying sufficient water to cover about 30 percent of the Earth with glacial ice. As climatic conditions change and deglaciation begins, ice melts; meltwaters flow into the oceans or remain on land as glacial lakes. Lake Agassiz, the largest lake in North America during Pleistocene deglaciation, was dammed to the north by the Laurentide ice sheet (LIS). This proglacial lake (a glacial lake directly in contact with glacier ice) and its drainage basin covered, at maximum, 2 million square kilometers, extending from the Rocky Mountains in Alberta to the Lake Superior basin, into South Dakota, and north to the LIS in Hudson Bay. Maximum volume was 163,000 cubic kilometers.

First recognized by W. H. Keating in 1823 and named by Warren Upham in 1880, Lake Agassiz honors Louis Agassiz, a promoter of continental ice theory. Lake Agassiz is known for its fluctuating size, shape, and depth; multiple large-volume outbursts (drawdowns); and differing meltwater outlets. Physical evidence documenting these variations include former beaches and wave-cut cliffs, lake outlets, and ancient--now dry--lake marshes.

The maximum elevation of the lake's surface was determined by a lake outlet at 70 meters above sea level. Lake outlets were controlled by the vacillating LIS. Lake Agassiz water would rise until it reached an outlet, at which point it would abruptly overflow. Runoff routes were the Mackenzie River Valley to the Arctic Ocean, Hudson Bay to the North Atlantic Ocean, the St. Lawrence River to the North Atlantic Ocean, and the Mississippi River Valley to the Gulf of Mexico. As deglaciation continued, the level of Lake Agassiz fell below the level of the eastern outlets; lands south of 53œ north latitude were above the level of the lake about eighty-five hundred years ago. Approximately one thousand years later, Lake Agassiz drained into the Tyrrell Sea, an area somewhat larger than today's Hudson and James bays. The lake existed for about five thousand years.

Short-term, often abrupt changes of climate are driven by reorganization of ocean circulation. Studies have shown that a freshwater influx of less than 100,000 cubic meters per second may slow or shut down the North Atlantic deep water (NADW). Part of the thermohaline circulation (THC), the NADW moves cold, salty, deepocean water south from North America. The same studies reveal that within a glacial-interglacial transition, a freshwater outburst of less than 10,000 cubic kilometers brought significant changes to ocean circulation, as well as colder temperatures. An outburst from Lake Agassiz of 9,500 cubic kilometers through the Great Lakes-St. Lawrence drainage system initiated the Younger Dryas cooling event 12,900 to 11,600 years ago.

Another cooling episode, the 8.2ka event, was preceded by an outburst of 163,000 cubic kilometers in one year from the Lake Agassiz-Lake Ojibway system. Near the end of the existence of Lake Agassiz, the waters of Lake Ojibway commingled with Lake Agassiz. The exit route for this enormous interglacial outburst was through a weakened LIS, then northward into the Hudson Strait. The Hudson Strait is 2,000 kilometers north of the Atlantic Ocean. It has been suggested that, despite the distance from the Atlantic Ocean, the rapid influx of cold freshwater brought about the 8.2ka event, the last major cooling event of the Pleistocene deglaciation.

Global warming may introduce large fluxes of cold freshwater to the North Atlantic Ocean. As the climate warms, Greenland glaciers could melt, and increased rain could fall, producing a high volume of freshwater that could slow or stop the NADW. Freshwater outbursts from Lake Agassiz during the past deglaciation document perturbations of the NADW and the THC, resulting in cold temperatures, especially in North America and northern Europe.

In addition, once highly reflective snow and ice have melted, the less reflective land surface will absorb more solar radiation and add more heat to Earth. Along with reduced snow cover is the loss of tundra--areas habitated by scrubby, often snow-covered vegetation located on vast regions of Europe and North America. Snow-covered tundra reflects sunlight, but, if typical tundra vegetation is replaced by taiga vegetation (the dark, evergreen forests of subarctic regions), more sunlight would be absorbed and more warming would occur.

Bibliography:

1) Alley, R. B. The Two-Mile Time Machine. Princeton, N.J.: Princeton University Press, 2000.

2) Broecker, Wallace S., and Robert Kunzig. Fixing Climate: What Past Climate Changes Reveal About the Current Threat--and How to Counter It. New York: Hill and Wang, 2008.

3) Clark, P. U., et al. "Freshwater Forcing of Abrupt Climate Change During the Last Glaciation." Science 293 (July 13, 2001): 283-287.

4) Teller, J. T., and L. Clayton, eds. Glacial Lake Agassiz. Geological Association of Canada Special Paper 26. Toronto: University of Toronto Press, 1983.

5) Teller, J. T., D. W. Leverington, and J. D. Mann. "Freshwater Outbursts to the Oceans from Glacial Lake Agassiz and Their Role in Climate Change During the Last Deglaciation." Quaternary Science Reviews 21 (2002): 879-887.

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