San Francisco - For scientists investigating global climate change, determining how much change is caused in the polar regions and how much is caused by increases in sea-surface temperatures in the tropics remains a key question.

For the past 20 years, many climatologists have believed the most important factor has been thermohaline circulation near the poles, where very cold, salty and dense water sinks, drawing surface water poleward. This process is occurring in both the North Atlantic between Greenland and Europe and around Antarctica; scientists call the systems the North Atlantic Conveyor and the Antarctic Convergence.

Tropical seawater warms as a result of solar insolation, or the rate of incoming sunlight at the Earth's surface, and also when carbon dioxide and other greenhouse gases increase in the atmosphere.

However, a major question is still the relative importance of the two processes in forcing global climate change.

Oregon State University geosciences department graduate students Shaun Marcott and Jeremy Shakun led sessions Monday that examined this question at the annual fall meeting of the American Geophysical Union in San Francisco .

Marcott and Shakun hope that by combining continental and oceanic records of global warming from 10,000 to 20,000 years ago, at the close of the last ice age, scientists will be better able to predict what our climate might be like over the next century.

It is known that the carbon dioxide in the atmosphere started increasing signifigantly 17,000 to 19,000 years ago, and that large-scale melting of glaciers started 19,000 years ago.

It is not clear why the Earth's climate system was pushed into a warming phase then.

The ultimate cause of ice ages is changes in the Earth's orbit (Milankovich cycles); however, once the Earth's climate system is in a glacial phase, a large environmental push is required to initiate massive, rapid melting and warming.

Research by Marcott, Shakun and other scientists is focusing on the nature and timing of warming through the end of the last Ice Age in order to understand how current human-induced warming might proceed.

Marcott is using the Cosmogenic Isotope Laboratory at OSU to date the periods of glacial recession in the mid-latitudes of North America. Meanwhile, Shakun is developing a similar chronology for glacial deposits in the Andes of Peru. They hope to determine whether mountain glaciers in the tropics began to recede before recession in North America.

The cosmogenic dating techniques they are using relies on isotopes such as Beryllium-10, a technique that is relatively new. The laboratory at OSU is a new facility established by Ed Brooks and Peter Clark of the geosciences department.

Along with the work of Marcott and Shakun, other researchers are studying the record of sea-surface temperatures over the same period of warming and deglaciation using fossil assemblages and chemistry of ocean sediments.