Accepted theory holds that variations in the earth's orbit, the wobbles and wiggles over thousands of years, control a good portion of the seasonal warming and cooling of the planet. These cycles -- called Milankovitch Cycles -- alter the difference between summer and winter.
Currently, the Earth is closer to the sun during the northern hemisphere winter than it was a few thousand years ago, and farther from the sun during the northern hemisphere summer, so summers should have been cooling while winters warmed.
"It's a nice story, but unfortunately, it doesn't seem to account for the past Greenland climate record," says Dr. Richard B. Alley, professor of geosciences. "Information from the Greenland ice cores does show that the summers have cooled, but also that the winters have cooled."
The Greenland ice coring projects produced two thermometers to measure temperatures in central Greenland's past. One, a combination of borehole temperatures and stable-isotope records, is very good for year-round temperatures. The other, the frequency of melt incidents, records temperatures in the summer.
"If we believe both temperature records, then 8,000 years ago, Greenland summers were a bit warmer than today, and Greenland winters were also warmer," Alley told attendees today (May 27) at the spring meeting of the American Geophysical Union in Baltimore. "In fact, the winters were perhaps twice as much warmer than the summers."
Because the thermometers indicate that the winters have cooled, some other cause must be found besides Milankovitch cycles. Alley suggests that ocean heat transport may be the key.
"If, over the last few thousand years, the ocean's transfer of heat to the area around Greenland has declined, then this would be a simple explanation for the overall cooling," says Alley. "This is not unlike what happens with the ocean circulation when rapid cooling events have occurred in the past, except more slowly."
During rapid events, such as the Younger Dryas, ocean heat transfer slows suddenly and Greenland gets cold. A large part of North America becomes cold, dry and windy. The area around the Great Lakes dries out as does the area around the Sahara Desert. Europe becomes colder. Similar changes have happened more slowly in many of these places over the last few thousand years. These climate trends may be a slow motion climate event, according to Alley.
"It appears that what is occurring is that events are happening slowly, rather than rapidly as in the past," says Alley. "The place to check out this possibility is in the oceanographic record."
EDITORS: Dr. Alley may be reached at (814) 863-1700 or Ralley@essc.psu.edu