Abstract

Abrupt climate changes centered on the north Atlantic were forced by freshening and enforced by the resulting sea-ice growth, as shown by a rapidly growing body of evidence. Cause-and-effect is especially clear for the event about 8200 years ago. The largest lake on Earth drained into the north Atlantic, probably in one bad summer, just before a prominent hemispheric cold-dry-windy climate anomaly, which has the pattern simulated in models forced by
north Atlantic freshening. Many older cold events similarly followed north Atlantic freshening. Paleoclimatic evidence for these events shows only moderate summertime cooling but exceptionally large wintertime cooling, including great extension of permafrost in Europe despite continued winds from the Atlantic. Sea ice is strongly implicated by data and models. The "switch" for the abrupt climate changes has been whether north Atlantic waters cooling in the winter
sank before they froze or froze before they sank, and this switch changed wintertime temperatures by tens of degrees. If future north Atlantic freshening weakens or stops the thermohaline circulation, a new ice age is not plausible. Furthermore, if global warming proceeds far enough that sea-ice growth would be suppressed despite a fresher ocean, then the climate anomalies would be greatly reduced. However, warming over the next decades may not be sufficient, so large climate anomalies remain possible for some time.

Biographical Sketch

Richard Alley is a Professor of Geosciences at Pennsylvania State University in State College, Pennsylvania. He earned Bachelor's (1980) and Master's (1983) degrees in Geology from Ohio State University, and earned his Ph.D. in Geology from the University of Wisconsin, Madison (1987). His research focuses on abrupt climate change, glaciers, ice sheet collapse and sea level change. He has participated in ice core drilling projects in Antarctica and Greenland and has won many awards for teaching and research.

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All seminars begin at 3:00 p.m. and are held in room 200 of the Oceanography/Physics building.