SCHEDULE
 

 
ATM S 211: Winter Quarter 2002 
 Climate and Climate Change
Notes for the lecture on Thursday February 14
 

A flickering climate
As the ice sheets retreated beginning about 15,000 years ago, some interesting things happened.  About 12,800 years ago Europe abruptly switched back into glacial conditions.  Ice sheets reformed, glaciers advanced down valleys, and much of the vegetation of northern Europe reverted to Arctic types.  There is some evidence that this cooling was echoed in other parts of the world.

What could have caused such an abrupt shift?  The prevailing theory, advanced in the 1980s by Dr Wallace Broecker, is that the thermohaline circulation shut down after a surge of meltwater down the newly opened St Lawrence River produced a freshwater "cap" in the North Atlantic.  After a time the thermohaline circulation restarted and Europe again received warmth from it.

This theory has recently been called into question by several workers, including Professor David Battisti of UW.  These workers cite several lines of evidence to the contrary, including (1) estimates that the thermohaline circulation delivers only a small fraction (on the order of 20%) of heat to Europe, the rest coming mostly from the atmosphere; (2) climate modeling simulations indicating that small changes in tropical sea-surface temperatures during glacial times would have a bigger effect on global climate, and especially in the North Atlantic, than they do at present.
 

Regional changes

The major northern hemisphere ice sheet extended into Washington state, scraping and deepening Puget Sound and when it retreated leaving mounds of glacial till that now form most of the hills in the region.  As the ice sheet receded, a glacial lake (Lake Missoula) in what is now western Montana repeatedly formed behind and then breached an ice dam, sending gigantic quantities of water across eastern Washington.  See the cool map and explanation and satellite images. See also a nice history of the scientific discovery of the scablands and the revolutionary explanation for them.

Pollen evidence from a lake core on the Olympic Peninsula indicates major vegetation changes about 10,000 years ago and 6,000 years ago.  Between those times, the region was much drier and parts of what are now the lush forests of the Puget Sound Lowlands were probably savannah.
 
 
 
 
 
 

 
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 Last Updated:
02/11/2002