Future global warming scenarios.

Abstract

In a study commissioned by the Pentagon, Peter Schwartz and Doug Randall ([1][1]) present a very alarming scenario regarding the short-term consequences of global warming. This scenario, which predicts a shutdown of the Atlantic Ocean's conveyor circulation in the next 10 to 15 years, is based on analogies to two large and abrupt climate changes, which occurred 12,700 and 8200 years ago. Both are thought to have been triggered by catastrophic releases of meltwater stored in lakes that formed along the southern margin of the retreating Canadian ice sheet. These floods appear to have squelched deep water formation in the North Atlantic and, by as yet unknown mechanisms, caused Earth's climate to plunge back toward its glacial condition. Clearly, if global warming were to cause a repeat of such an abrupt change, the consequences would be akin to those alluded to in the warning to the Pentagon, namely, a large cooling of northern Europe. But there is no reason to believe that the impacts could occur in a mere decade, nor would they be so awesome. As the one who first pointed out the link between the Atlantic's conveyor circulation and abrupt climate changes, I take serious issue with both the timing and the severity of changes proposed in the Pentagon scenario. Computer simulations do suggest that a greenhouse-induced warming would increase the delivery of precipitation and river runoff to the North Atlantic and, further, that given a large enough warming, this excess fresh water could cause the conveyor to sag and, in the extreme, shut down. However, the time required for this to happen is more likely a century, not a decade. Further, no full-fledged global model has yet reproduced the immense impacts coincident with the two meltwater floods. We suspect that the required amplifier involves sea ice formation in the North Atlantic. If indeed this is the case, then as the globe warms, amplification by this mechanism becomes ever less likely. Exaggerated scenarios serve only to intensify the existing polarization over global warming. What is needed is not more words but rather a means to shut down CO2 emissions to the atmosphere. Although we are powerless to accomplish this by 2015, we certainly have the wherewithal to do it by 2075. 1. 1.[↵][2] 1. P. Schwartz, 2. D. Randall , “Abrupt climate change,” report prepared by Global Business Network (GBN) for the Department of Defense, available at [www.gbn.org/ArticleDisplayServlet.srv?aid=26231][3]. [1]: #ref-1 [2]: #xref-ref-1-1 View reference 1. in text [3]: http://www.gbn.org/ArticleDisplayServlet.srv?aid=26231