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So how would that produce cooling? Ordinarily the conveyor is propelled by the pull created by masses of water sinking in the North Atlantic. When this pull diminishes, the movement of warm water north in the Gulf Stream could slow or stall, driving down temperatures in Europe and North America, and possibly elsewhere.
It has happened before. Roughly 12,000 years ago, at the end of the last Ice Age, a natural warming sent freshwater from melting glaciers flowing out of the St. Lawrence River into the North Atlantic, all but shutting down the Gulf Stream and plunging Europe into a 1,300-year deep freeze. The more that becomes known about this period, named the Younger Dryas (after a tundra plant), the more scientists fear that the rapid melting of sea ice could cause the same catastrophe again. Only next time, writes geophysicist Penn State's Richard Alley in a forthcoming book, Two-Mile Time Machine, the effects would be much greater, "dropping northern temperatures and spreading droughts far larger than the changes that have affected humans through recorded history." Would this be "the end of humanity?" he asks rhetorically. "No," he replies. "An uncomfortable time for humanity? Very."
A sudden chill would shorten growing seasons, and the resulting changes in precipitation could be even more damaging. Colder air is dryer air, and Alley points out that during the Younger Dryas, the monsoon weakened in Asia and the Sahara expanded. Harvey Weiss, a Yale archaeologist who has studied the role of climate in human history, notes that it's not changes in temperature that bring down civilizations but changes in precipitation.
Protecting civilization is the goal of the Kyoto Protocol, but the treaty allows 12 more years for implementation, on the assumption that climate change will be gradual. That assumption looks shaky. Studies of deep underground ice layers in Greenland, which reveal a record of climate changes over hundreds of thousands of years, show that major climate shifts, like the onset of the Younger Dryas, can come very abruptly--within a few decades.
It probably won't be possible to avoid some climate change this century, up or down--and there's still a chance that the earth's systems will compensate for any that occurs--but the possibility that climate turns rapidly and unpredictably should spur us into doing whatever is practical to turn from fossil fuels--fast. If done right it can be a boon. Energy conservation usually increases profits. In developing nations it's often cheaper to use alternatives like wind power to electrify new areas.
At the entrance to the Churchill Northern Studies Centre, a base for investigations of regional climate change, a rusting rocket is a mute reminder of the complex's earlier life as part of defenses against Soviet nuclear attack. That threat never materialized, and now, belatedly, scientists venture from the base to study a threat that has materialized but against which no adequate defense has been mounted. Despite the danger that climate change poses, the resources currently devoted to studying this problem--and combatting it--are inconsequential compared with the trillions spent during the cold war. Twenty years from now, we may wonder how we could have miscalculated which threat represented the greater peril.