ARCTIC ICE MELTDOWN WILL LEAD TO A GLOBAL FREEZE

The lighter freshwater wouldn't sink, interrupting the vertical circulation at a crucial point in the cycling of heat through the ocean ~ as if you're grabbing this conveyor belt and slowing it down.

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. "