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These isolated dramas play out far from the mid-latitudes of the planet, where the vast majority of people live, but they could soon have serious implications for all of us. What is really at risk in the Arctic is part of the thermostat of the earth itself. The difference in temperatures between the tropics and the poles drives the global climate system. The excess heat that collects in the tropics is dissipated at the poles, about half of it through what has been nicknamed the ocean conveyor, a vast deepwater current equivalent to 100 Amazon Rivers. Much of the rest of the heat is conveyed as energy in the storms that move north from the tropics. If the poles continue to warm faster than the tropics, the vigor of this planetary circulatory system may diminish, radically altering prevailing winds, ocean currents and rainfall patterns. One consequence: grain production in the breadbaskets of the U.S. and Canada could be in jeopardy if rainfall becomes less steady and predictable. Already, severe and unpredictable storms across the northern hemisphere may be a sign that the global system is changing.
Even greater climate change could be on the way. Growing numbers of scientists fear that the warming trend will so disrupt ocean circulation patterns that the Gulf Stream, the current that warms large parts of the northern hemisphere, could temporarily shut down. If that happens, global warming would, ironically, produce global cooling--and bring on a deep freeze.
Such a calamity could be self-inflicted. Many scientists believe that the current warming is related to the increased burning of fossil fuels, such as gasoline and coal, which overloads the atmosphere with carbon dioxide and other greenhouse gases. That's why 160 countries signed the 1997 Kyoto Protocol, which requires industrial nations to reduce their greenhouse emissions to an average of 5.2% below 1990 levels between the years 2008 and 2012. But even that weak treaty remains controversial, and governments have made little progress toward implementing the pact. The U.S. Senate hasn't even considered ratifying it. Opponents seize on the possibility that the warming we're seeing may not be our doing but just part of the natural variation in climate.
Partly in response to this deadlock, NASA climatologist James Hansen last week unveiled an alternative strategy. Instead of pursuing the politically unpopular goal of drastically reducing consumption of fossil fuels, he suggests going after other greenhouse gases, such as methane, which he thinks has accounted for as much warming as carbon dioxide in the past century, even though it is present in the atmosphere in much smaller quantities.
Without action, major changes appear inevitable. Should surface water temperatures in the high Arctic rise just a few degrees, the sea ice could disappear entirely, but even a partial melting could devastate the northern hemisphere's climate. A combination of melting ice, increased precipitation and runoff from melting glaciers on land could leave a layer of buoyant freshwater floating atop the denser salt water, at a point in the North Atlantic where water ordinarily cools and sinks. 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 a conveyor belt and slowing it down.