The Earth's heat-distribution system has already begun shifting massively in response to rising levels of greenhouse gases. Precipitation patterns, the change of seasons, storm intensity, sea ice, glaciers, temperatures on the tundras--all are in flux. As scientists nervously monitor sea and air currents for signs of major shifts, many believe that today's proliferation of weather extremes may be the prelude to another epochal transition--a possibility first flagged by the great oceanographer Wallace Broecker in the journal Science in 1997.

How bad could it get? Imagine Europe suffering floods and heat waves on a vastly greater scale than those endured in 2002 and 2003, while northern regions experience intermittent deep freezes as atmospheric and ocean circulations struggle to find new equilibrium. At the same time, droughts and floods not seen since ancient times would afflict some of the most densely populated regions on earth. The probability of drought in the American breadbasket would rise, and along with it the possibility that the U.S. grain surplus--which accounts for the dominant share of world grain exports--would disappear.

A flickering climate wouldn't just clobber countries with the wealth and technological resources to try to cope. It would affect every part of the planet, and in so doing reduce the resiliency of the global community. With every nation dealing with local emergencies, it would be more difficult to mobilize resources to aid victims in other areas, and there would be fewer resources to mobilize.

Municipalities around the world would struggle under the burden of greatly increased demands on funds to maintain and repair basic infrastructure. Forget about safety nets--FEMA and its ilk would be bankrupt. In the world's tightly coupled markets, financial tsunamis would surge through the system, leaving banks and corporations insolvent. Financial panics, largely absent for more than 70 years, would return with a vengeance.

Here at home, a flickering climate would impose an enormous tax on every individual and business. Property values in most places would plummet as buyers disappeared and costs of insurance and maintenance soared. The upper-middle-class American family, today so well protected against external shocks, would find its layers of insulation gradually stripped away as fuel, food, jobs, and social order became less certain. Katrina's aftermath exposed how quickly extreme weather can reduce an orderly society to dysfunction.

Some of the calamities that may happen--droughts that last more than a century, an advance of arctic zones southward, incessant and epic storms--simply overwhelm the imagination when we try to envision them in a world of six billion people depending on an exquisitely balanced food system. Earlier civilizations destroyed by climate did not have modern technologies or markets as a bulwark against nature's stresses. But changing climate won't challenge only markets and economies; it will stress the environment too, and by decimating ecosystems, we have undermined crucial buffers against weather extremes.