HERE'S THE BEST CASE: The U.S. breaks its political logjam and passes ambitious caps on carbon, pledging to reduce emissions quickly and dramatically. This reconfigures economic gravity, so that money now flows toward the sun and wind and insulation, not coal. That money starts producing engineering breakthroughs, lowering the price of everything from solar panels to plug-in hybrids so rapidly that the technologies spread as quickly as, say, the Wii Remote.

Meanwhile, seeing that rapid change is possible, we in the rich world rise to our obligations and embark on a kind of global Marshall Plan to deploy the same new gear all over the world, allowing China and India and everyone else to develop without burning their coal. Meanwhile, we make noble changes in our habits—the rising price of gas leads to the demand for decent mass transit, and once the bus is there we actually decide to, you know, take it. We become less American, more European, and we encourage the developing world to aspire to Oslo and not Orange County.

Oh, and the earth responds. Seeing our good intentions and rapid progress, nature backs off a little. Sea levels rise in inches, not feet. We build some levees and we evacuate some low-lying Pacific atolls. It could happen. The Intergovernmental Panel on Climate Change, that noble band of climate experts, calculated last spring that we could build all the technology we needed to hold carbon emissions down at an eminently reasonable cost; in the next 20 years it would knock about one 12th of 1 percent off gross world product annually. As The Economist, that Trotskyite rag, put it, "The world would hardly notice."

But then there's the other kind of crisis, the one that begins with the Very Concerned Doctor sitting down across from you. And now we're talking about a different kind of regimen. The changes aren't just daunting; they're damaging—technicians are X-ing targets on your body with grease pencil and your hair is falling out. A lump in one place turns into a shadow in a dozen others, and the "treatments" start seeming very nearly as bad as the problem, and your friends are scanning the Internet for news of new, novel cures.

If you were laying down odds right about now, you'd be hard-pressed not to lean toward this scenario. Late last summer, the sheets of Arctic sea ice began to thin at a markedly faster rate: There was a week when an area the size of Florida melted every single day. And the same kind of bad lab results were coming back from many different systems: tundra permafrost, temperate-forest soils, Amazon aridity. Small flare-ups caught everyone's attention: Wildfire blazed across a parched Southern California, drought dropped Lake Lanier so low that Atlanta started measuring its water supply in weeks. Worse than we'd feared, and spreading faster. There seems little doubt now that this century will be at least as tough as the scientific consensus has been warning: crop-threatening heat waves, rapid spread of mosquito-borne disease, the whole litany.

But now we've begun to fear that it may be very much worse even than we had feared. New data from Greenland and the west Antarctic suggests that those great ice sheets are becoming dangerously unstable—and with that comes the possibility of sea level rising not merely a few feet this century, but a few meters. Which takes us from civilization-challenging to civilization-threatening. Greenland alone has about 25 feet of sea-level rise locked in its mile-thick glaciers. Boot up Google Earth if you want to see what this would mean for your local coastline. No wonder serious scientists are starting to talk about, say, orbiting giant shades to cast cooling geometric shadows, or flooding the upper atmosphere with sulfur dioxide. Desperate measures …