The last time world leaders talked about halting global warming, in Kyoto in 1997, they lacked a consensus. The U.S. Senate had spurned the talks by a vote of 95–0, eliminating any chance that the United States, then the world's biggest emitter of carbon dioxide, would take a leadership role. And China, soon to become the biggest emitter, was exempt from having to make painful cuts. As we move toward new talks in December in Copenhagen, the key players seem to be engaged for the first time. In the United States, the Waxman—Markey bill, which aims to aggressively cap and reduce greenhouse-gas emissions, has passed the House and is awaiting action in the Senate. The big worry now is that the planet may not adhere to the diplomatic timetable.
When it comes to climate, what counts is not only what humans do to reduce the buildup of greenhouse gases, but also how the earth responds. Currently half the carbon we release into the atmosphere gets absorbed by land and sea—much of it by plants, which take in carbon dioxide in the process of photosynthesis. This cycle has the potential to change at any time, and the consequences could dwarf any measures agreed to at Copenhagen to halt the temperature rise. At issue is the balance between two natural phenomena. One is beneficial: as carbon-dioxide levels in the air rise, plants grow more quickly, absorbing more carbon in return. Scientists can measure this in the lab, but they don't know how much more fertile the new, carbon-enhanced environment will be for plants. The other is "a monster in the dark," says Stephen Pacala, an environmental scientist at Princeton. As temperatures rise, permafrost, which holds an enormous amount of carbon from long-dead plants, tends to dry out, allowing decay and a release of carbon into the atmosphere. If this phenomenon, called "outgassing," were to kick in, it could inundate the atmosphere with carbon dioxide, perhaps doubling or tripling the effect of the past century of human industry.
Outgassing is one of the "dangerous anthropogenic warming" effects that the Copenhagen summit is trying to head off. Nobody knows for sure what might trigger it, but preventing a global temperature increase of more than 2 degrees Celsius is considered essential. To stay below that limit, the consensus is that we should establish a maximum level of carbon in the atmosphere and do whatever is necessary to stay below it. A few years ago, scientists thought that a doubling of carbon concentrations over preindustrial times, to 550 parts per million, was a reasonable line in the sand; in recent years they've revised that figure downward, to 450 ppm, which is what Copenhagen (and the Waxman-Markey bill) aim for. But reaching that would require a drastic 80 percent cut in emissions by midcentury. And a minority of scientists, led by NASA's outspoken expert, James Hansen, say even that's not enough: they think the concentration limit is 350 ppm—and we're already at 387 ppm. Meanwhile, observations, though not conclusive, have been pointing in the wrong direction: temperatures are rising quickly at the poles, the north polar ice cap is in retreat, permafrost is showing troubling signs of change, and ocean currents may be weakening the uptake of carbon. As politicians negotiate and the rest of us feel good about driving hybrids and using fluorescent bulbs, our fate may be riding on an obscure contest between plants and permafrost.