Credit: Mauricio Alejo

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Rivers fed by melting snow and glaciers supply water to over one-sixth of the world's population--well over a billion people. But these sources of water are quickly disappearing: the Himalayan glaciers that feed rivers in India, China, and other Asian countries could be gone in 25 years (after this story appeared in print this claim was retracted by scientists: see correction). Such effects of climate change no longer surprise scientists. But the speed at which they're happening does. "The earth appears to be changing faster than the climate models predicted," says ­Daniel Schrag, a professor of earth and planetary sciences at Harvard University, who advises President Obama on climate issues.

Atmospheric levels of carbon dioxide have already climbed to 385 parts per million, well over the 350 parts per million that many scientists say is the upper limit for a relatively stable climate. And despite government-led efforts to limit carbon emissions in many countries, annual emissions from fossil-fuel combustion are going up, not down: over the last two decades, they have increased 41 percent. In the last 10 years, the concentration of carbon dioxide in the atmosphere has increased by nearly two parts per million every year. At this rate, they'll be twice preindustrial levels by the end of the century. Meanwhile, researchers are growing convinced that the climate might be more sensitive to greenhouse gases at this level than once thought. "The likelihood that we're going to avoid serious damage seems quite low," says Schrag. "The best we're going to do is probably not going to be good enough."