Quotes of the Day

The language of science, like that of the United Nations, is by nature cautious and measured. That makes the dire tone of the just-released final report from the fourth assessment of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), a network of thousands of international scientists, all the more striking. Global warming is "unequivocal." Climate change will bring "abrupt and irreversible changes." The report, a synthesis for politicians culled from three other IPCC panels convened throughout the year, read like what it is: a final warning to humanity. "Today the world's scientists have spoken clearly, and with one voice," said U.N. Secretary-General Ban Ki Moon, who attended the publication of the report in Valencia, Spain. Climate change "is the defining challenge of our age."

The work of the IPCC, which was co-awarded the Nobel Peace Prize last month with Al Gore, underscores just how momentous that challenge will be. The report predicted that at a warming trend of 3.6 degrees Farenheit — now considered almost unavoidable, due to the greenhouse gases already emitted into the atmosphere — could put up to 30% of species on the planet at risk for extinction. A warming trend of 3 degrees would puts millions of human beings at risk from flooding, wetlands would be lost and there would be a massive die-off of sea corals. Sea levels would rise by 28 to 43 cm, and most frightening of all, the report acknowledged the possibility that the melting of the Greenland ice sheet, which would release enough fresh water to swamp coastal cities, could occur over centuries, rather than millennia. "If you add to this the melting of some of the ice bodies on Earth, this gives a picture of the kinds of issues we are likely to face," said Rajendra Pachauri, the IPCC's chairman.

As if the potential consequences of climate change weren't scary enough, the IPCC emphasized just how little time we have left to try to change the future. The panel reported that the world would have to reverse the rapid growth of greenhouse gases by 2015 to avert the worst consequences. The clock was running. "What we will do in the next two, three years will determine our future," said Pachauri. "This is the defining challenge."

That puts the pressure on the world's leaders to finally do something about global warming. They'll have their last, best chance next month, when energy ministers from around the world travel to Bali, Indonesia, for the annual meeting of the U.N.'s Framework on Climate Convention. There policymakers will begin attempting to negotiate a successor to the Kyoto Protocol, which expires in 2012. "The breakthrough needed in Bali is for a comprehensive climate deal that all nations can embrace," said Ban.

All the nations in the world will play a role in those negotiations, but their success and failure will come down to two countries: the U.S. and China. If the world's two biggest carbon emitters can agree to cap their greenhouse gas emissions — neither signed on for limits under Kyoto — we may stand a chance of averting the grimmest consequences of climate change. If they fail, then the IPCC has already written our future. We'll find out in Bali.

• Bryan Walsh