A global deal to combat climate change lurches toward reality in Lima, Peru, this week—and yet any politically feasible agreement will be insufficient to restrain continued warming of global average temperatures, perhaps uncomfortably high. Although recent pledges by China, the 28 countries of the European Union and the U.S. are the first signs of the possibility of restraining the endless growth of greenhouse gas pollution on a long-term basis, atmospheric concentrations of carbon dioxide have crossed the threshold of 400 parts per million—and will reach 450 ppm in less than two decades at present growth rates. The estimated one trillion metric tons of carbon the atmosphere can absorb could be burned through in even less time, particularly if India, as it develops, picks up where China leaves off by burning coal without any attempt to capture the CO2 before the greenhouse gas spews from smokestacks.

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