The latest report from the United Nations Intergovernmental Panel on Climate Change, issued in April, suggested several ways to allot responsibilities. If one starts counting in the 18th century and counts only emissions from industry and energy generation, the United States is responsible for more than a quarter of all greenhouse gases that humanity has put into the air. China, by contrast, is responsible for 10 percent.But if one starts counting in 1990, when the world first became aware that CO2 was a problem, and includes greenhouse gases emitted from changes in land use, the United States is responsible for only 18 percent, and China’s share rises to 15 percent. Rich and poor countries, unsurprisingly, disagree on the proper measure.
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