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Summary
Last week's fifth assessment from the Intergovernmental Panel on Climate Change (IPCC) might appear to suggest that, in spite of decades of intensive study, scientists haven't made one whit of progress on the biggest question in climate: By how much will a doubling of atmospheric carbon dioxide (CO2) levels warm the world? But in fact, climate scientists have greatly tightened constraints on their traditional estimate of how sensitive climate may be to added greenhouse gases. And they are advancing a new way of gauging how warm it may get by century's end that is easier to estimate and of more use to policymakers.
