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“Human influence extremely likely to be the dominant cause of observed warming since the middle of the last century,” was the headline from Friday’s release of the first instalment of the Intergovernmental Panel on Climate Change’s fifth assessment report. “Extremely likely”—indicating a 95%-100% likelihood—was ratcheted up one notch from the 2007 fourth assessment report’s “very likely.” Yet compared to 2007, the IPCC widened its estimate of the responsiveness of the climate system to carbon dioxide by reducing the lower band to a 1.5°C increase from 2°C, qualifying the new estimate as only “likely.”
