The proportionality of global warming to cumulative carbon emissions

To date, efforts to describe and predict the climate response to human CO2 emissions have focused on climate sensitivity: the equilibrium temperature change associated with a doubling of CO2. But recent research has suggested that this 'Charney' sensitivity, so named after the meteorologist Jule Charney who first adopted this approach in 1979, may be an incomplete representation of the full Earth system response, as it ignores changes in the carbon cycle, aerosols, land use and land cover. Matthews et al. propose a new measure, the carbon-climate response, or CCR. Using a combination of a simplified climate model, a range of simulations from a recent model intercomparison, and historical constraints, they find that — independent of the timing of emissions or the atmospheric concentration of CO2 — emitting a trillion tonnes of carbon will cause 1.0–2.1 °C of global warming, a CCR value that is consistent with model predictions for the twenty-first century.