Three-quarters of climate change is manmade

Nature : At least 74% of the temperature rise over the past 60 years is almost certainly due to human activity, according to a pair of researchers at the Swiss Federal Institute of Technology in Zürich. Reto Knutti and Markus Huber report in Nature Geoscience that greenhouse gases contributed 0.6–1.1 °C to the warming observed since the middle of the 20th century, with the most statistically likely contribution being about 0.85 °C. About half of that was offset by the cooling effect of aerosols, which influence the climate by scattering light. Changes in solar radiation accounted for about 0.07 °C of the rise in temperature. One method for assessing global warming, known as optimal fingerprinting, compares observed patterns of surface air temperature over time with the modeled climate response to greenhouse gases, solar radiation, and aerosols from volcanoes and other sources. Knutti and Huber, however, used a simpler method, which is based on Earth’s total energy budget and which entails running their model many thousands of times with different combinations of parameters. Their findings are consistent with results produced by other attribution methods, including optimal fingerprinting.