claim: "a paper...in Science magazine concludes that the climate sensitivity—how much the earth’s average temperature will rise as a result of a doubling of the atmospheric concentration of carbon dioxide—likely (that is, with a 66% probability) lies in the range 1.7°C to 2.6°C, with a median value of 2.3°C.
evidence: New approaches retrieve data such as CO 2 content of past atmospheres from fossil leaf stomata and isotope composition, measuring cellular CO2 concentrations.