Interpreting Differential Temperature Trends at the

Estimated global-scale temperature trends at EarthÕs surface (as recorded by thermometers) and in the lower troposphere (as monitored by satellites) di-verge by up to 0.14¡C per decade over the period 1979 to 1998. Accounting for differences in the spatial coverage of satellite and surface measurements re-duces this differential, but still leaves a statistically signiÞcant residual of roughly 0.1¡C per decade. Natural internal climate variability alone, as simu-lated in three state-of-the-art coupled atmosphere-ocean models, cannot com-pletely explain this residual trend difference. A model forced by a combination of anthropogenic factors and volcanic aerosols yields surface-troposphere tem-perature trend differences closest to those observed. Over the past century, atmospheric tempera-tures within a few meters of Earth’s surface have been measured at thousands of locations worldwide (1). The record of temperature changes in the free atmosphere is consider