Causes of twentieth-century temperature change near the Earth's surface

Observations of the Earth's near-surface temperature show a global-mean temperature increase of approximately 0.6 K since 1900 (ref. 1), occurring from 1910 to 1940 and from 1970 to the present. The temperature change over the past 30-50 years is unlikely to be entirely due to internal climate variability(2-4) and has been attributed to changes in the concentrations of greenhouse gases and sulphate aerosols(5) due to human activity. Attribution of the warming early in the century has proved more elusive. Here we present a quantification of the possible contributions throughout the century from the four components most likely to be responsible for the large-scale temperature changes, of which two vary naturally (solar irradiance and stratospheric volcanic aerosols) and two have changed decisively due to anthropogenic influence (greenhouse gases and sulphate aerosols). The patterns of time/space changes in near-surface temperature due to the separate forcing components are simulated with a coupled atmosphere-ocean general circulation model, and a linear combination of these is fitted to observations. Thus our analysis is insensitive to errors in the simulated amplitude of these responses. We find that solar forcing may have contributed to the temperature changes early in the century, but anthropogenic causes combined with natural variability would also present a possible explanation. For the warming from 1946 to 1996 regardless of any possible amplification of solar or volcanic influence, we exclude purely natural forcing, and attribute it largely to the anthropogenic components.