Correlations between patterns of 19th and 20th century surface temperaturechange and HadCM2 climate model ensembles

We examine the similarity between patterns of changes in decadally-averaged surface air temperature simulated by ensemble experiments with the HadCM2 coupled climate model and observed patterns over the period 1860-1999, usin g a spatial pattern correlation method. The analysis is conducted for two a nthropogenic (greenhouse gases with/without sulphate aerosols) and two natu ral (solar and volcanic) modelled forcing histories. For each scenario an e nsemble of at least four model runs is used. We compare transient model sig nal and observed patterns at corresponding times, and establish significanc e using a 1000 year model control run to construct sampling distributions f or random noise patterns. We find evidence of an anthropogenic signal in ob served surface temperature patterns emerging over recent decades, particula rly in northern winter and spring seasons. Signals from greenhouse gas forc ing alone or in combination with sulphate aerosols are both plausible in ou r analysis, but the highest significance is achieved using seasonally-defin ed greenhouse gas plus aerosol signals. There is little evidence of any det ectable solar signal, but we do find two periods (late 19th century; 1960s- 90s) during which a weak volcanic signal is detectable. Using ensemble simu lations makes the conclusions quite robust, but the somewhat different inte rpretations from this analysis compared to optimal fingerprint analysis of the same simulations reveals some sensitivity to the choice of methodology.