MONTE-CARLO CLIMATE-CHANGE FORECASTS WITH A GLOBAL COUPLED OCEAN-ATMOSPHERE MODEL

Four time-dependent greenhouse warming experiments were performed with the same global coupled atmosphere-ocean model, but with each simulat ion using initial conditions from different ''snapshots'' of the contr ol run climate. The radiative forcing - the increase in equivalent CO2 concentrations from 19852035 specified in the Intergovernmental Panel on Climate Change (IPCC) scenario A - was identical in all four 50-ye ar integrations. This approach to climate change experiments is called the Monte Carlo technique and is analogous to a similar experimental set-up used in the field of extended range weather forecasting. Despit e the limitation of a very small sample size, this approach enables th e estimation of both a mean response and the ''between-experiment'' va riability, information which is not available from a single integratio n. The use of multiple realizations provides insights into the stabili ty of the response, both spatially, seasonally and in terms of differe nt climate variables. The results indicate that the time evolution of the global mean warming signal is strongly dependent on the initial st ate of the climate system. While the individual members of the ensembl e show considerable variation in the pattern and amplitude of near-sur face temperature change after 50 years, the ensemble mean climate chan ge pattern closely resembles that obtained in a 100-year integration p erformed with the same model. In global mean terms, the climate change signals for near surface temperature, the hydrological. cycle and sea level significantly exceed the variability among the members of the e nsemble. Due to the high internal variability of the modelled climate system, the estimated detection time of the global mean temperature ch ange signal is uncertain by at least one decade. While the ensemble me an surface temperature and sea level fields show regionally significan t responses to greenhouse-gas forcing, it is not possible to identify a significant response in the precipitation and soil moisture fields, variables which are spatially noisy and characterized by large variabi lity between the individual integrations.