The relative contribution of historical stratospheric ozone depletion and historical increases in atmospheric CO2 concentration has been assessed using the CSIRO T63/L18 AGCM. This model experiments are part of the Climate of the 20th Century (C20C) Project, where an extensive suite of experiments where undertaken. The CSIRO T63/L18 AGCM experiment used HadISST 1.1 sea surface temperature and sea ice datasets for period 1871 to 2003, with solar variability, increasing concentration of CO2 and depletion of stratospheric ozone included according to the C20C project guidelines. The preliminary analyses of the model results showed a good agreement between observed (ERA40) and simulated mean sea level pressure (MSLP) trends for period 1971 to 2003. Both the observed and simulated data demonstrated a decline in MSLP at high latitudes within the Southern Hemisphere and corresponding increase in the Southern Hemisphere mid-latitudes. In both cases a characteristic wave number three patterns is present in trend data at mid-latitudes. The resultant analyses suggest that stratospheric ozone depletion had the strongest contribution to the trend in MSLP distribution during all months, except for winter where the CO2 effect was strongest. The results also demonstrated that stratospheric ozone depletion had contributed strongly to the positive trend in the Southern Annular Mode. The analyses of simulated rainfall from these experiments indicated a strong impact of stratospheric ozone depletion and CO2 increase on the patterns summer rainfall (NDJFM period) distribution during the past decade in Australia. The impacts of increasing CO2 and ozone depletion on rainfall anomalies appear to be of opposite sign across many regions of globe and in particular eastern Australia and South Pacific Convergence Zone, with CO2 forcing resulting in increased summer rainfall, while ozone forcing led to rainfall decrease in eastern Australia.

The observed drying trends in eastern Australia and positive rainfall anomalies in NW Australia over the past decade are best explained by the combination of SST, solar, CO2 and ozone forcing in the ensemble of model simulations. Overall the stratospheric ozone depletion has been the decisive agent in maintaining the persistent drought conditions in eastern Australia during the past decade. These conditions have resulted in widespread water use restrictions in almost all major urban areas of eastern Australia in recent years. This new findings have far reaching implications for future options for population growth in coastal eastern Australia.