The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) in 2001 reported that there has been an increase of about 0.6C in the global-mean temperature over the last century. The IPCC Assessment concluded that most of the observed warming over the last 50 years was likely to have been due to anthropogenic factors; increasing greenhouse gases and sulphate aerosols in the atmosphere. All studies that have successfully detected an anthropogenic influence on recent climate change have considered global scale variations. It is harder to identify the effects of anthropogenic climate change on regional scales than on global scales because of the greater magnitude of the internal climate variability relative to forced climate change signals.

The results from a regional climate change detection and attribution study over Australia will be described. Observed climate variability and change in the Australian region will be compared with climate model simulations of natural and anthropogenically-forced climate variations over the 20th century. A number of simple indices of climate variability and change are defined, including the area-mean temperature over land, the land-ocean temperature contrast, and the mean diurnal cycle and annual cycle of temperature over land. These indices are all expected to show a common response to increasing greenhouse gases but are reasonably independent for natural climate variations.

The simulated interannual variability and correlation structure of the indices, averaged over the Australian region, compare reasonably well with the observed indices. The observed trends over the last 50 years in all the indices over Australia are consistent with simulated trends in model experiments that include increasing greenhouse gas and sulphate aerosol concentrations. The observed mean warming over the last 50 years is not consistent with internal climate variability. Hence, it is likely that a significant fraction of the observed warming in the Australian region over the last 50 years is due to human activity.