Probabilistic predictions of climate change for Australia and southern Africa using the reliability ensemble average of IPCC CMIP3 model simulations

Abstract

“Reliability ensemble averaging” (REA) is applied to a multimodel ensemble of coupled atmosphere‐ocean general circulation models (AOGCMs) from the third phase of the Coupled Model Intercomparison Project (CMIP3) to produce mean and probabilistic climate change projections for Australia and southern Africa. CMIP3 was conducted in preparation for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC). REA produces a weighted average of the ensemble of climate change results, based on a measure of each model's bias and each model's degree of convergence in the predicted climate change. The methodology allows an assessment of the reliability of the projected climate change, the quantification of model uncertainty range, and the production of climate change projections in probabilistic form. Temperature and precipitation changes for three emission scenarios (B1, A1B, and A2) are analyzed, with a special focus on the A2 scenario. Regional differences in temperature and rainfall changes are identified, and threshold probabilities and probability density functions (PDFs) for key subregions are produced. In summer for temperature the A2 scenario results show a narrow PDF over southwestern Australia, reflecting agreement between models, partially caused by down weighting of outlier models due to their large biases. In contrast, the PDF for southeastern Australia is much broader, and that for tropical Australia has a bimodal character. There are significant decreases in rainfall under the A2 scenario in winter in the southwest (25–30% decrease) (common in sign to all AOGCM simulations) and the southeast (15–25% decrease) of Australia. There are no changes above natural variability in summer, hence no significant changes in the magnitude of Australian monsoon rainfall. Over southern Africa the region of maximum warming is over the Kalahari Desert in summer and winter. Under the A2 scenario in summer, there is a significant increase in rainfall over Kenya and Tanzania and a significant drying (∼10–20%) over parts of Namibia and Botswana. In winter, most of southern Africa south of 15°S shows significant decreases in rainfall.