Climate-change scenarios for Australia and New Zealand

Current global climate models (GCMs) do not provide reliable detailed regional predictions of climate change. However, with a careful approach, GCM results cart be used to develop scenarios of regional climate change. Such scenarios ave descriptions of a range of plausible, but not all possible, future climates for a region which can be used for guidance in impact studies. This paper describes our approach to developing climate-change scenarios for Australia and New Zealand, and the caveats and uncertainties inevitably attached to them. The scenarios presented for mean precipitation and temperature change over Australia and New Zealand allow for three important sources of uncertainty: the range of greenhouse-gas emission scenarios considered by the Intergovernmental Panel on Climate Change as plausible, the range of uncertainty regarding the sensitivity of the global climate to increased greenhouse forcing, and differences between GCMs in the simulated regional climate change per degree Celsius of global warming. GCM results were used directly for preparing the Australian region scenarios, but for the smaller and very mountainous New Zealand, a statistical spatial interpolation technique was also used. For Australia in 2030, the warming scenarios are 0.5-2.5 degrees C for inland areas, 0.1-5 degrees C for northern coastal areas and 0.5-2.0 degrees C for southern coastal areas. Scenarios for New Zealand are warmings of 0.5-2.0 degrees C in inland Canterbury and Otago and 0.5-1.5 degrees C elsewhere. Projected changes in summer precipitation for any location within Australia range from little change to as much as a 20% increase by 2030. Winter precipitation changes may be as much as 20% by 2030, but the direction of change depends on the region. Regardless of season, precipitation increases (of zero up to around 20% by 2030) predominate in the New Zealand scenarios except around Wellington, the east coast of the North Island, South Canterbury and Otago. According to these scenarios, changes in 2070 are about twice the magnitude of the 2030 changes. GCM results and other information are used also to provide some guidance on how other aspects of climate such as diurnal temperature range, humidity sea level, and tropical cyclones may change. Significantly, there is considerable evidence pointing to a future increase in rainfall intensity and in the frequency of heavy rainfall events. Although quantification of this is difficult, the GCM's results suggest scenarios of up to a halving of-the return period of heavy rainfall events by 2030 and up to a fourfold reduction by 2070.