Potential impacts of projected climate change on safe carrying capacities for extensive grazing lands of northern Australia

Abstract

Climate change has the potential to significantly affect the productivity of the extensive grazing lands of northern Australia. One area that may be impacted is the estimated safe carrying capacity of grazed pastures. This is being examined within the Northern Grazing Systems program being coordinated by Meat and Livestock Australia. Here we report the potential impact of different climate scenarios on the safe carrying capacities of grazing lands using the GRASP pasture simulation model. GRASP has previously been used to investigate many aspects of grazing land management, from estimating 'safe' carrying capacities through to impacts of projected climate change. The model has been adapted to more realistically represent the process of pasture degradation. Additionally, growth parameters were modified to represent the effects of elevated CO 2. These changes are described. The updated model has been used to estimate impacts of projected climate change on pasture condition, soil loss and animal production for a range of land types and locations across northern Australia. Initial simulations were undertaken for the Maranoa and Fitzroy regions of Queensland and the Victoria River District of the northern Territory. The working definition employed for a 'safe' carrying capacity is the fixed stocking rate that would maintain good pasture condition (at least 70% perennials average) over a 30-50 year period. The simulation runs are based on current climate and two possible 'future climates' selected from 392 files per region provided by the Queensland Climate Change Centre of Excellence. Mean values of temperatures and rainfall for all files were compared with the current climate file, and ranked in terms of change in temperature and change in rainfall. The selected files represented (1) 3°C increase with decile 9 rainfall; (2) 3°C increase with decile 1 rainfall. These represented upper and lower limits of the degree of change in rainfall at a temperature increase that is plausible by 2050. For nitrogen-limited land types, estimated safe carrying capacities actually improved or experienced no change for the two future climates. By contrast, the safe carrying capacities for some other land types decreased by more than 50%, especially under the 3°C increase in temperature with decile 1 rainfall. Pasture production responses tended to be proportionally greater than the projected changes in rainfall, as were carrying capacities but to an even greater extent. Some reasons for the variable responses and implications for the extensive grazing industries are discussed.