Regional Climate Modeling in the South West of Western Australia (SWWA) to Examine Future Changes in Rainfall and Temperature Extremes

Temperature and precipitation extremes are important factors of climate change because they have a very significant environmental impact, despite their relative infrequency. In the case of agriculture and native vegetation, changes in the frequency and intensity of extreme events may impact the viability of regions as croplands, managed forests or for conservation purposes. The south west of Western Australia (SWWA) has been identified as a hotspot, acutely at risk of climate change (Malcolm et al. 2006) however we know very little about how these changes are likely to manifest at the regional scale, in particular with respect to extremes. Forestry and agriculture are important industries in the region and understanding how they may be affected by variations in climate extremes will help to develop management strategies that will enable these industries to adapt to changing conditions.

Current research indicates that the temporal distribution of extreme events is as important as the spatial distribution, especially when considering the necessary conditions for forestry and agriculture. For example, forest mortality events are exacerbated when heatwaves and dry conditions coincide or occur consecutively (Brouwers et al. 2012). Furthermore, the growing season of cereal crops in the SWWA is at risk of being shortened by the shifting of the last frost and first heat events to earlier in the season (Zheng et al 2012). It is therefore clear that assessments of future changes in climate on agriculture and forestry need to focus on climate extremes, with a particular focus on the magnitude and timing of these events.

When evaluating the merits of regional climate simulations, one of the most compelling arguments for this high resolution, dynamical downscaling approach is its ability to simulate the extremes of temperature and precipitation with greater skill than lower resolution models. This is particularly true for precipitation, where the spatial distribution of rainfall is improved by the fine scale representation of topography, especially when the scale of the model allows for the explicit resolution of convection. Furthermore, the representation of frost, which is heavily influenced by local effects, can also be improved by a high-resolution simulation.

In this research we examine the significance of changes in extremes for the SWWA in terms of their spatiotemporal distribution over the next 45 years. To establish the impact of these changes on forestry and agriculture in the region, we use a selection of bioclimatic indices and the CCI/CLIVAR/JCOMM Expert Team on Climate Change Detection and Indices metrics to explore the future viability of forested areas and arable land. In order to achieve this, an ensemble regional climate simulation using 4 CMIP3 general circulation models and WRFV3.3 was performed over Western Australia. 30-year simulations were run historically (1970-1999) and into the future (2030-2059) using the A2 emissions scenario at a horizontal grid spacing of 50 km, 10 km and 5 km. Simulations are explicitly resolving convection at the highest resolution. The ensemble member outcomes of the future simulations are combined and then compared with the results of the historical ensemble simulations, employing the delta change method of bias correction.

References: Brouwers, N. C., Mercer, J., Lyons, T., Poot, P., Veneklaas, E., & Hardy, G. (2013). Climate and landscape drivers of tree decline in a Mediterranean ecoregion. Ecology and evolution, 3(1), 67-79. Malcolm, J. R., Liu, C., Neilson, R. P., Hansen, L., & Hannah, L. E. E. (2006). Global warming and extinctions of endemic species from biodiversity hotspots.Conservation biology, 20(2), 538-548. Zheng, B., Chenu, K., Fernanda Dreccer, M., & Chapman, S. C. (2012). Breeding for the future: what are the potential impacts of future frost and heat events on sowing and flowering time requirements for Australian bread wheat (Triticum aestivium) varieties?. Global Change Biology, 18(9), 2899-2914.