The use of uncorrected regional climate model output to force impact models: A case study for wheat simulations

Computationally-expensive regional climate models (RCM) are increasingly being used to generate local climate data for climate change impact studies. These studies usually process RCM output to remove errors in the simulated climate. However, this paper investigates the suitability of raw output from simulations of a single RCM as input to a biophysical impact model. Our study analyses errors in wheat yields simulated for New South Wales (NSW), Australia by the Agricultural Production Systems Simulator (APSIM) model forced with output from 2 RCM simulations with horizontal resolutions of approximately 50 and 10 km over NSW and with output from the global climate model (GCM) simulation that they downscale. Overall, across the NSW wheat belt, the ∼50 km simulation has a better simulation of mean yields for the 1990-2010 period than the GCM simulation, and the ∼10 km simulation has a better simulation than the ∼50 km simulation. The average mean yield from APSIM simulations forced with observations is 3.5 t ha-1. The average magnitudes of errors in mean yields for the GCM, ∼50 km and ∼10 km simulations are 1.2, 1.0 and 0.5 t ha-1 respectively. We suggest that the improvement in the simulation of mean yields with increasing climate model resolution is largely due to an improvement in the simulation of mean rainfall totals for the growing season. However, for a given value of mean growing season total rainfall, all 3 climate model simulations have a climate that is more conducive to high yields than the observed climate. This difference must be due to errors in other aspects of the simulated climates

Climate projections for southern Australian cool-season rainfall: insights from a downscaling comparison

The projected drying of the extra-tropics under a warmer climate has large implications for natural systems and water security in southern Australia. The downscaling of global climate models can provide insight into regional patterns of rainfall change in the mid-latitudes in the typically wetter cool season. The comparison of statistical and dynamical downscaling model outputs reveals regions of consistent potential added value in the climate-change signal over the 21st century that are largely related to finer resolution. These differences include a stronger and more regionalised rainfall decrease on west coasts in response to a shift in westerly circulation and a different response further from the coast where other influences are important. These patterns have a plausible relationship with topography and regional drivers that are not resolved by coarse global models. However, the comparison of statistical and dynamical downscaling reveals where the method and the configuration of each method makes a difference to the projection. This is an important source of uncertainty for regional rainfall projections. In particular, the simulated change in atmospheric circulation over the century is different in the dynamical downscaling compared to the global climate model inputs, related in part to a different response to patterns of surface warming. The dynamical downscaling places the border between regions with rainfall increase and decrease further north in winter and spring compared to the global climate models and therefore has a different rainfall projection for southeast mainland Australia in winter and for Tasmania in spring

Climate Change Impact on Water and Salt Balances: An Assessment of the Impact of Climate Change on Catchment Salt and Water Balances in the Murray-Darling Basin, Australia

Climate change has potentially significant implications for hydrology and the quantity and quality of water resources. This study investigated the impacts of climate change and revegetation on water and salt balance, and stream salt concentration for catchments within the Murray-Darling Basin, Australia. The Biophysical Capacity to Change model was used with climate change scenarios obtained using the CSIRO DARLAM 125 (125 km resolution) and Cubic Conformal (50 km resolution) regional climate models. These models predicted up to 25% reduction in mean annual rainfall and a similar magnitude of increase in potential evapotranspiration by 2070. Relatively modest changes in rainfall and temperature can lead to significant reductions in mean annual runoff and salt yield and increases in stream salt concentrations within the Basin. The modelled reductions in mean annual runoff were up to 45% in the wetter/cooler southern catchments and up to 64% in the drier/hotter western and northern catchments. The maximum reductions in salt yield were estimated to be up to 34% in the southern catchments and up to 49% in the northern and western catchments. These changes are associated with average catchment rainfall decreases of 13 to 21%. The results suggest that percentage changes in rainfall will be amplified in runoff. This study demonstrates that climate change poses significant challenges to natural resource management in Australia

Regional climate changes as simulated in time-slice experiments

Three 30 year long simulations have been performed with a T42 atmosphere model, in which the sea-surface temperature (SST) and sea-ice distribution have been taken from a transient climate change experiment with a T21 global coupled ocean-atmosphere model. In this so-called time-slice experiment, the SST values (and the greenhouse gas concentration) were taken at present time CO2 level, at the time of CO2 doubling and tripling. The annual cycle of temperature and precipitation has been studied over the IPCC regions and has been compared with observations. Additionally the combination of temperature and precipitation change has been analysed. Further parameters investigated include the difference between daily minimum and maximum temperature, the rainfall intensity and the length of droughts. While the regional simulation of the annual cycle of the near surface temperature is quite realistic with deviations rarely exceeding 3 K, the precipitation is reproduced to a much smaller degree of accuracy. The changes in temperature at the time of CO2 doubling amount to only 30-40 of those at the 3 * CO2 level and show hardly any seasonal variation, contrary to the 3 * CO2 experiment. The comparatively small response to the CO2 doubling can be attributed to the cold-start of the simulation, from which the SST has been extracted. The strong change in the seasonality cannot be explained by internal fluctuations and cold start alone, but has to be caused by feedback mechanisms. Due to the delay in warming caused by the transient experiment, from which the SST has been derived, the 3 * CO2 experiment can be compared to the CO2 doubling studies performed with mixed-layer models. The precipitation change does not display a clear signal. However, an increase of the rain intensity and of longer dry periods is simulated in many regions of the globe. The changes in these parameters as well as the combination of temperature- and precipitation change and the changes in the daily temperature range give valuable hints, in which regions observational studies should be intensified and under which aspects the observational data should be evaluated. © 1995 Kluwer Academic Publishers