Impact of land cover change on regional climate and El Nino in Australia

Impact of land cover change on regional climate and El Nino in Australia

Climate impacts of land cover change (LCC) are still a subject of discussion despite growing evidence that LCC affects global and regional climates (Zhao et al. 2001; Timbal and Arblaster 2006; Pielke et al. 2002; Narisma and Pitman 2003). To investigate the climate impacts of LCC,
this paper presents results for two sets of 10 member ensemble experiments using CSIRO atmospheric GCM.

The CSIRO climate model is a fully coupled atmosphere, land surface, sea ice and ocean model, with a model horizontal resolution of ~1.8o grid increment and 18 vertical levels. In this study, we used the uncoupled version where ocean and sea ice components were represented by observed seasonally varying sea surface temperatures and sea ice data. We quantified changes in land surface parameters and impacts on mean climate from the pre-European (1788) to
modern day (1990) land covers and impact of ENSO on the strength of surface temperature anomalies during second half of 20th century. Preclearing land surface parameters were generated by extrapolating modern-day values of remnant
native vegetation to the pre-European extents of each land cover class. The extrapolation was performed for Australian continent at 8 km spatial resolution and the fine scale parameters then aggregated using Shuttleworth’s (1991) approach to coarse resolution aggregated for model.

The largest differences between pre- European and modern day surface parameters were in eastern Queensland, southwest Western Australia, and New South Wales/Victoria. In eastern Queensland, vegetation fraction and leaf area index during the summer decreased by 14% and 20% respectively. This corresponded to a surface albedo increase of 4%. Stomatal resistance increased by 3% and surface roughness decreased by 54%. In New South Wales/Victoria, similarly large decreases in vegetation fraction (19%) and LAI (23%) caused an albedo increase of 7%, while there was a corresponding 46% reduction in surface roughness. In southwest Western Australia, replacement of native woodlands with predominantly winter grains and crops resulted in a modest decrease in vegetation fraction (5%) and LAI (2%) during inter, while stomatal resistance decreased by 15%. However, surface albedo increased by 14% due to higher reflectance of sandy soils.

Overall, the largest impact of land cover change on land surface parameters occurred in eastern Australia. Our results showed that modification of vegetation in eastern Queensland and New South Wales/Victoria contributed to increases in areaaveraged surface temperatures of 0.19°C and 0.63°C in the present-day compared to the pre- European values. In New South Wales/Victoria, area-averaged total rainfall decreased by 2.5%, while in eastern Queensland there was a rainfall decrease of 5.2%. The impact of LCC on the winter climate of southwest Western Australia was a cooling of 0.14°C, even though albedo increased. This coincided with a small increase in winter rainfall by 0.6% and soil moisture by about 12%. Our winter response of increased rainfall is at odd with Pitman et al (2004) and Timbal and Arblaster (2006) although annual average rainfall, showing a decline of 0.24%, is consistent with their results. Nevertheless, even this decline is too small to be attributable to the influence of LCC. Further studies are needed to address the exact cause of this discrepancy.

Composites of summer surface temperatures during the five strongest El Niño and La Niña episodes from 1950-2003 showed significant warming under present-day land cover conditions. However, increases in surface temperatures in eastern Australia were the highest during both episodes. On average, the strongest warming (≈ 3.6ºC) occurred during 1982/1983 El Niño in eastern Australia and southwest Western Australia, the regions of largest land cover change. The surface temperature were similar for 1997/98 and 2002/2003 El Niño years, indicating the land surface forcings act to amplify the effect of El Niño on the surface climate of Australia.

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