Australian land cover has been significantly altered since European settlement, primarily for agricultural utilisation, with native vegetation widely replaced or modified for cropping and intensive animal production. While there have been numerous investigations into the regional and near surface climate impacts of Australian land cover change, these investigations have not included the climate impacts of larger-scale changes in atmospheric circulation and their associated feedbacks, or the impacts of longer-term soil moisture feedbacks. In this research the CSIRO General Circulation Model (GCM) was used to investigate the climate impacts of Australian land cover change, with larger-scale and longer-term feedbacks included.
To avoid the common problem of overstating the magnitude and spatial extent of changes in land surface conditions prescribed in GCM land cover change experiments, the current Australian land surface
properties were described from finer-scale, satellite derived land cover datasets, with land surface conditions extrapolating from remnant native vegetation to pre-clearing extents to recreate the pre-clearing land surface properties. Aggregation rules were applied to the fine-scale data to generate the land surface parameters of the GCM, ensuring the equivalent sub-grid heterogeneity and land surface biogeophysics were captured in both the current and pre-clearing land surface parameters.
The differences in climate simulated in the pre-clearing and current experiments were analysed for changes in Australian continental and regional climate to assess the modelled climate impacts of Australian land cover change. The changes in modelled climate were compared to observed changes in Australian precipitation over the last 50 and 100 years to assess whether modelled results could be detected in the historical record. The differences in climate simulation also were
analysed at the global scale to assess the impacts of local changes on larger scale circulation and climate at distance from the land cover changes.
The Australian continental and regional analyses demonstrated that Australian land cover change did have statistically significant impacts on air temperature and precipitation simulated in the CSIRO GCM. The statistically significant DJF warming and drying modelled over south east Queensland, with causal links back to historical land cover change in the region, corresponded with strong drying trends over the last 50 years for the region. As this region and the areas to the north and west, continue to be actively cleared, this has significant implications for land use management planning in Queensland.
The statistically significant increase in JJA precipitation modelled over south west Western Australia was opposite to the observed drying trends identified from 100 and 50 year analyses of
observed precipitation. This result was significant for the region, as it demonstrated that the increased JJA latent heat fluxes over agricultural land had the potential to increase cloud cover and precipitation. This finding supported field studies and satellite observations over the region that showed winter latent heat fluxes were higher over agricultural land than over adjacent native vegetation, and preferentially formed cumulus clouds with higher water content over the agricultural land. The modelling results therefore suggest the strong drying trend over south west Western Australia has been in response to other climatic forcing, rather than from historical land cover change.
The global analysis identified global scale changes in atmospheric circulation responding to the changes in circulation around Australia. A change in the DJF Australian monsoon flow appeared to influence the wind flow across the Indian and Pacific oceans, with impacts on air
temperature and precipitation in Asia, Europe and North America, as well as in Australia. The changes in cloud cover, soil moisture and snow cover over these areas resulted in larger changes in surface fluxes than occurred over the regions of Australian land cover change. A northward shift in JJA mid latitude westerly wind flow around Australia, had impacts over Australia and further to the north, resulting in a northward shift and increased flow in the JJA Asian monsoon. The increased Asian monsoon flow impacted the wind flow and circulation over the Indian and Pacific oceans, with impacts on air temperature, and precipitation over Asia, Europe and North America. Again the changes in cloud cover and soil moisture over these areas resulted in larger changes in surface fluxes than occurred over the regions of Australia land cover change.