Professor Dev Niyogi of Purdue University has alerted me to a new paper (subscription required) by B. Timbal and J. M. Arblaster in GEOPHYSICAL RESEARCH LETTERS entitled “Land cover change as an additional forcing to explain the rainfall decline in the south west of Australia” which further supports the need to include land use / land cover change in any assessment of the human forcing of the climate system (SWA is “southwest Australia”).
The abstract reads,
“A fully coupled climate model forced with natural and anthropogenic atmospheric forcings is used to investigate the rainfall decline in the south west of Australia. Results are compared between experiments with two different land covers. We found that vegetation cover affects modelled rainfall in the region and enhances the model response to anthropogenic atmospheric forcings, which were found in a previous study to explain part of the observed rainfall decline. This result is observed directly in model rainfall and using a statistical downscaling technique which relates local rainfall to mean sea level pressure and large-scale rainfall. While the rainfall response to anthropogenic forcings is driven mostly by the changes in pressure, the land cover influences directly the modelled rainfall (large-scale and total) and thus indirectly the downscaled rainfall. ”
The paper cautions that further study is needed,
“On the balance of evidence, the rainfall decline in SWA that is attributable to anthropogenic atmospheric forcing….. is likely to have been enhanced by the large-scale land clearance which occurred in SWA over the 20th century. However the magnitude of the enhancement must be treated with caution. Firstly it is a model response from a single simulation and secondly the effective land clearance in our simulation is about 4 times what was observed. Despite these legitimate uncertainties, the credibility of this result is reinforced by its consistency with a previous study (PI04). This should encourage further studies with a more realistic land clearance (magnitude and timing).”
The PI04 study is the 2004 Journal of Geophysical Research paper Pitman, A. J., G. T. Narisma, R. A. Pielke Sr., and N. J. Holbrook, Impact of land cover change on the climate of southwest Western Australia.
If this first order land use/land cover climate forcing is not included in the new IPCC assessment, it will be a serious error of omission.