Background
A reduction of rainfall in southwest Western Australia in the middle of the past century reduced inflows to the Perth water supply by about 42%; and, according to Pitman et al., some people have concluded that "the decline in rainfall in this region was associated with a change in the large-scale atmospheric circulation, and that the warming in this region was associated with the enhanced greenhouse effect (IOCI, 2002)."

What was done
Exploring an alternative hypothesis that focuses on land cover change as the cause of the decline in rainfall and associated warming, the authors of this newer study used three high-resolution mesoscale models to simulate five different July climates for both natural and current land cover conditions.

What was learned
Pitman et al. found that the calculated changes in precipitation caused by the historical land cover change were similar to the observed precipitation changes in both magnitude and pattern.  They then reasoned that since the simulated precipitation changes were consistent for each of the three models for each of the five July climatic conditions studied, "it is extremely unlikely that this simulated pattern matches the observed pattern by coincidence."  Analogously, they concluded that "since we did not vary sea surface temperatures, carbon dioxide or the lateral boundary conditions between the simulations using current and natural land cover, the changes we simulate cannot be explained in these terms."

With respect to what did matter, they report that "a change in just the roughness length and the zero plane displacement height [resulting from the removal of forest trees] could explain the rainfall changes."  Specifically, they say that the "current land cover reduces frictional drag and leads to increased horizontal winds that advect moisture being carried onshore from the Indian Ocean further inland," where strong convergence leads to increased vertical velocities and "the combination of more moisture and increased vertical velocities increases precipitation," which is exactly what observations show to be the case there today.

Last of all, the four scientists say they "find a pattern of warming in two of the three models, suggesting that attributing the warming in this region to the enhanced greenhouse effect is premature."

What it means
Readily understandable effects of land cover change in southwest Western Australia are capable of describing temperature and precipitation changes experienced there in recent decades much better than do general circulation models that simulate significant CO2-induced global warming.

Reference
Indian Ocean Climate Initiative (IOCI).  2002.  Climate Variability and Change in Southwest Western Australia.  Indian Ocean Climate Initiative Panel.  Perth, Western Australia.