In a contemporary study of the rainfall reduction experienced in southwest Western Australia in the middle of the past century, which reduced inflows to the Perth water supply by about 42%, Pitman et al. (2004) reported that certain people had 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)." Exploring an alternative hypothesis that invoked land cover change as the cause of the warming and the associated decline in rainfall, the four researchers used three high-resolution mesoscale models to simulate five different July climates for both natural and current land cover conditions. In doing so, they found that calculated changes in precipitation caused by historical land cover change were similar to the observed precipitation changes in both magnitude and pattern. Therefore, they reasoned that since the simulated precipitation changes were consistent for each of the three models for each of the five July climatic conditions they studied, "it is extremely unlikely that this simulated pattern matches the observed pattern by coincidence." Analogously, they concluded that since they did not vary sea surface temperatures, carbon dioxide, or the lateral boundary conditions between their simulations using current and natural land cover, the changes they simulated "could not be explained in these terms."

With respect to what did matter, Pitman et al. found 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," as they determined 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 showed to be the case there at the time of their study. In addition, they discovered "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." Consequently, it would appear that 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.

Two years later, Christy et al. (2006) constructed time series of regional surface air temperature for the period 1910-2003 for two adjacent regions of central California: the irrigated San Joaquin Valley (18 stations) and the nearby non-irrigated Sierra Nevada (23 stations). This they did because, in their words, "the centroids of observations of the valley and mountain stations are separated by only 60 km horizontally and less than 1000 meters vertically," and they therefore felt that the two regions' "long-term climate trends should be very similar if no differential forcing develops." Over the period of their study, however, irrigated land in the San Joaquin Valley rose from 242,000 hectares in 1899 to 1,250,000 hectares by 1982; and the four researchers sought to determine if the historical development of irrigated agriculture in the San Joaquin Valley might have had a significant effect on its surface air temperature.

When all was said and done, Christy et al. found that "the central San Joaquin Valley [had] experienced a significant rise of minimum temperatures (~3°C in June, July, August and September, October, November), a rise that [was] not detectable in the adjacent Sierra Nevada." Therefore, they suggested that the large valley warming was "caused by the massive growth in irrigated agriculture," whereby "human engineering of the environment has changed a high-albedo desert into a darker, moister, vegetated plain, thus altering the surface energy balance in a way [they] suggest has created the results found in this study." In addition, they say that "the lack of long-term warming in the generally underdeveloped Sierra Nevada (annual mean trend, 1910-2003, -0.02° ± 0.1°C decade^-1) coupled with significant nighttime-only warming in the valley, suggests a regional inconsistency compared with twentieth-century simulations of climate forced by human influences other than land use changes."

Put another way, one could say that with no sign of any greenhouse gas-induced warming over the underdeveloped Sierra Nevada from 1910 to 2003, there is no compelling reason to believe that any of the warming experienced in the nearby San Joaquin Valley over the same period was greenhouse gas-induced, which suggests that a good portion of the global warming of the past century may well have had a significant anthropogenic -- but non-greenhouse gas-induced -- component associated with it, especially in places where irrigated agriculture was developed over the same time span.

Simultaneously, de Laat and Maurellis (2006) sought to strengthen their earlier conclusions by adding two more data sets and analyzing them with an additional statistical method, while looking more closely for additional evidence of the industrialization-warming correlation in climate model simulations of enhanced greenhouse gas-induced warming. As a result of these efforts, they "established that the correlation between observed near-surface temperature trends and CO2 emissions presented in de Laat and Maurellis (2004) occurs in a variety of data sets in a completely consistent way." They also confirmed that surface and satellite temperature measurements "display the same kind of temperature trend enhancements," and that they "are quite large and cover a sizable fraction of the globe (~10%)." In addition, the Dutch scientists confirmed "the absence of the above correlation in climate model simulations of enhanced greenhouse gas warming." Thus, they concluded that, over the last two decades, non-greenhouse gas anthropogenic processes "have contributed significantly to surface temperature changes," which finding strengthens their earlier conclusion that "observed surface temperature changes might be a result of local surface heating processes and not related to radiative greenhouse gas forcing."

One year later, LaDochy et al. (2007) introduced their study of the subject by noting that "when speculating on how global warming would impact the state [of California], climate change models and assessments often assume that the influence would be uniform (Hansen et al., 1998; Hayhoe et al., 2004; Leung et al., 2004)." Therefore, to assess the validity of this assumption, the three researchers used temperature trends in California climate records over the period 1950-2000 to assess the extent of warming in the various sub-regions of the state. Then, by looking at human-induced changes to the landscape, they attempted to evaluate the importance of those changes with respect to temperature trends, determining their significance compared to changes caused by changes in the air's composition, such as atmospheric CO2 concentration.

This work revealed, in their words, that "most regions showed a stronger increase in minimum temperatures than with mean and maximum temperatures," and that "areas of intensive urbanization showed the largest positive trends, while rural, non-agricultural regions showed the least warming." In fact, they report that northeast interior basins of the state actually experienced cooling. Large urban sites, on the other hand, exhibited rates of warming "over twice those for the state, for the mean maximum temperatures, and over five times the state's mean rate for the minimum temperature." As a result, LaDochy et al. concluded that "if we assume that global warming affects all regions of the state, then the small increases seen in rural stations can be an estimate of this general warming pattern over land," which implies that "larger increases," such as those found in areas of intensive urbanization, "must then be due to local or regional surface changes."

Returning to Australia, McAlpine et al. (2007) used an uncoupled version of a climate model with ocean and sea ice components, represented by observed seasonally-varying sea surface temperatures and sea ice data, to carry out two sets of model simulations for the period 1949-2003, where the first experiment employed pre-European land cover characteristics of Australia and the second employed modern-day land cover characteristics of the continent, as derived from corresponding land cover maps. The results indicated that "replacing the native woody vegetation with crops and grazing in southwest Western Australia and eastern Australia has resulted in significant changes in regional climate, with a shift to warmer and drier conditions, especially in southeast Australia, the nation's major agricultural region." More specifically, they report that the anthropogenic-induced land cover change resulted in "a statistically significant warming of the surface temperature, especially for summer in eastern Australia (0.4-2°C) and southwest Western Australia (0.4-0.8°C), a statistically significant decrease in summer rainfall in southeast Australia, and increased surface temperature during the 2002/2003 El Niño drought event."

Noting that these "simulated changes in Australia's regional climate suggest that land cover change is likely a contributing factor to the observed trends in surface temperature and rainfall at the regional scale," the seven scientists went on to say that "trying to attribute historical changes in mean and seasonal surface temperature and rainfall using a single radiative forcing such as greenhouse gases is simplistic and needs to be re-evaluated." Indeed, they state that their work "suggests the importance of land cover change as a contributing factor to the observed changes in Australia's regional climate." And, we might add, it has likely been a major contributing factor. In addition, this phenomenon has probably played a large role in forcing historical climate change in other parts of the world as well, with the consequence that much of modern global warming that has been attributed to anthropogenic greenhouse gas emissions may instead have been caused by this more direct influence of humanity on nature.

Two years later, but still "down under," Deo et al. (2009) compared the output data from a pair of ensemble simulations (10 members each) using a climate model (the CSIRO Mark 2 AGCM) forced with observed sea surface temperature and sea ice data for the period 1951-2003, where the only difference between the ensemble experiments was the land cover change (LCC) that occurred between pre-European (1788) and modern day (1990) conditions. In reporting their findings, Deo et al. write that "the conversion of native forests to crops and grazing pastures in eastern New South Wales and Victoria, the region with the most extensive LCC, has resulted in a significant decrease in vegetation fraction, leaf area index and surface roughness, and an increase in albedo," such that "the long-term (1951-2003) summer (DJF) and area-averaged latent heat flux decreased by 4.8 Wm^-2 while the sensible heat flux increased by 1.1 Wm^-2," leading to "a warmer land surface." In addition, they found that during strong El Niño events the changes were much larger. During the 1982/83 event, for example, they calculated that "the summer values of area-averaged sensible heat flux increased by 18.8 Wm^-2 with a compensating decrease in latent heat flux of 20.3 Wm^-2."

In light of these findings, the six scientists concluded that (1) "the conversion of native vegetation to crops and pastures has resulted in an increased fraction of available energy at the land surface used for sensible heating, which has contributed to higher average surface temperatures and more hot days," and that (2) "the increased number of hot days has contributed to a drier lower atmosphere, resulting in a decrease in regional rainfall and evapotranspiration." Clearly, these temporal changes -- which result in "an increase in the number of dry and hot days, a decrease in daily rainfall intensity and wet-day rainfall, and an increase in the decile-based drought duration index" -- should be carefully considered before attributing such real-world phenomena to the historical increase in the atmosphere's CO2 concentration.

And in light of all of the findings discussed in this Summary, we can confidently concur with the sentiment expressed by de Laat and Maurellis (2004) that a goodly portion of observed surface temperature increases over the past few decades likely are, as they expressed it, "a result of local surface heating processes and not related to radiative greenhouse gas forcing."

References
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