What was done
The authors 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 centroid 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 ha in 1899 to 1,250,000 ha by 1982. Hence, the four researchers sought to determine if the historical development of irrigated agriculture in the San Joaquin Valley had had a significant effect on its surface air temperature.

What was learned
When all was said and done, Christy et al. determined that "the central San Joaquin Valley has experienced a significant rise of minimum temperatures (~3°C in June, July, August and September, October, November), a rise that is not detectable in the adjacent Sierra Nevada."

What it means
The climate scientists suggest that the large valley warming they detected 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 broadly developed over the same time span.