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— You've heard of urban heat islands. Now researchers have confirmed the existence of their opposite: cool farm patches.
Whereas urban development generates pockets of hot air, irrigated fields tend to cool things down, they say - and there is evidence that the effects have been felt in California for over a century.
In areas of intensive irrigation, such as the Central Valley in California, US, these "cool farms" have counteracted global warming, say Céline Bonfils and David Lobell of the Lawrence Livermore National Laboratory in California. But they warn that a reduction in irrigation could spell the end of the relief that these regions have enjoyed.
Bonfils and Lobell compared irrigation and temperature data for California between 1915 and 2000, during which time the area of irrigated land in the Central Valley doubled. They found that maximum daytime temperatures in the area were between 0.9°C and 1.6°C cooler during this period than areas that were only modestly irrigated.
Extrapolating back to when irrigation began in 1887, they calculate that intensively irrigated parts of the Central Valley are 1.8°C to 3.2°C cooler than they would otherwise have been.
Chilly spillover
That cooling occurs because much of the solar energy that hits irrigated ground during the day goes to evaporate the extra water in the soil and in plants instead of heating the air, explains Lara Kueppers of the University of California at Santa Cruz.
Kueppers had predicted the cool farms effect from climate modelling studies (Geophysical Research Letters, DOI: 10.1029/2006GL028679). The models suggest that the cool air spreads to areas outside the irrigated fields, although the extent of this "spillover" is still unclear.
Bonfils and Lobell say the cool farms effect could explain why minimum and maximum winter temperatures steadily rose in California between 1915 and 2000, whereas maximum summer temperatures did not.
The warmer winter temperatures can only be explained by the greenhouse effect, and the authors speculate that the cool farms effect may have masked the impact of global warming on summer temperatures. Irrigation is mostly carried out during the summer.
Dwindling supplies
The cool times may not last, however. The amount of irrigation in California has stabilised since 1980, Bonfils and Lobell point out, because expanding urban areas have laid increasing claims on dwindling water supplies. In the US overall, irrigation decreased for the first time - by 2% - between 1998 and 2003.
A rollback of the cooling effect of irrigation in the face of continued global warming could mean that California will be hit by substantial warming, say the researchers. The same is likely to be true of other regions of the world. India, Pakistan and China have become huge irrigators over the past 50 years, but the growth of irrigated areas is slowing down.
This may mean that irrigated regions, which now provide about 40% of global food production, will feel more than their share of warming in the future. In turn, this will inevitably have an impact on food security.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0700144104)
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