You've heard of urban heat islands that generate pockets of hot air. Now researchers confirm the existence of their opposite: cool farm patches that tend to cool things down, reports New Scientist. These have been felt in California for more than a century, in areas of intensive irrigation, like the Central Valley, where "cool farms" have counteracted global warming. Researchers from the Lawrence Livermore National Laboratory, extrapolating back to when irrigation began in 1887, calculate that intensively irrigated parts of the Central Valley are ~3.0 to 5.75°F cooler than they would otherwise have been. The cooling happens because much of the solar energy hitting irrigated ground during the day goes to evaporate the extra water in the soil and plants instead of heating the air. The cool farms could explain why minimum and maximum winter temperatures steadily rose in California between 1915 and 2000, while maximum summer temps did not. The warmer winters can only be explained by the greenhouse effect, and the authors speculate the cool-farms effect may have masked the impact of global warming on summer temps — since irrigation is mostly carried out during the summer.
But the cool times may not last. 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. This could also mean that irrigated regions around the world, which now provide about 40% of global food production, will feel more than their share of warming in the future, with the obvious impact on food security. . . So, plant your water gardens now. JULIA WHITTY