Irrigation cooling effect: Regional climate forcing by land‐use change

TLDR

Regional detection of greenhouse warming depends on long‑term temperature records, yet land‑use changes—particularly urban heat islands—can confound observed temperature trends. The study aims to demonstrate that irrigation cooling, an effect opposite to urban heat islands, exists across regions. Using a regional climate model, the authors simulate and quantify this irrigation cooling effect. The modeled ICE exhibits strong seasonal variability, cooling dry‑season monthly means and maxima, leaving rainy‑season and minimum temperatures largely unchanged, and in California its magnitude rivals projected greenhouse‑gas warming, implying that past irrigation expansion may have masked the full warming signal.

Abstract

Regional detection of a greenhouse warming signal relies on extensive, long‐term measurements of temperature. The potentially confounding impact of land‐cover and land‐use change on trends in temperature records has mostly focused on the influence of urban heat islands. Here we use a regional climate model to show that a regional irrigation cooling effect (ICE) exists, opposite in sign to urban heat island effects. The magnitude of the ICE has strong seasonal variability, causing large dry‐season decreases in monthly mean and maximum temperatures, but little change in rainy‐season temperatures. Our model produced a negligible effect on monthly minimum temperature. In California, the modeled regional ICE is of similar magnitude, but opposite sign, to predictions for future regional warming from greenhouse gases. Given our results for California and the global importance of irrigated agriculture, past expansion of irrigated land has likely affected observations of surface temperature, potentially masking the full warming signal caused by greenhouse gas increases.