Irrigation expansion in North China Plain has historically decelerated regional warming and mitigated temperature extremes

Since the 1950s, global irrigated area has expanded dramatically, with complex effects on regional climate worldwide. The North China Plain (NCP) is among the most intensively irrigated regions in the world, but the effects of historical irrigation expansion on climate extremes over multi-decadal timescale are largely uncertain. Combining statistical methods with model simulations, we found that NCP experienced a decreasing trend of 0.2–0.25 ℃ decade⁻¹ (p < 0.1) in daily maximum temperature (T_max) during May-June of 1961–2000 along with irrigation expansion, which is distinct from other regions experiencing strong warming such as most of western China. The cooling effect on T_max is 0.092 ℃ decade⁻¹ (p < 0.01), relatively lower than that in California’s Central Valley but comparable to the trend in Northwest China and larger than the trend in Tibetan Plateau. The correlation coefficients between irrigation expansion and temperature change from 1960 to 2000 for T_max and mean air temperature (T_mean) are –0.58 and –0.33 (p < 0.01), respectively, suggesting the ability of irrigation to alleviate regional warming and temperature extremes. Such effect varies over time, continuously strengthening from 1961 to 1980 because of intensive irrigation expansion, but then remaining relatively unchanged or weakening during 1980–2005 with moderate expansion. After 2005, the cooling effect is not detectable, which implies that it is completely canceled out by other forcings such as greenhouse gas warming, compensation of urban area expansion, small irrigation expansion rate and decline in irrigation water use. Despite that, irrigation is still able to reduce the number of extreme heat days after 1980. Compared with other factors, we found that irrigation expansion is the second most important contributor (27%) to the decrease in T_max during the study period, after aerosol pollution (54%). This work emphasizes the ability of irrigation expansion to adapt agriculture to climate change over the past decades, and highlights the need for sustainable irrigation expansion in the future.