The Role of Irrigation Expansion on historical climate change: Insights from CMIP6

To produce food for a growing world population, irrigated areas have increased from approximately 0.63 million km(2) of land in 1900 to 3.1 million km(2) of land in 2005. Despite this massive expansion, irrigation is still overlooked in most state-of-the-art Earth system models (ESMs) involved in the Coupled Model Intercomparison Project phase 6 (CMIP6). To our knowledge, only three CMIP6 models represent irrigation activities: CESM2, GISS-E2-1-G, and NorESM2-LM. Here, we investigate the role of irrigation on historical climate at global and regional scales by analyzing trends of key surface climate variables in CMIP6 simulations during 1900-2014. The three models including irrigation show distinct behavior from the 15 models without irrigation over intensively irrigated areas (i.e., >50% of grid cell area is equipped by irrigation): both annual (months that correspond to monthly air temperature higher than 274 K) mean latent heat flux (LHF) and soil moisture increase over time, in contrast to the models without irrigation that show no trend or even a negative trend. The positive LHF trend over intensively irrigated areas in the irrigation-on models is consistent with three satellite-based LHF products. While annual (considering the warmest month in a year) warming trends are found in these regions for most of the no-irrigation models, the increase in LHF induces a cooling trend for the models with irrigation, which was not confirmed by observational air temperature data sets. These findings, supported by satellite data, indicate the importance of improved representation of land management in the next generation of ESM.