Past and future projected radiative effects from irrigation

Irrigation is the most dominant freshwater use practice on Earth, yet its effect on the top-of-atmosphere (TOA) radiative budget remains uncertain because past estimates have relied on idealised experiments or single-model frameworks. Here, we use historical simulations (1902–2014) from the Irrigation Model Intercomparison Project (IRRMIP) to provide the first multi-model quantification of irrigation-induced effective radiative forcing (ERF), defined as the change in net TOA radiative flux including rapid atmospheric adjustments. Across seven Earth system models, under prescribed sea surface temperatures (SST), the global multi-model mean (MMM) irrigation-induced ERF is positive but small (∼ 0.023 W m ⁻² ). While the global signal is small, it masks regional heterogeneity: South Asia and West Central Asia exhibit higher positive ERF (∼0.23 W m ⁻² ), with spatial maps revealing coherent forcing hotspots of up to +5 W m ⁻² over the Indo-Gangetic Plain that are consistent across all models, while East Asia shows negative ERF (∼-0.03 W m ⁻² ). The clear-sky ERF is greater than the all-sky ERF in most of the regions, indicating that cloud adjustments following irrigation may exert a negative contribution to the ERF. Correlations between historical irrigation ERF variability and its potential drivers suggest that reduced outgoing longwave radiation is the dominant control at the global scale, as well as in South Asia and West Central Asia, while near-surface temperature, precipitable water, albedo, and cloud cover contribute with substantial regional and inter-model variability. We additionally examine future changes in net TOA radiation imbalance (Δ R _n,TOA ) from a fully coupled CMIP-style model (SSP1-2.6 and SSP3-7.0, 2015–2070) to assess whether the historical ERF pattern persists under future irrigation. The global mean Δ R _n,TOA is more positive under the high irrigation demand pathway (SSP3-7.0) than under SSP1-2.6, consistent with projected differences in irrigation expansion between scenarios. These results demonstrate that irrigation's radiative influence, while small in the global mean, is regionally substantial and scenario-dependent, with implications for the attribution of regional climate change and for the design of land-based mitigation trajectories.