Projecting future change of monsoon rainfall is essential for water resource management, food security, disaster mitigation, and infrastructure planning. Here we assess the future change and explore the causes of the changes using 15 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble projects that, under the shared socioeconomic pathway (SSP) 2–4.5, the total land monsoon rainfall will likely increase in the Northern Hemisphere (NH) by about 2.8% per one degree Celsius of global warming (2.8% °C−1) in contrast to little change in the Southern Hemisphere (SH; −0.3% °C−1). In addition, in the future the Asian–northern African monsoon likely becomes wetter while the North American monsoon becomes drier. Since the humidity increase is nearly uniform in all summer monsoon regions, the dynamic processes must play a fundamental role in shaping the spatial patterns of the global monsoon changes. Greenhouse gas (GHG) radiative forcing induces a “NH-warmer-than-SH” pattern, which favors increasing the NH monsoon rainfall and prolonging the NH monsoon rainy season while reducing the SH monsoon rainfall and shortening the SH monsoon rainy season. The GHG forcing induces a “land-warmer-than-ocean” pattern, which enhances Asian monsoon low pressure and increases Asian and northern African monsoon rainfall, and an El Niño–like warming, which reduces North American monsoon rainfall. The uncertainties in the projected monsoon precipitation changes are significantly related to the models’ projected hemispheric and land–ocean thermal contrasts as well as to the eastern Pacific Ocean warming. The CMIP6 models’ common biases and the processes by which convective heating drives monsoon circulation are also discussed.

An accurate prediction of land monsoon precipitation (LMP) is critical for the sustainable future of the planet as it provides water resources for more than two-thirds of the global population. Here, we show that 24 CMIP6 (Phase Six of the Coupled Model Intercomparison Project) models’ ensemble mean projects, under the SSP2-4.5 scenario, summer LMP will very likely increase in South Asia (∼4.1 %/oC), likely increase in East Asia (∼4.6 %/oC) and northern Africa (∼2.9 %/oC) but likely decrease in North America (∼−2.3 %/oC). The annual mean LMP in three Southern Hemisphere monsoon regions will likely remain unchanged due to significantly decreased winter precipitation. Regional mean LMP changes are dominated by the change in upward moisture transport with moderate contribution from evaporation and can be approximated by the changes of the product of the mid-tropospheric ascent and 850-hPa specific humidity. Greenhouse gases (GHG)-induced thermodynamic effects increase moisture content and stabilize the atmosphere, which tends to offset each other. The spatially uniform increase of humidity cannot explain markedly different regional LMP changes. Intermodel spread analysis demonstrates that the GHG-induced circulation changes (dynamic effects) are primarily responsible for the regional differences. The GHGs induce a “warm land-cool ocean” that strengthens the Asian monsoon, and a “warm North Atlantic and Sahara” that enhances the northern African monsoon, as well as an equatorial central Pacific warming that weakens the North American monsoon. CMIP6 models generally capture realistic monsoon rainfall climatology, but commonly overproduce summer rainfall variability. The models’ biases in projected regional SST and land-sea thermal contrast likely contribute to the models’ uncertainties in the projected monsoon rainfall changes.

Eurasia has experienced more frequent bitter winters over the past two decades, which concurred with a prominent "Warm Arctic-Cold Siberia" (WACS) pattern that is unexpected from global warming. Here we show, by analysis of 117-year observations and climate model's millennial simulations, that the WACS is an internal mode of winter temperature variability, which cannot be excited by greenhouse gases and solar forcing. Observational and simulated results suggest that frequent occurrences of that WACS pattern are instigated by warm phases of Atlantic Multidecadal Oscillation (AMO). North Atlantic warming may activate the WACS by generating a background Atlantic-Eurasian wave train characterized by enhanced Ural Mountain ridge and East Asian trough, which is conducive to recurrent WACS pattern. The wave train-induced the Barents Sea ice melting can act as an amplifier, reinforcing the WACS. Although increased greenhouse gases favor a uniform warming pattern, they may contribute to WACS formation by affecting AMO.Plain Language Summary During the past two decades, Eurasia has experienced frequent bitter winters against the rapid warming over the Arctic, which have caused severe damage to livelihoods and socio-ecological systems. The prevailing paradigm attributes cold Eurasia to accelerated Barents-Kara Sea ice melt as part of global warming. Here we show, by analysis of 117-year observations and climate model's millennial simulations, that the WACS is an internal mode of winter temperature variability instigated by a warm phase of Atlantic Multidecadal Oscillation (AMO). North Atlantic warming stimulates the WACS by generating a background planetary wave train that is conducive to the frequent occurrence of the WACS. Although increased greenhouse gases favor a uniform warming pattern, they may contribute to WACS formation by affecting AMO. The findings deepen our understanding of the interactive influences of natural variability and anthropogenic forcing on climate change and shed light on future change of northern winter climate under increasing anthropogenic forcing.