Soil moisture-atmosphere coupling accelerates global warming

Anthropogenic greenhouse gas (GHG) emissions since the Industrial Revolution have caused measurable global warming, but warming rates vary regionally. Land-atmosphere coupling, influenced by factors like soil moisture, snowpack, and vegetation, modulates energy and mass fluxes to the atmosphere, affecting weather patterns and climate anomalies. Soil moisture-atmosphere coupling (SA) has been linked to increased high-temperature extremes and heat waves. However, the extent to which SA will impact global warming's rate and heat extremes under different emission scenarios remains unclear. This study investigates the SA effect on global warming using six global climate models (CESM2, CMCC-ESM2, EC-Earth3, IPSL-CM6A-LR, MIROC6, and MPI-ESM1-2-LR) from the Land Surface, Snow and Soil Moisture Model Intercomparison Project (LS3MIP), the Scenario Model Intercomparison Project (ScenarioMIP), and the historical experiment in Coupled Model Intercomparison Project phase 6 (CMIP6). The study focuses on extra-tropical summer conditions under various warming scenarios. Experiments with fixed soil moisture (LFMIP-pdLC) isolate the SA effect by comparing fully coupled experiments (historical, SSP1-2.6, and SSP5-8.5) with the fixed soil moisture experiment. Three time periods (1995-2014, 2040-2059, and 2080-2099) represent modern, mid-term, and long-term future conditions.

Previous research highlights the role of land-atmosphere coupling in regional climate variability. Studies have shown links between SA and increased intensity and frequency of high-temperature extremes and heat waves. However, a comprehensive understanding of SA's impact on global warming's rate, intensity of heat extremes, and its time evolution across different emission scenarios has been lacking. This study builds upon this existing research by using a multi-model approach and focusing on the isolation of the SA effect to provide a more complete picture.

Six global climate models from CMIP6 were used to analyze SA's impact on surface air temperature during extra-tropical summer. The Land Feedback Model Intercomparison Project with prescribed Land Conditions experiment (LFMIP-pdLC) from LS3MIP, where soil moisture is fixed to its 1980-2014 climatological state, allowed the isolation of SA effects. Comparisons were made between fully coupled experiments (historical, SSP1-2.6, and SSP5-8.5) and the LFMIP-pdLC experiment. Three time horizons (1995-2014, 2040-2059, 2080-2099) were considered. The 90th percentile of surface air temperature was used to define extreme high-temperature events. The SA effect on extreme high-temperature probability and intensity was determined by comparing coupled experiments to the LFMIP-pdLC experiment. The SA-induced changes in surface air temperature were decomposed into radiative forcing terms (surface albedo, evapotranspiration, shortwave transmissivity, air emissivity, aerodynamic resistance, and residual terms) using the surface energy balance. Multi-model means were used to reduce model uncertainty, with daily data used for probability distribution function and extreme high-temperature analyses. The study used monthly data derived from all six models, except for latent heat flux calculation where MIROC6 was excluded due to data limitations. For the probability distribution function and extreme high-temperature analysis, daily data from individual models were used instead of multi-model means.

SA amplifies global warming across much of Earth's land surface under all emission scenarios. SA-induced warming is projected to weaken significantly under the most stringent GHG mitigation pathway (SSP1-2.6). While SA's contribution to extreme high-temperature likelihood is less than 10% under modern conditions, this influence strengthens in the mid-term future, particularly over northern China and northernmost South America. By the end of the century, SA's role in extreme high-temperature events is projected to rise significantly (approaching 20% in mid-high-latitude Northern Hemisphere and subtropical Southern Hemisphere under high emission scenario). Over North America (NA) and Europe (EUR), SA causes a rightward shift and flattening of the surface air temperature probability distribution function, resulting in a substantial increase in extreme high-temperature probability (+52.5% for NA and +30.8% for EUR). SA also influences the intensity of extreme high-temperature events, with projections showing an increase of >1.5°C globally and as much as 8.0°C over NA and EUR under the high-emission scenario. Under the low-emission scenario, the relationship between SA and the intensity and probability of extreme high-temperature events weakens. GHG-driven warming dries the soil column, reducing evapotranspiration and increasing sensible heat flux, which enhances shortwave radiation and triggers nonlinear warming. This is most pronounced in the Northern mid-latitudes (especially EUR and NA) and Southern subtropical regions. The enhanced sensitivity of evapotranspiration to soil drying under high emissions accelerates the amplified warming. The decomposition of SA-induced surface air temperature changes shows that changes in evapotranspiration and shortwave transmissivity are primary drivers. Over EUR and NA in the long-term future under high emissions, combined positive radiation from evapotranspiration and shortwave transmissivity significantly increase compared to the modern period.

The findings demonstrate that SA acts as a positive feedback mechanism, accelerating GHG-driven warming. The strength of this feedback is strongly influenced by emission scenarios; mitigation efforts are crucial to reduce the severity of SA-induced warming and extreme high-temperature events. The study highlights the importance of maintaining ecosystem stability to sustain soil moisture within appropriate limits, particularly in NA and EUR, where the impact of SA is projected to be most severe. The local effects of SA on surface air temperature are emphasized, but future research should investigate the combined impact of local and non-local SA effects on global land warming. While acknowledging limitations related to model parameterizations and scenario uncertainties, the results are robust due to the high reliability of the models in simulating historical soil moisture and the consistency of results across different models.

This study projects that SA significantly amplifies GHG-driven global warming, particularly under high-emission scenarios. The ‘warmer climate-drier soil’ feedback accelerates this amplification, leading to increased frequency and intensity of extreme high-temperature events. Mitigation efforts are essential to minimize this risk, and maintaining ecosystem stability to control soil moisture levels is crucial, especially in vulnerable regions like NA and EUR. Further research should explore the combined effects of local and non-local SA on global land warming.

The study acknowledges uncertainties associated with model parameterizations of SA and scenario uncertainties due to limited knowledge about future radiative forcing. However, the use of multiple models and focus on historical simulation reliability increases confidence in the results. The analysis focused on the local effects of SA, and further research is needed to explore the non-local impacts on large-scale atmospheric circulation.