Soil moisture-induced changes in land carbon sink projections in CMIP6

Abstract. The terrestrial biosphere absorbs about one third of anthropogenic CO₂ emissions, thereby significantly slowing human–induced climate change. Its capacity to act as a carbon sink strongly depends on climate conditions, particularly soil moisture (SM), which can constrain plant growth and amplify land–atmosphere feedbacks. Therefore, accurately capturing these effects in Earth System Models (ESMs) is critical.

Using dedicated experiments of the Land Feedback Intercomparison Project (LFMIP, an experiment within the Land Surface, Snow, and Soil Moisture Model Intercomparison Project, LS3MIP) from the latest generation of ESMs from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we show that projected SM changes substantially reduce the land carbon sink by the end of the century (2070–2099). This reduction is mainly driven by SM variability, highlighting the importance of SM extremes, which are projected to become more frequent and intense under climate change. Our results confirm those of the previous model generation based on the Global Land-Atmosphere Climate Experiment–Coupled Model Intercomparison Project phase 5 (GLACE–CMIP5). The results show that the strong negative impact of SM changes on the land carbon sink shown for GLACE–CMIP5 is less severe in LFMIP. A more in–depth analysis reveals that this is due at least in part to the specific ESM sampling of the respective experiments, with participating ESMs from CMIP5 generally showing a stronger drying trend. Despite agreement on the negative impact of SM on the land carbon sink in most tropical and mid–latitude ecosystems in both sets of multi–model experiments, there are large intermodel differences in the projected magnitudes.

As SM can influence land carbon uptake both directly and indirectly via land–atmosphere coupling, we conduct a contribution analysis on the impact of direct and indirect SM effects on carbon uptake, which reveals that SM–atmosphere interaction dominate SM–induced changes globally. However, models show disagreement on the magnitude of these effects. Intermodel differences arise mainly from varying sensitivities of GPP to SM–related direct and indirect effects, suggesting that differences likely stem from varying representations of water–stress related processes across ESMs.

Our findings highlight SM–atmosphere coupling as a critical factor for future land carbon uptake. Improving the representation of water stress processes, plant hydraulics, and vegetation characteristics in ESMs is essential for reducing uncertainty in projections. Maintaining and possibly extending the experimental set up to a larger set of models in future CMIP generations will be key to advancing our understanding of SM–carbon interactions and consequently of the evolution of the land carbon sink under human–induced climate change.