Assessing soil moisture-induced changes in land carbon sink projections of CMIP6 models

The terrestrial biosphere absorbs about one third of anthropogenic carbon dioxide emissions and thereby dampens human-induced climate change. However, its capacity to act as a carbon sink depends on climate conditions, including temperature and water availability. Uncertainties in both future climate conditions and the response of the terrestrial biosphere lead to greatly diverging projections of the land carbon sink among state-of-the-art Earth System Models (ESMs).

Previous research identified soil moisture (SM) as a critical factor that can restrict land carbon uptake through water limitation and the intensification and prolongation of heat extremes. Green et al. (2019) demonstrated the severe negative impact of reduced SM on long-term land carbon sink projections of the 5th Coupled Model Intercomparison Project (CMIP5) using dedicated experiments isolating the effects of SM.

Here, we use equivalent experiments performed with four ESMs participating in CMIP6 to investigate the impact and uncertainty of SM-induced changes in land carbon sink projections by the end of the century (2070-2099). Our results demonstrate a substantial reduction in the negative impact of SM on the global land carbon sink compared to the previous model generation. Models agree on a SM-induced reduction in land carbon uptake in summer, consistent with an overall SM decline across models, while intermodel uncertainty remains high in spring, particularly regarding the effects of SM variability at mid-to-high latitudes. Additionally, high uncertainty in SM-induced impact on annual carbon uptake persists in the tropics and northern mid-latitudes, driven by differences in the sensitivity of carbon uptake to SM but also disagreement in SM projections across models.

We extend our analysis to a larger ensemble of CMIP6 models that have not performed the SM experiments. To this end, we employ the methods of Schwingshackl et al. (2018), which utilize the distinct link between SM and the evaporative fraction in the different SM regimes. Using this relationship we emulate the impact of SM on the land carbon sink in regions where land carbon uptake is controlled by SM.

The study aims to gain insights into SM-induced impacts and related uncertainties in land carbon sink projections of CMIP6 models, highlighting the ongoing challenge of accurately projecting SM-induced changes in the land carbon sink.

 

References:

Green, J. K., Seneviratne, S. I., Berg, A. M., Findell, K. L., Hagemann, S., Lawrence, D. M., & Gentine, P. (2019). Large influence of soil moisture on long-term terrestrial carbon uptake. Nature, 565(7740), 476-479. https://doi.org/10.1038/s41586-018-0848-x 

Schwingshackl, C., Hirschi, M., & Seneviratne, S. I. (2018). A theoretical approach to assess soil moisture–climate coupling across CMIP5 and GLACE-CMIP5 experiments. Earth System Dynamics, 9(4), 1217-1234. https://doi.org/10.5194/esd-9-1217-2018