Weaker than expected future ocean carbon uptake due to carbon-climate feedbacks

The global ocean and terrestrial carbon dioxide (CO₂) sinks have removed approximately half of the total anthropogenic carbon emissions emitted to the atmosphere since 1850. Robust estimates of future carbon uptake are paramount to determine Paris Agreement compatible remaining greenhouse gas emission budgets including negative emission pathways to balance hard to abate emissions. Missing carbon-climate feedbacks in state-of-the-art greenhouse gas concentration-driven Earth System Models (ESMs), however, render future carbon cycle estimates uncertain. Here, historical and future ocean and land carbon uptake estimates from emissions-driven CMIP6 experiments conducted with AWI-ESM-1-REcoM are presented.

In the emissions-driven model setup, carbon-climate feedbacks and differences in the initial distribution of terrestrial vegetation lead to a reduced carbon source from anthropogenic land use changes, a smaller atmospheric CO₂ growth and a substantially weaker oceanic and terrestrial carbon uptake increase until the 1970s, compared to the concentration-driven model setup. Thereafter, the terrestrial CO₂ sink increases stronger in the emissions-driven setup, leading to similar atmospheric CO₂ growth in both model setups by the end of the historical period. In the future, ocean and land carbon sinks respond distinctively to both model setup and scenario forcing before peak emissions, between peak emissions and peak atmospheric CO₂, and before and after net zero emissions. The land sink in particular continues to increase stronger than the ocean sink after peak atmospheric CO₂. By the end of the 21st century, carbon-climate feedbacks yield atmospheric CO₂ concentrations considerably lower by 17 to 42 ppm and a weaker ocean carbon sink in the emissions-driven model setup, with the largest differences in strong mitigation scenarios. As emissions-driven ESM setups are recommended for the upcoming CMIP7, these model results stress the need to improve our understanding of the future evolution of the global carbon sinks.