Carbon restoration potential on global land under water resource constraints

Ecosystem restoration is a critical nature-based solution to mitigate climate change. However, the carbon sequestration potential of restoration, defined as the maximum achievable carbon storage, has likely been overestimated because previous studies have not adequately accounted for the competition between ecosystem water demands for maximizing carbon sequestration and human water needs. Here we used a comprehensive process-based model combined with extensive land-use data and evaporation recycling accounting for land-atmosphere feedback to estimate the water requirements associated with ecosystem restoration. We found that achieving the carbon sequestration potential of restoration would significantly reduce global water availability per capita by 26%, posing considerable risks to water security in water-stressed and highly populated regions. If human water use is safeguarded, the achievable carbon sequestration potential would be reduced by a third (from 396 PgC to 270 PgC). Brazil, the United States and Russia have the largest achievable potentials. Future projections accounting for changes in climate, atmospheric CO2, land use and human population under the shared socioeconomic pathway (SSP) scenarios SSP119, SSP245 and SSP585 suggest an increase in this achievable potential to 274-302 PgC by the end of the century, with China expected to have the largest potential. Our findings provide a nuanced understanding of the trade-offs and synergies between carbon sequestration goals and water security, offering an empirical framework to guide the sustainable implementation of ecosystem restoration strategies.