Potential of land-based terrestrial carbon sinks to mitigate climate change

Enhancing terrestrial ecosystem carbon sinks is one of the effective strategies to achieve carbon neutrality targets for climate change mitigation. However, efforts to enhance the carbon sink mainly focused on natural climate solutions, including protecting, restoring, and managing terrestrial ecosystems. The contributions of artificial measures, such as rock weathering, urban alkaline material weathering, and desert saline alkali soil water to terrestrial ecosystem carbon sinks have often been overlooked by previous estimates of terrestrial carbon sinks. To account for major carbon sink measures, we proposed a new concept of “four-color” terrestrial carbons based on their formation/sink mechanisms, i.e., green carbon (the carbon sequestered by forest, grassland, and inland wetland), black carbon (carbon sink through the addition of carbon-rich materials, such as straw, organic fertilizers, and biochar to the soil), blue carbon (the carbon stored in coastal ecosystems), and white carbon (carbon sink by chemical processes). We reviewed the potential measures for enhancing the different carbon sinks to provide a framework for achieving carbon neutrality targets.

We identified 15 measures for enhancing the sink of the different carbon groups. The enhancement approaches for green carbon sinks mainly include protecting, restoring, and managing forest, grassland, and inland wetland ecosystems. Adding carbon-rich materials to cropland soil is the major measure to enhance black carbon sink. The enhancement of blue carbon sink mainly focuses on ecosystem protection and restoration. We proposed enhancing silicate and carbonate rock weathering, irrigation and salt washing in arid regions, and utilizing urban alkaline materials as measures to enhance white carbon sink. With proper implementation of the above measures, we estimated C sequestration of 16.7 Pg CO₂ yr^-1 for green carbon, 5.8 Pg CO₂ yr^-1 for black carbon, 1.1 Pg CO₂ yr^-1 for blue carbon, and 7.7 Pg CO₂ yr^-1 for white carbon. Our results showed the potential to enhance the “four-color” carbon sinks globally by 31.4 Pg CO₂ yr^-1, higher than the estimate in the IPCC AR6. However, large uncertainties still exist in the estimation of current and future carbon sink potential in terrestrial ecosystems due to the different approaches used in different studies, large spatiotemporal variation, and insufficient data of carbon storage and sink. Therefore, we emphasize the need for strengthening monitoring and basic data acquisition and establishing rapid and accurate quantification techniques for terrestrial carbon sinks. Furthermore, future research should focus on the potential and multiple enhancement measures of white carbon under different climates and its response to global change.