Data were obtained from the literature to identify past changes in and the present status of the coastal carbon cycle. They indicate that marine coastal ecosystems driving the coastal carbon cycle cover, on average, 5.8% of the Earth’s surface and contributed 55.2% to carbon transport from the climate-active carbon cycle to the geological carbon cycle. The data suggest that humans not only increase the CO2 concentration in the atmosphere but also mitigate (and before 1860 even balanced) their CO2 emissions by increasing CO2 storage within marine coastal ecosystems. Soil degradation in response to the expansion and intensification of agriculture is assumed to be a key process driving the enhanced CO2 storage in marine coastal ecosystems because it increases the supply of lithogenic matter that is known to favour the burial of organic matter in sediments. After 1860, rising CO2 concentrations in the atmosphere indicate that enhanced CO2 emissions caused by land-use changes and the burning of fossil fuel disturbed what was a quasi-steady state before. Ecosystem restoration and the potential expansion of forest cover could mitigate the increase of atmospheric CO2 concentrations, but this carbon sink to the atmosphere is much too weak to represent an alternative to the reduction of CO2 emission in order to keep global warming below 1.5–2.°C. Although the contribution of benthic marine coastal ecosystems to the global CO2 uptake potential of ecosystem restoration is only around 6%, this could be significant given national carbon budgets. However, the impact on climate is still difficult to quantify because the associated effects on CH4 and N2O emissions have not been established. Addressing these uncertainties is one of the challenges faced by future research, as are related issues concerning estimates of carbon fluxes between the climate-active and the geological carbon cycle and the development of suitable methods to quantify changes in the CO2 uptake of pelagic ecosystems in the ocean.

Blue carbon is identified as a natural climate solution as it provides multiple ecosystem services, including climate mitigation, adaptation, and other co-benefits. There remain ongoing challenges for blue carbon as a natural climate solution, particularly as blue carbon ecosystems are at risk from climate change. Concepts of uniformitarianism were applied to consider how the present and past behaviour of blue carbon ecosystems can inform decision-makers of blue carbon risks. Climate change may increase the capture and storage of blue carbon in the short to medium term; this is largely due to negative feedbacks between elevated atmospheric carbon dioxide and temperature, and supplemented by natural processes of sediment supply and accumulation. Opportunities for retreat and increasing carbon storage as sea levels rise are likely to be greater where sea level has a longer history of relative stability, largely in the Southern Hemisphere. Landward retreat will be crucial where millennia of sea-level rise have limited the capacity for in situ blue carbon additionality; this may be thwarted by highly developed coastal zones and coastal squeeze effects. Negative feedbacks may fail under higher emissions, greater warming and rates of sea-level rise exceeding ~5–7 mm yr.−1; this tipping point may be surpassed within the next century under a high emissions scenario. Retreat of blue carbon ecosystems to higher elevations where they are afforded protection from the effects of sea-level rise will be critical for blue carbon additionality. Carbon markets are prepared to incentivise restoration of blue carbon ecosystems as they adapt to climate change; however, knowledge gaps remain, particularly regarding the behaviour of blue carbon ecosystems in the global south. Given the momentum in blue carbon research, scientists and practitioners are well placed to continue addressing blue carbon risks.