Past changes in and present status of the coastal carbon cycle

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.