Roles of marginal seas in absorbing and storing fossil fuel CO2

Abstract

We review data on the absorption of anthropogenic CO₂ by Northern Hemisphere marginal seas (Arctic Ocean, Mediterranean Sea, Sea of Okhotsk, and East/Japan Sea) and its transport to adjacent major basins, and consider the susceptibility to recent climatic change of key factors that influence CO₂ uptake by these marginal seas. Dynamic overturning circulation is a common feature of these seas, and this effectively absorbs anthropogenic CO₂ and transports it from the surface to the interior of the basins. Amongst these seas only the East/Japan Sea has no outflow of intermediate and deep water (containing anthropogenic CO₂) to an adjacent major basin; the others are known to be significant sources of intermediate and deep water to the open ocean. Consequently, only the East/Japan Sea retains all the anthropogenic CO₂ absorbed during the anthropocene. Investigations of the properties of the water column in these seas have revealed a consistent trend of waning water column ventilation over time, probably because of changes in local atmospheric forcing. This weakening ventilation has resulted in a decrease in transport of anthropogenic CO₂ from the surface to the interior of the basins, and to the adjacent open ocean. Ongoing measurements of anthropogenic CO₂, other gases and hydrographic parameters in these key marginal seas will provide information on changes in global oceanic CO₂ uptake associated with the predicted increasing atmospheric CO₂ and future global climate change. We also review the roles of other marginal seas with no active overturning circulation systems in absorbing and storing anthropogenic CO₂. The absence of overturning circulation enables anthropogenic CO₂ to penetrate only into shallow depths, resulting in less accumulation of anthropogenic CO₂ in these basins. As a consequence of their proximity to populated continents, these marginal seas are particularly vulnerable to human-induced perturbations. Maintaining observation programs will make it possible to assess the effects of human-induced changes on the capacity of these seas to uptake and store anthropogenic CO₂.