Chapter 13 – ocean systems

The ocean comprises ~71% of the Earth’s surface area and is in constant interaction with the atmosphere above and the land surface at the coastal interface, allowing a continuous exchange of greenhouse gases (GHGs) between the spheres. The ocean plays an important role in absorbing and storing carbon dioxide (CO2) from fossil fuel combustion, land-use change, and cement production. Since the industrial revolution, the ocean has stored ~31% of human emitted CO2 adding to a total storage of anthropogenic CO2 of 152 ± 20 Pg C (PgC = Petagrams of carbon) from 1850 to 2007 and is currently removing about 2.6 ± 0.6 Pg C of excess CO2 every year from the atmosphere. On longer timescales (i.e., centuries to millennia), the ocean carbon sink acts as a primary regulator of the Earth’s climate. While the ocean carbon sink mitigates climate change, absorption of anthropogenic CO2 leads to ocean acidification with potentially harmful effects for marine ecosystems. The ocean also contributes to the cycles of other greenhouse gases. Specifically, it is a weak source of methane. The contribution of the ocean to the net global methane budget, however, is substantially smaller than the oceanic uptake of CO2. The ocean was a net source of methane (CH4) to the atmosphere of ~13 Tg CH4 year−1 (Tg CH4 = Teragrams of CH4) with a possible range of 9–22 Tg CH4 year−1 over the period 2000–20. Hence, the methane fluxes from the ocean to the atmosphere are an order of magnitude smaller than the anthropogenic emissions over the same period. Likewise, the ocean comprises a natural source of nitrous oxide (N2O) of ~ 3.4 Tg N year−1 between 2007 and 2016, although with a substantial possible range between 2.5 and 4.3 Tg N year−1.

Landschützer P., Keppler L. & Ilyina T., 2022. Ocean systems. In: Poulter B., Canadell J. G., Hayes D. J. & Thompson R. L. (Eds.), Balancing greenhouse gas budgets, pp 427-452. Amsterdam: Elsevier. Chapter (restricted access).

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