Carbon dioxide pumped into the air since the Industrial Revolution appears to have changed the way the coastal ocean functions, according to a new analysis published this week in Nature. A comprehensive review of research on carbon cycling in rivers, estuaries and continental shelves suggests that collectively this coastal zone now takes in more carbon dioxide than it releases. The shift could impact global models of carbon’s flow through the environment and future predictions related to climate change.
We need to better understand the role of the coastal ocean in carbon dioxide exchange between the atmosphere and the ocean, said study co-author Wei-Jun Cai, professor of oceanography in the University of Delawares School of Marine Science and Policy within the College of Earth, Ocean, and Environment. That will give us a much better capacity to predict future global carbon budgets and fluxes due to climate change and other anthropogenic factors.
Cai and other environmental scientists have been examining the complex dynamics that move different forms of carbon through coastal waters. Numerous variables, from rainfall to temperature to plant photosynthesis, can influence how much carbon is present in water at any given time.
Carbon is not stationary, Cai said. It flows and changes among its different forms.
The multiple sources and processes at play make coastal carbon challenging to study, however, and Cai said it has traditionally been overlooked in global carbon budget calculations. The annual estimate of how much anthropogenically-released carbon dioxide is trapped by land, for example, has been determined by subtracting the amount taken up by the ocean from the amount put into the air.
If there is another reservoir — the coastal ocean — that also takes up carbon dioxide, then that changes the balance, Cai said.The coastal zone may be relatively small compared to the open ocean, but the researchers point out that it represents a disproportionately large amount of the carbon dioxide exchanged between air and water.
That suggests that the coastal ocean may have its own mechanism for holding carbon dioxide — something Cai first suspected in 2005 on a cruise off the coast of Georgia. There he was surprised to see that sea surface carbon dioxide levels were about the same as 10 years prior, even though there were significantly greater amounts of the greenhouse gas in the atmosphere.Conventional wisdom would hold that sea surface carbon dioxide should rise in tandem with levels in the atmosphere, as is the case in most of the ocean basin.