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The world's oceans are a massive carbon "sink," taking carbon dioxide from the atmosphere and absorbing it into their churning depths. But this sink is showing signs of strain.

Why it matters: If the oceans slow their carbon uptake, there would be more planet-warming carbon dioxide in the air, which would speed up global warming significantly.

What they did: For a new study, published in the journal Science on Thursday, an international research team determined how much carbon dioxide the oceans have been absorbing.

What they found: The study finds that oceans have taken more than 100 billion tons of CO2 between 1994 and 2007, which is about one-third of total emissions during that period.

  • This shows the oceans roughly kept up with the ever-increasing rate of human-caused emissions during the 1994–2007 period.
  • While the overall share of emissions absorbed by oceans has not changed, the rate at which they are absorbing carbon dioxide has increased fourfold between 1994 to 2007, when compared to the period from 1800 to 1994, the study found.

"If it wasn't for this uptake by the oceans, the atmospheric CO2 concentration would be as much as 480 ppm and the global atmospheric temperatures would be considerably warmer," study co-author Richard Feely of NOAA tells Axios.

"This means that the ocean has been providing humanity with an ecosystem service that can be valued at more than $1 trillion."
— study co-author Nicolas Gruber of ETH Zurich tells Axios, assuming a carbon price of $10 per ton of CO2.

The research also shows that ocean acidification, which is occurring from chemical reactions as seawater absorbs CO2, is beginning to affect marine life well below the surface.

  • Since the dawn of the Industrial Revolution, the pH of the ocean surface waters has declined by about 0.11 pH units, Feely says. The greatest decrease in pH is in the high latitudes.
  • Ocean acidification poses a major threat to calcifying organisms, such as sea butterflies and mussels, with indications that problems are already showing up in ecosystems.

Another study, published recently in Geology, provides a new long-term history of how carbon has accumulated in deep-sea sediments throughout geologic time. Carbon is absorbed this way as dead diatoms and plankton descend through the water column, accumulating slowly but steadily as "marine snow" on the seafloor.

"The more acidic the ocean becomes, the smaller the volume of dead carbonate plankton shells sinking through the water column that will make it to the seafloor without dissolving completely on their way down," study co-author Dietmar Muller tells Axios.

"In other words, we are continuously reducing the capacity of the oceans to store away atmospheric CO2 in deep-sea sediments."

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