Global Study Determines the Amount of Manmade CO2 Emissions Absorbed by the Ocean from 1994 to 2007

Not all of the carbon dioxide (CO2) produced during the burning of fossil fuels stays in the atmosphere and adds to global warming. The ocean and the ecosystems on land absorb substantial quantities of these manmade CO₂ emissions from the atmosphere.

Using the results of extensive ocean CO2 measurements, we can now estimate how much manmade CO2 the ocean has taken up in the last few decades. (Photograph: andrej67/iStock)

The ocean absorbs CO2 in two steps: first, the CO2 mixes in the surface water. Later, the ocean’s overturning circulation spreads it: ocean currents and mixing processes move the dissolved CO2 from the surface down to the ocean’s depths, where it collects over time.

Carbon sink in the ocean

This overturning circulation is the motivating force behind the oceanic sink for CO2. The size of this sink is very crucial for the atmospheric CO2 levels: without this sink, the concentration of CO2 in the atmosphere and the extent of anthropogenic climate change would be significantly higher.

Establishing what share of the manmade CO2 the oceans absorb has long been a key concern for climate researchers. An international team of researchers led by Nicolas Gruber, Professor for Environmental Physics at ETH Zurich, has currently established this oceanic sink over a period of 13 years. As stated in the recent issue of Science, the scientists have discovered that the ocean has absorbed from the atmosphere as much as 34 gigatons (billions of metric tons) of manmade carbon between 1994 and 2007. This figure matches 31% of all anthropogenic CO2 released during that period.

The marine sink is intact

This percentage of CO2 absorbed by the oceans has stayed comparatively stable than the preceding 200 years, but the absolute quantity has expanded considerably. This is because as long as the atmospheric concentration of CO2 rises, the oceanic sink fortifies more or less proportionally: the more CO2 is in the air, the more is absorbed by the oceans—until it becomes ultimately saturated. Thus far, that level has not been reached.

Over the examined period, the global ocean continued to take up anthropogenic CO2 at a rate that is congruent with the increase of atmospheric CO2.

Nicolas Gruber, Professor for Environmental Physics, ETH Zurich.

These data-based research findings also ratify numerous earlier, model-based estimates of the ocean sink for manmade CO2. “This is an important insight, giving us confidence that our approaches have been correct,” Gruber adds. The results additionally allow the scientists to come to conclusions regarding the CO2 sink of the ecosystems on land, which are more tough to define.

Regional differences in the absorption rate

While the general results indicate an intact ocean sink for manmade CO2, the scientists also found in the various ocean basins significant deviations from the uptake anticipated from the rise in atmospheric CO2. The North Atlantic Ocean, for example, absorbed 20% less CO2 than anticipated between 1994 and 2007. “This is probably due to the slowdown of the North Atlantic Meridional Overturning Circulation in the late 1990s, which itself is most likely a consequence of climate variability,” Gruber explains. But this lower sink in the North Atlantic was offset by a significantly larger uptake in the South Atlantic, such that the uptake by the whole Atlantic developed as anticipated. The scientists observed similar variations in the Pacific, in the Southern Ocean, and in the Indian Ocean.

We learned that the marine sink does not just respond to the increase in atmospheric CO2. Its substantial sensitivity to climate variations suggests a significant potential for feedbacks with the ongoing change in climate.

Nicolas Gruber, Professor for Environmental Physics, ETH Zurich.

Results of two surveys

The outcomes are based on an international survey of CO2 and other physical and chemical properties in the different oceans, measured from the surface down to depths of up to 6 km. Researchers from seven nations took part in the globally coordinated program that began in 2003. Internationally, they performed over 50 research cruises up to 2013, which were then synthesized into a universal data product.

For their investigations, the scientists used a new statistical technique formulated by Gruber and his former Ph.D. student, Dominic Clement. This technique permitted them to differentiate between the variations in the manmade and the natural CO2 components that constitute the changes in the total concentration of dissolved CO2 in the water. Natural CO2 indicates the amount of CO2 that was present in the oceans before industrialization.

Gruber had already contributed to a related study around the turn of the millennium. Using observations acquired from the very first global CO2 survey carried out between the late 1980s and the mid-1990s that research projected that the ocean had absorbed about 118 gigatons of carbon from the beginning of industrialization from 1800 until 1994. His present team of scientists prolonged this investigation up to 2007, permitting them not only to determine the budget for manmade CO2 from 1994 to 2007, but also to measure the intactness of the ocean carbon sink.

Increasing CO2 content acidifies marine habitats

By controlling the rate of global warming, the oceanic sink for manmade CO2 offers a key service for humanity, but it has its downside: ocean water becomes acidified when CO2 is dissolved in it.

Our data has shown that this acidification reaches deep into the ocean’s interior, extending in part to depths of more than 3000 m.

Nicolas Gruber, Professor for Environmental Physics, ETH Zurich.

This can have grave consequences for a number of sea organisms. Calcium carbonate unexpectedly dissolves in acidified environments, which poses a danger to corals and mussels whose shells and skeletons are composed of calcium carbonate. The ocean’s varying chemical composition can also influence physiological processes such as the breathing of fish. Gruber is committed: “Documenting the chemical changes imparted on the ocean as a result of human activity is crucial, not least to understand the impact of these changes on marine life.”

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