UH STUDY: Oceans Serve as Buffer From Full Impact of Human-Produced CO2

Rosette with sampling bottles for collecting ocean water. March, 2019. PC: N Gruber, ETH Zurich

The world’s oceans are helping to combat global warming. An international team of scientists, including University of Hawai‘i at Mānoa oceanography Professor Christopher Sabine, has shown that the ocean is absorbing carbon dioxide (CO2), generated during the combustion of fossil fuels, from the atmosphere at an increasing rate from the 1990s to the early 2000s.

As reported in the latest issue of Science, the researchers found the ocean has taken up as much as 34 gigatonnes (billions of metric tonnes) of human produced carbon between 1994 and 2007. This figure corresponds to 31% of all human-produced CO2 emitted during that time.

“The uptake and storage in the ocean of human produced CO2 has significantly decreased the climate change effects the planet has seen so far,” said Sabine. “Getting a handle on how much more the ocean can take is critical for predicting future climate change impacts.”

Change in human produced CO2 in oceans (1994-2007). Yellow = large increase. March, 2019. PC: N Gruber

The ocean takes up CO2 in two steps. First, the CO2 dissolves in the surface water. Afterward, the ocean’s overturning circulation—ocean currents and mixing processes—slowly transport the dissolved CO2 into the ocean’s interior, where it accumulates over time. This overturning circulation is the driving force behind the oceanic sink for CO2 because it makes room for the surface ocean to absorb more CO2. Without this sink, the concentration of CO2 in the atmosphere and the extent of anthropogenic climate change would be considerably higher.

Starting in 2003, scientists from seven countries, including Sabine, participated in an internationally coordinated program wherein they carried out more than 50 research cruises globally over ten years. They measured CO2 and other chemical and physical properties from the surface down to depths of six kilometres to create a three-dimensional picture of ocean chemistry.

To determine the increase in the oceanic CO2 content as a result of the oceanic uptake of human produced CO2 from the atmosphere, they contrasted the measured CO2 concentration from this new survey to a synthesis, led by Sabine, of observations obtained from a global CO2 survey conducted between the late 1980s and the mid-1990s.

As the atmospheric concentration of CO2 rises, the absolute quantity of CO2 taken up by the oceans increases. The oceanic sink strengthens more or less proportionally: The more CO2 is in the atmosphere, the more is absorbed by the oceans—until it eventually becomes saturated.

So far, that point 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,” said Nicolas Gruber, professor for environmental physics at ETH Zurich and lead author of the new study.

“By comparing our early assessment to this recent analysis, we have confidence that the marine carbon sink is still intact, which is reassuring, but it comes at a price,” said Sabine, who joined the UH Mānoa School of Ocean and Earth Science and Technology in 2018.

By moderating the rate of global warming, the oceanic sink for human produced CO2 provides an important service for humanity, however, CO2 dissolved in the ocean makes seawater more acidic.

This can have serious consequences for many marine organisms. Calcium carbonate shells and coral skeletons are vulnerable in acidifying environments. The changing chemical composition of the ocean can also impact physiological processes such as the breathing of fish.

Sabine noted, “Documenting the accumulation of human produced CO2 in the ocean and the resulting acidification is particularly important for Hawai‘i where our tourism and fishing industries depend on healthy ecosystems. A clear understanding of the stress we are putting on our marine ecosystems from burning fossil fuels will help us to better manage our resources sustainably.”