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The ocean has absorbed about 30% of carbon dioxide emissions from human activities since the Industrial Revolution, significantly slowing the pace of climate change.

But as emissions continue to rise, scientists have warned that the ocean could eventually become too saturated with carbon to absorb more. Has the day arrived?��

Using a computer simulation, ��ɫ��Ƶ oceanographer Nicole Lovenduski and her collaborators found that the recent slowdown in the ocean’s carbon absorption is likely a result of natural fluctuations instead of human-driven climate change. The was published in Environmental Research Letters.

Nicole Lovenduski/��ɫ��Ƶ

“The ocean has been doing us this huge favor for so long by taking up man-made carbon,” said Lovenduski, director of the Institute of Arctic and Alpine Research and professor in the Department of Atmospheric and Oceanic Sciences.” If less is going into the ocean, that means more is left in the atmosphere.”

Scientists previously found that between 2004 and 2014, the ocean absorbed about 2 gigatons less human-generated, or anthropogenetic, carbon than it did between 1994 to 2004, even though the atmospheric carbon levels had continued to rise over the past decades. This amount is roughly one-third of the U.S.’ annual carbon emissions.

“Generally, when there’s more carbon in the atmosphere, the ocean would try to take up more to maintain its chemical equilibrium,” Lovenduski said.

But climate models have suggested that once the planet warms up above a certain point, the ocean would start to absorb less carbon. This is because warm water takes up less CO2 than cold water. Warmer temperatures also slow down ocean currents that help bring cold seawater from Earth’s colder regions to the tropics and from the deep sea to the surface.

Scientists were unsure if the slowdown since 2004 was a result of global warming.

Measuring anthropogenic carbon concentrations in the ocean is no small feat. Lovenduski and other oceanographers have gone out on boats to drop large bundles of roughly 30 canisters into designated spots across all five oceans to collect water samples from different depths.

Holly Olivarez (front) and colleagues during a survey expedition in 2021. (Courtesy of Holly Olivarez)

Because these surveys are time-consuming and labor-intensive, scientists only conduct them once every decade.

That means that, by chance, the ocean could be experiencing special conditions during the sampling years, Lovenduski said. Some of these natural fluctuations could influence how much carbon the seawater absorbs. For example, during an El Niño event, less carbon-rich water from the deep ocean rises to the surface in the tropical Pacific Ocean, which means measurements taken during such event could complicate estimates of how much carbon the ocean is absorbing.

Lovenduski and her team set out to investigate the causes of the recent slowdown in ocean carbon uptake.

The research team, including first author, , who was a graduate student at ��ɫ��Ƶ, used a computer model to simulate how forces outside the oceans, such as rising atmospheric CO2 and volcanic eruptions, and internal factors, like El Niño events, have influenced how much carbon the ocean took up between 1994 and 2014.

When the model only considered external factors, the ocean’s carbon storage continued to rise in step with anthropogenic emissions. The growth rate only slowed when the team added the effects of internal factors to the model. The impact of forces like El Niño events was particularly prominent in places like the tropical Pacific Ocean, subpolar North Atlantic Ocean and the Southern Ocean.

Scientists drop bundles of roughly 30 canisters into designated spots across all five oceans to collect water samples from different depths. (Courtesy of Holly Olivarez)��

“It turns out that the way we measure the ocean could have imparted that change in carbon storage growth rate,” Lovenduski said. “Maybe the slowdown from climate change is already happening, but our paper suggests that we need to rule out the internal factors before we can say that with confidence.”

She added that the study highlights a major gap in ocean monitoring. Scientists sample the atmosphere hourly, but the chemistry of the ocean only once a decade. Without a more accurate picture of how much carbon the ocean is absorbing, it would be difficult to estimate how fast the planet is truly warming.

“We are only measuring a tiny little sliver of the ocean, and we're only doing it once every 10 years. We need to fill the location gap and time gap,” she said.

Lovenduski hopes that in the future, scientists can deploy autonomous robots to different parts of the ocean to analyze anthropogenic carbon concentrations in seawater. The technology will allow them to measure carbon levels in the ocean continuously and across more locations.��