Study shows weaker ocean circulation could promote CO₂ buildup in atmosphere

As climate change progresses, the ocean’s reverse circulation is expected to weaken significantly. With such a slowdown, scientists estimate that the ocean will absorb less carbon dioxide from the atmosphere.

However, slower circulation should also result in less carbon being removed from the deep ocean that would otherwise be released into the atmosphere. Overall, the ocean should retain its role in reducing carbon emissions from the atmosphere, even if at a slower pace.

A new study by an MIT researcher published in Nature Communications Scientists may need to rethink the relationship between ocean circulation and its long-term ability to store carbon. As the ocean weakens, it could release more carbon from the deep ocean into the atmosphere.

The reason is related to a previously uncharacterized feedback between available iron in the ocean, upwelled carbon and nutrients, surface microorganisms, and a little-known class of molecules commonly known as “ligands.”

When the ocean circulates more slowly, all of these actors interact in a self-perpetuating cycle that ultimately increases the amount of carbon the ocean releases into the atmosphere.

“By isolating the impact of this feedback, we see a fundamentally different relationship between ocean circulation and atmospheric carbon levels, with implications for climate,” said study author Jonathan Lauderdale, a researcher in MIT’s Department of Earth, Atmospheric, and Planetary Sciences.

“What we thought was happening in the ocean has been completely turned upside down.”

According to Lauderdale, the results show that “we cannot rely on the ocean to store carbon in the deep ocean in response to future circulation changes. We need to be proactive in reducing emissions now, rather than relying on these natural processes to buy time to mitigate climate change.”

Box flow

In 2020, Lauderdale led a study that explored ocean nutrients, marine organisms and iron, and how their interactions influence phytoplankton growth around the world.

Phytoplankton are microscopic plant-like organisms that live on the ocean surface and feed on carbon and nutrients from the ocean depths, as well as iron from desert dust.

The more phytoplankton can grow, the more carbon dioxide they can absorb from the atmosphere through photosynthesis, which plays an important role in the ocean’s ability to sequester carbon.

For the 2020 study, the team developed a simple “box” model, representing conditions in different parts of the ocean as general boxes, each with a different balance of nutrients, iron and ligands – organic molecules thought to be byproducts of phytoplankton.

The team modeled a general flow between the boxes to represent the broader circulation of the ocean: how seawater flows and is then brought back to the surface in different parts of the world.

This modeling revealed that even if scientists “seeded” the oceans with extra iron, it would not have much effect on overall phytoplankton growth. The reason was a limit set by the ligands.

It turns out that, left to its own devices, iron is insoluble in the ocean and therefore unavailable to phytoplankton. Iron only becomes soluble at “useful” levels when it is bound to ligands, which keep the iron in a form that plankton can consume.

Lauderdale found that adding iron to one ocean region to consume additional nutrients deprives other regions of the nutrients that phytoplankton need to grow. This reduces ligand production and the supply of iron to the original ocean region, limiting the amount of additional carbon that would be absorbed by the atmosphere.

Unexpected change

Once the team published its study, Lauderdale worked the box model into a form he could make publicly accessible, including carbon exchange in the ocean and atmosphere and expanding the boxes to represent more diverse environments, such as conditions similar to those in the Pacific, North Atlantic and Southern Oceans.

During this process, he tested other interactions within the model, including the effect of varying ocean circulation.

He ran the model with different circulation strengths, expecting to see less atmospheric carbon dioxide with weaker ocean overturning—a relationship that previous studies have confirmed, going back to the 1980s. But what he found instead was a clear, opposite trend: The weaker the ocean circulation, the more CO2 is present.₂ accumulated in the atmosphere.

“I thought there was a mistake,” Lauderdale recalls. “Why were atmospheric carbon levels following the wrong trend?”

When he checked the model, he found that the parameter describing ocean ligands had been left “on” as a variable. In other words, the model calculated ligand concentrations as if they varied from one ocean region to another.

On a whim, Lauderdale turned off this parameter, which set ligand concentrations to constant values ​​across all modeled ocean environments, an assumption that many ocean models typically make. This change reversed the trend, returning to the assumed relationship: weaker circulation led to reduced atmospheric carbon dioxide. But which trend was closer to the truth?

Lauderdale studied the scant data available on ocean ligands to see whether their concentrations were more constant or variable in the ocean itself. He found confirmation in GEOTRACES, an international study that coordinates measurements of trace elements and isotopes in the world’s oceans, which scientists can use to compare concentrations across regions.

Indeed, the concentrations of the molecules vary. If ligand concentrations do indeed change from region to region, then this surprising new result is likely representative of the ocean reality: weaker circulation leads to increased carbon dioxide concentrations in the atmosphere.

“It was this weird trick that changed everything,” Lauderdale says. “The ligand change revealed this completely different relationship between ocean circulation and atmospheric CO.”₂ that we thought we understood well.

Slow cycle

To see what might explain this reverse trend, Lauderdale analyzed biological activity and concentrations of carbon, nutrients, iron, and ligands from the ocean model under different circulation strengths, comparing scenarios in which ligands were variable or constant across boxes.

This revealed a new phenomenon: the weaker the ocean circulation, the less carbon and nutrients the ocean draws from the depths. The phytoplankton present at the surface would then have fewer resources to grow and would therefore produce fewer by-products (including ligands).

With fewer ligands available, less iron on the surface would be available for use, which would further reduce the phytoplankton population. There would then be less phytoplankton available to absorb carbon dioxide from the atmosphere and consume carbon brought up from the deep ocean.

“My work shows that we need to look more closely at how ocean biology can affect climate,” Lauderdale says. “Some climate models predict a 30 percent slowdown in ocean circulation due to melting ice sheets, particularly around Antarctica.”

“This dramatic slowdown in the overturning circulation could actually be a big problem: in addition to a host of other climate problems, not only would the ocean absorb less anthropogenic CO2₂ from the atmosphere, but this could be amplified by a net outgassing of carbon from the deep ocean, leading to an unexpected increase in atmospheric CO.₂ and additional unexpected global warming.

This article is republished with kind permission from MIT News (web.mit.edu/newsoffice/), a popular site covering the latest research, innovation, and teaching at MIT.