Research shows weaker ocean circulation can boost CO₂ buildup in the atmosphere

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

However, a slower circulation should also pump less carbon out of the deep ocean that would otherwise be released back into the atmosphere. On balance, the ocean should continue to play its role in reducing carbon emissions from the atmosphere, albeit at a slower rate.

A new study from an MIT researcher published in Nature communication thinks scientists may need to rethink the relationship between the ocean’s circulation and its long-term capacity to store carbon. As the ocean weakens, it could instead release more carbon from the deep ocean into the atmosphere.

The reason for this is a previously unknown feedback loop between the ocean’s iron, upwelling carbon and nutrients, surface microorganisms, and a little-known class of molecules commonly known as ‘ligands’.

When the ocean flows more slowly, all of these factors work together in a self-perpetuating cycle that ultimately increases the amount of carbon the ocean releases back into the atmosphere.

“By isolating the impact of these feedbacks, we see a fundamentally different relationship between ocean circulation and atmospheric carbon, 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 overturned.”

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

Box current

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

Phytoplankton are microscopic plant-like organisms that live on the surface of the water. They feed on carbon and nutrients that rise from the deep ocean and iron that drifts in from desert dust.

The more phytoplankton can grow, the more carbon dioxide it can take up from the atmosphere through photosynthesis, which plays a major role in the ocean’s ability to capture carbon.

For the 2020 study, the team developed a simple “box” model, in which conditions in different parts of the ocean are represented as generic 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 larger ocean circulation — the way seawater sinks and then rises to the surface in different parts of the world.

This modeling revealed that even if scientists were to “seed” the oceans with extra iron, that iron would not have much effect on global phytoplankton growth. The reason was a limit imposed by ligands.

It turns out that iron, left on its own, is insoluble in the ocean and therefore unavailable to phytoplankton. Iron only becomes soluble at “useful” levels when it is coupled with ligands, which keep 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 nutrients that the phytoplankton there need to grow. This reduces ligand production and the supply of iron back to the original ocean region, limiting the amount of additional carbon that would be taken up from the atmosphere.

Unexpected switch

After the team published their research, Lauderdale refined the box model into a form he could make publicly available. Among other things, he mapped carbon exchange in the ocean and atmosphere and expanded the boxes to represent more diverse environments, such as conditions similar to those in the Pacific, North Atlantic, and Southern Ocean.

During this process he also 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 circulation — a relationship that previous studies have supported, going back to the 1980s. But what he found instead was a clear and opposite trend: The weaker the ocean circulation, the more CO₂ built up in the atmosphere.

“I thought there had been a mistake,” Lauderdale recalled. “Why were atmospheric carbon levels going in the wrong direction?”

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 a change from one ocean region to another.

On a whim, Lauderdale turned this parameter “off,” making the ligand concentrations constant in each modeled ocean environment, an assumption that many ocean models typically make. That one change reversed the trend, back to the assumed relationship: weaker circulation led to less atmospheric carbon dioxide. But which trend was closer to the truth?

Lauderdale looked at the scant available data on ocean ligands to see if their concentrations were more constant or variable in the real ocean. 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 from region to region.

The concentrations of the molecules did indeed vary. If ligand concentrations change from one region to another, then his surprising new result was likely representative of the real ocean: weaker circulation leads to more carbon dioxide in the atmosphere.

“It’s this one weird trick that changed everything,” Lauderdale says. “The ligand switch revealed this completely different relationship between ocean circulation and atmospheric CO₂ which we thought we understood pretty well.”

Slow cycle

To determine what might explain the reversal trend, Lauderdale analyzed biological activity and the concentrations of carbon, nutrients, iron, and ligands from the ocean model at different circulation strengths. He compared scenarios in which the ligands in the different boxes varied or were constant.

This revealed a new feedback: the weaker the ocean circulation, the less carbon and nutrients the ocean takes up from the depths. Phytoplankton at the surface would then have fewer resources to grow and would consequently produce fewer byproducts (including ligands).

With fewer ligands available, there would be less iron available at the surface, further reducing the phytoplankton population. There would then be less phytoplankton available to absorb carbon dioxide from the atmosphere and consume carbon that has risen from the deep ocean.

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

“This huge delay in the circulation reversal could actually be a big problem: on top of a host of other climate problems, the ocean would not only absorb less human-caused CO2, but also less CO2.₂ from the atmosphere, but this could be amplified by a net outgassing of carbon from the deep ocean, which could lead to an unexpected increase in atmospheric CO₂ and unexpected further warming of the climate.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that reports on research, innovation, and education at MIT.