A slower ocean circulation due to warming will emit more CO2

8 Jul. () –

As ocean circulation weakens due to climate change, it could release more carbon from the deep ocean into the atmosphere, according to a new study in Nature Communications.

The reason has to do with a previously uncharacterized feedback between available iron in the ocean, upwelling carbon and nutrients, surface microorganisms, and a poorly understood class of molecules generally 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 that the ocean returns to 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,” he says. it’s a statement study author Jonathan Lauderdale, a research scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences.

“What we thought was happening in the ocean has completely reversed.”

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 must be proactive in reducing emissions now, rather than relying on these natural processes to buy time to mitigate climate change.”

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 consume a diet of carbon and nutrients that rise up from the deep ocean and iron that reaches the atmosphere through desert dust.

The more phytoplankton can grow, the more carbon dioxide they can absorb from the atmosphere through photosynthesis, and this 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 (the way seawater sinks and is then propelled back to the surface in different parts of the world).

This model revealed that even if scientists “seeded” the oceans with additional iron, that iron would not have much effect on global phytoplankton growth. The reason was due to a limit set by the ligands.

It turns out that, left alone, 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 iron supply to the original ocean region, limiting the amount of additional carbon that would be absorbed from the atmosphere.

Once the team published their study, Lauderdale worked on the box model to make it accessible to the public, including carbon exchange between 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 Ocean.

In the process, he tested other interactions within the model, including the effect of variations in ocean circulation.

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

“I thought there was some mistake,” Lauderdale recalls. “Why were atmospheric carbon levels trending the wrong way?”

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 was calculating ligand concentrations that changed from one ocean region to another.

Following a hunch, Lauderdale turned off this parameter, which set ligand concentrations as constant in each modeled ocean environment, an assumption that many ocean models often make.

That change reversed the trend and returned to the assumed relationship: a weaker circulation led to a reduction in atmospheric carbon dioxide. But which trend was closer to the truth?

Lauderdale analyzed the scant data available on ocean ligands to see whether their concentrations were more constant or variable in the real ocean. She found confirmation in GEOTRACES, an international study that coordinates trace element and isotope measurements in the world’s oceans.which scientists can use to compare concentrations from one region to another.

In fact, the concentrations of the molecules varied. If ligand concentrations change from region to region, then their surprising new result was likely representative of the real ocean: weaker circulation leads to more carbon dioxide in the atmosphere.

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

To see what might explain the reversed trend, Lauderdale analyzed the biological activity and concentrations of carbon, nutrients, iron and ligands in the ocean model under different circulation intensities, comparing scenarios where ligands were variable or constant in the different cells.

A BIG PROBLEM

This revealed a new feedback: the weaker the ocean circulation, the less carbon and nutrients the ocean draws up from the depths. Any phytoplankton at the surface would then have fewer resources to grow on and would produce fewer byproducts (including ligands) as a result.

With fewer ligands available, less iron on the surface would be usable, further reducing the phytoplankton population. So, There would be less phytoplankton available to absorb carbon dioxide. from the atmosphere and consume carbon rising from the depths of the ocean.

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

“This massive slowdown in ocean circulation could actually be a big problem.”:In addition to a host of other climate problems, not only would the ocean absorb less anthropogenic CO2 from the atmosphere, but this could be amplified by a net outgassing of carbon from the deep ocean, leading to an unforeseen increase in atmospheric CO2 and even greater climate warming.”