New research challenges current thinking on the ocean’s role in storing carbon.
A new MIT study challenges previous understanding of the ocean’s role in climate change by demonstrating that weaker oceanic circulation might not decrease, but instead increase atmospheric CO2 levels. This finding arises from the interaction of various oceanic components like iron, ligands, and microorganisms, which together may lead to an unexpected rise in CO2 if ocean circulation diminishes.
Ocean Circulation and Climate Change
As climate change advances, the ocean’s overturning circulation is predicted to weaken substantially. With such a slowdown, scientists estimate the ocean will pull down less carbon dioxide from the atmosphere. However, a slower circulation should also dredge up less carbon from the deep ocean that would otherwise be released back into the atmosphere. On balance, the ocean should maintain its role in reducing carbon emissions from the atmosphere, if at a slower pace.
Rethinking Oceanic Carbon Storage
However, a new study by an MIT researcher finds that scientists may have to rethink the relationship between the ocean’s circulation and its long-term capacity to store carbon. As the ocean gets weaker, it could release more carbon from the deep ocean into the atmosphere instead.
The reason has to do with a previously uncharacterized feedback between the ocean’s available iron, upwelling carbon and nutrients, surface microorganisms, and a little-known class of molecules known generally as “ligands.” When the ocean circulates more slowly, all these players interact in a self-perpetuating cycle that ultimately increases the amount of carbon that the ocean outgases back to the atmosphere.
Implications for Climate Action
“By isolating the impact of this feedback, we see a fundamentally different relationship between ocean circulation and atmospheric carbon levels, with implications for the climate,” says study author Jonathan Lauderdale, a research scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences. “What we thought is going on in the ocean is completely overturned.”
Lauderdale says the findings show that “we can’t count on the ocean to store carbon in the deep ocean in response to future changes in circulation. We must be proactive in cutting emissions now, rather than relying on these natural processes to buy us time to mitigate climate change.”
His study was published recently in the journal Nature Communications.
Reevaluating Phytoplankton’s Role
In 2020, Lauderdale led a study that explored ocean nutrients, marine organisms, and iron, and how their interactions influence the growth of phytoplankton around the world. Phytoplankton are microscopic, plant-like organisms that live on the ocean surface and consume a diet of carbon and nutrients that upwell from the deep ocean and iron that drifts in from desert dust.
The more phytoplankton that can grow, the more carbon dioxide they can absorb from the atmosphere via photosynthesis, and this plays a large 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 that are thought to be byproducts of phytoplankton. The team modeled a general flow between the boxes to represent the ocean’s larger circulation — the way seawater sinks, then is buoyed back up to the surface in different parts of the world.
Challenges of Ocean Seeding
This modeling revealed that, even if scientists were to “seed” the oceans with extra iron, that iron wouldn’t have much of an effect on global phytoplankton growth. The reason was due to a limit set by ligands. It turns out that, if left on its own, iron is insoluble in the ocean and therefore unavailable to phytoplankton. Iron only becomes soluble at “useful” levels when linked 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 robs other regions of nutrients that phytoplankton there need to grow. This lowers the production of ligands and the supply of iron back to the original ocean region, limiting the amount of extra carbon that would be taken up from the atmosphere.
Reversing Assumptions in Ocean Modeling
Once the team published their study, Lauderdale worked the box model into a form that he could make publicly accessible, including ocean and atmosphere carbon exchange and extending the boxes to represent more diverse environments, such as conditions similar to the Pacific, the North Atlantic, and the Southern Ocean. In the 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 supported, dating back to the 1980s. But what he found instead was a clear and opposite trend: The weaker the ocean’s circulation, the more CO2 built up in the atmosphere.
New Insights from Variable Ligand Concentrations
“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 as changing from one ocean region to another.
On a hunch, Lauderdale turned this parameter “off,” which set ligand concentrations as constant in every modeled ocean environment, an assumption that many ocean models typically make. That one change reversed the trend, back to the assumed relationship: A weaker circulation led to reduced atmospheric carbon dioxide. But which trend was closer to the truth?
Lauderdale looked to the scant available data on ocean ligands to see whether their concentrations were more constant or variable in the actual ocean. He found confirmation in GEOTRACES, an international study that coordinates measurements of trace elements and isotopes across the world’s oceans, that scientists can use to compare concentrations from region to region. Indeed, the molecules’ concentrations varied. If ligand concentrations do change from one region to another, then his surprise new result was likely representative of the real ocean: A weaker circulation leads to more carbon dioxide in the atmosphere.
“It’s this one weird trick that changed everything,” Lauderdale says. “The ligand switch has revealed this completely different relationship between ocean circulation and atmospheric CO2 that we thought we understood pretty well.”
Exploring the Effects of Circulation on Climate
To see what might explain the overturned trend, Lauderdale analyzed biological activity and carbon, nutrient, iron, and ligand concentrations from the ocean model under different circulation strengths, comparing scenarios where ligands were variable or constant across the various boxes.
This revealed a new feedback: The weaker the ocean’s circulation, the less carbon and nutrients the ocean pulls up from the deep. Any phytoplankton at the surface would then have fewer resources to grow and would produce fewer byproducts (including ligands) as a result. With fewer ligands available, less iron at the surface would be usable, further reducing the phytoplankton population. There would then be fewer phytoplankton available to absorb carbon dioxide from the atmosphere and consume upwelled carbon from the deep ocean.
“My work shows that we need to look more carefully at how ocean biology can affect the climate,” Lauderdale points out. “Some climate models predict a 30 percent slowdown in the ocean circulation due to melting ice sheets, particularly around Antarctica. This huge slowdown in overturning circulation could actually be a big problem: In addition to a host of other climate issues, not only would the ocean take up less anthropogenic CO2 from the atmosphere, but that could be amplified by a net outgassing of deep ocean carbon, leading to an unanticipated increase in atmospheric CO2 and unexpected further climate warming.”
Reference: “Ocean iron cycle feedbacks decouple atmospheric CO2 from meridional overturning circulation changes” by Jonathan Maitland Lauderdale, 8 July 2024, Nature Communications.
DOI: 10.1038/s41467-024-49274-1
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5 Comments
It’s not “unexpected” if they know about it, now, is it?
Do they know it, though? Do they know anything? Seems to me like they’re discovering previously unknown climate variables all the time. SciTechDaily is full of articles like that. Which means the existing climate models are just garbage, yet the worldwide policies are now being brought on based on them.
All knowledge is ultimately based on whether or not we see repetition of observed phenomena. If we see certain phenomena repeating under certain circumstances, we expect the repetition to continue. We consider the expected outcomes to be “known” to some extant or another.
This applies to climate modeling. Looking back on the history of climate modeling (AND system modeling in general), we can clearly see that as we repeatedly attempt to create better and more robust models of systems, the predicted results tend to converge to a set of outcomes that are reproducible. This trend is not pure monotonic. The results can overshoot, undershoot, or go “sideways”. However, the fact is that the overall pattern is there. We rely on this pattern as the best way to improve our knowledge. And when we update our knowledge with faith in this process, we will sometimes have our expectations reversed.
It is easy to reject this method because it is not 100% convergent for every iteration of the process, and sometimes the iterations eventually converge to a radically different than they had been converging to earlier. However, this is the best system we have. Do you have a better method for creating knowledge? This method has advanced humanity since we began In fact, it is implicitly built-in to most organisms in one way or another. Each organisms of detection-response (e.g., homeostasis, defense, hunting) works because the organisms are designed to repeatedly respond the same way to stimuli. This is essentially because the organisms evolved to have a certain kind of pattern matching. That doesn’t mean each response to stimuli is optimized, or even helpful, but this underlies evolution and survival itself. So, if you have a better method of learning and optimizing our decisions, please let the world know what it is. You will get fame and glory, surely. Unless it’s a time machine you event, though, I doubt your “better” and “more constructive” method will be actually be better or more constructive. I will keep putting my faith in the process of repeatedly improving models and trying new ones and basing my knowledge each day on the latest state of knowledge we get from them. Apparently, nearly everyone else prefers that method as well. Good luck, Boba.
Nice try, bud, but you’ve forgotten one thing – computer models are NOT science. They may be based on scientific findings and empirical data, but science they’re not. They’re just elaborated guessworks. No knowledge can be “created” based on guesswork.
And you’re basically admitting yourself that the models are all over the place. No, they’re “not 100%” accurate. They’re anywhere between 0% and 99%, and we don’t know exactly where. You can hope they’re close to 100% but I know they’re not even in their teens, because f*cking Manhattan is still not under the f*cking water.
So, of course we should reject climate models as a basis for policy-making. Not only it’s easy to do, it’s common sense. Not that the politicians ever cared about common sense, though.
The overall “pattern” you’re talking about, or a certain agreement between various climate models, can just as easily be attributed to confirmation bias. No one wants to go against the grain and risk future funding.
“A new MIT study challenges previous understanding of the ocean’s role in climate change by demonstrating that weaker oceanic circulation MIGHT not decrease, but instead increase atmospheric CO2 levels.”
Once again a scientific study is couched in lawyer terms — “might” — instead of the numbers that are the veritable language of science. What should be presented is the probability (along with error bars) of atmospheric CO2 levels increasing, given specific conditions or at least assumed scenarios, and the behavior under different conditions. Just saying that something “might” change sign in an unspecified situation is not of much value in predicting measurable changes that can be used in a null-hypothesis to test the idea. What other things might impact the system? What are reasonable feedback loops that could influence, in particular buffer, the system behavior. I think that this report is premature. It is at though the author was under some pressure to write something, even if it was just a status report on preliminary findings, which is how I would characterize this. “No wine before its time!”
“On a hunch, Lauderdale turned this parameter “off,” which set ligand concentrations as constant in every modeled ocean environment, an assumption that many ocean models typically make. That one change reversed the trend, back to the assumed relationship: …”
Instead of depending on an ‘On-Off’ switch, it looks like they need to do some sensitivity studies to see how the results vary with a range of ligand concentrations. This initial work is interesting, but it is incomplete.
IMHO, the report shares knowledge. Scitechdaily constantly mines journals and other media channels for new material, which can be as preliminary as this result. Then Scitechdaily publishes whatever they can to keep us visiting the site. That includes publishing titles/headlines that are designed to generate consumption. While report’s authors are obviously living in the world of “publish or perish” and are probably under not-insignificant pressure to publish something, it doesn’t necessarily cause harm since it is a form of sharing knowledge or at least functioning as a reminder of sorts. What might be significantly unproductive in this early phase of the research is how the media handles the report. IMHO.