As we face the challenges of climate change, it’s crucial to understand the potential impact on the ocean’s overturning circulation. Scientists predict a substantial weakening of this circulation, which could lead to the ocean pulling down less carbon dioxide from the atmosphere.
However, a slower circulation may also mean less carbon being released from the deep ocean into the atmosphere. This suggests that the ocean could still significantly reduce carbon emissions, albeit at a slower pace.
A recent study by an MIT researcher challenges our previous understanding and suggests that the relationship between the ocean’s circulation and its long-term carbon storage capacity may need to be re-evaluated. As the circulation weakens, more carbon could be released from the deep ocean into the atmosphere.
This phenomenon is tied to a complex interplay between the ocean’s iron content, upwelling carbon and nutrients, surface microorganisms, and a group of molecules known as “ligands.” When the ocean circulates more slowly, these factors interact in a way that increases the amount of carbon released back into the atmosphere. Understanding these dynamics is crucial for informing future climate change mitigation strategies.
“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,” says study author Jonathan Lauderdale, a research scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences.
In 2020, an important study by Lauderdale examined the complex relationship between ocean nutrients, marine organisms, and iron, and their impact on phytoplankton growth. The study revealed that adding iron to enhance phytoplankton growth is constrained by the presence of ligands, which limit the availability of iron for phytoplankton consumption. This highlights the intricate balance of nutrients, iron, and ligands in regulating global phytoplankton growth and carbon sequestration.
After the team published their study, Lauderdale diligently adapted the box model to be easily accessible to the public. He incorporated ocean and atmosphere carbon exchange and expanded the boxes to represent a wider range of environments, including conditions similar to the Pacific, the North Atlantic, and the Southern Ocean. During this process, he also tested various interactions within the model, including the impact of different ocean circulation patterns.
Lauderdale conducted model runs with varying circulation strengths, anticipating a decrease in atmospheric carbon dioxide with weaker ocean overturning, a relationship that had been supported by previous studies dating back to the 1980s. To his surprise, he discovered a clear and contrary trend: weaker ocean circulation led to an increase in atmospheric CO2 buildup.
Upon examining the model, Lauderdale discovered that the parameter for ocean ligands had been left as a variable, leading to varying ligand concentrations in different ocean regions. After disabling this parameter, the model assumed constant ligand concentrations in all ocean environments, a common assumption in many ocean models. This change reversed the trend, indicating that weaker circulation led to reduced atmospheric carbon dioxide.
To verify this, Lauderdale looked into data from the GEOTRACES study, which indicated that ligand concentrations varied across different regions of the ocean. This finding suggested that the 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 has revealed this completely different relationship between ocean circulation and atmospheric CO2 that we thought we understood pretty well.”
Lauderdale’s analysis of oceanic biological activity and nutrient concentrations under varying circulation strengths uncovered an intriguing finding. A previously unnoticed feedback loop emerged: as the ocean’s circulation weakens, it brings up fewer carbon and nutrients from the deep.
This results in reduced resources for surface phytoplankton, leading to a decline in their population. With fewer phytoplankton, the absorption of carbon dioxide and upwelled carbon from the deep ocean diminishes, reinforcing the impact of weakened circulation.
“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.”
Journal reference:
- Jonathan Maitland Lauderdale. Ocean iron cycle feedbacks decouple atmospheric CO2 from meridional overturning circulation changes. Nature Communications, 2024; DOI: 10.1038/s41467-024-49274-1