Carbon dioxide gas is persistently saturating our world, originating from both natural sources, such as respiration, and human activity, such as deforestation and the burning of fossil fuels. Regardless of its source, carbon dioxide (CO₂) levels have been skyrocketing throughout our planet over the past century. So where does it all go?

Recent research has showed notable elevation in oceanic absorption of carbon dioxide in the past decade, prompting scientists to speculate as to the cause. Geographer Timothy DeVries at UC Santa Barbara, along with colleagues Mark Holzer of the University of New South Wales in Sydney and François Primeau of UC Irvine, analyzed ocean data over the past three decades in an attempt to discern what is causing this effect.

The researchers compiled existing data in the form of tracer data, which come from ship measurements at regular intervals or robotic instruments called floats. Both tools collect information and measure various properties of the ocean over time, such as temperature, salinity and chlorofluorocarbons (man-made gases that get into the ocean). Their distribution in the ocean traces out the ocean currents, allowing the researchers to use a model to reconstruct what the currents have been doing over the span of three decades: the 1980s, 1990s and 2000s.

The model conveyed ocean circulation, and the researchers subsequently added in the carbon cycle. “The carbon cycle model is simple: If there is more CO₂ in the atmosphere than in the ocean, then the ocean picks it up, and if there is more CO₂ in the ocean than the atmosphere, the CO₂ goes back into the atmosphere,” DeVries said.

The amount of carbon dioxide absorbed by the ocean during the 2000s increased compared with that absorbed during the 1990s. An earlier report by Devries found that the large scale circulation of the ocean was weaker during the 2000s than during the 1990s, especially in the upper ocean, and therefore bought less carbon dioxide rich water from the ocean depths to the surface. / Courtesy of Sara Mikaloff Fletcherson

This model allowed the researchers to make a connection between oceanic carbon dioxide absorption and ocean circulation. It revealed that there has been a slowing down of ocean circulation over the past decade, specifically overturning circulation, which brings surface water into the deep ocean and back.

“In the deeper ocean, we have more CO₂ because various natural processes allow it to accumulate down there,” DeVries said. “So when you slow down the overturning, you’re bringing up less of that carbon dioxide to the surface, resulting in less CO₂ escaping back into the atmosphere.”

DeVries explained that there are two equivalent ways of viewing this process: Less carbon dioxide is escaping from the ocean due to slower circulation, or more carbon dioxide is being absorbed by the ocean from the atmosphere because the surface is now saturated with less carbon dioxide.

“We can’t really tell the difference between more is going in and less is coming out, but the net effect is the same: The ocean takes up more CO₂,” DeVries said.

The constructed model allowed the researchers to analyze the data and infer what the ocean circulation has been doing, but it did not tell them why the overturning circulation has slowed down. Could it be caused by human activity, or is it just natural variability?

Climate models predict that in the not too distant future, ocean circulation is going to slow down, like what scientists have observed.

“This is because the surface of the ocean is getting warmer as the atmosphere is getting warmer and also because ice sheets melt and they run off fresh water into the ocean,” DeVries said. “Both of those things tend to slow down the overturning circulation because you’re getting a more stable situation where you have less dense water on top of more dense water.”

These predictions are based off the assumption that the slower ocean circulation is anthropogenic, but DeVries clarifies that it could just be a result of the natural variability of the ocean.

“As recently as the 1990s, not too long ago, the ocean circulation was a lot more vigorous. So it could just be a natural cycle,” DeVries said. “Unfortunately at this point, we just don’t know why the slowing down of circulation is happening.”

Regardless of the reasons, at first glance, this increase in uptake of carbon dioxide by the ocean could seem like a good thing. More carbon dioxide in the ocean means less carbon dioxide in the atmosphere, which means less overall warming due to the greenhouse gas effect.

However, DeVries points out the nature of the double-edged sword: the more carbon dioxide in the ocean, the more acidic the ocean. Furthermore, ocean acidification is detrimental for many organisms in the ocean, including the magnificent reef-building corals.

Although there are still many more questions to be answered in the future, DeVries’ research successfully illuminated the effects of weaker ocean circulation on oceanic absorption of carbon dioxide. He plans to continue analyzing oceanic data and creating a better model that accounts for fluctuations during the various decades. This is in order to make more subtle distinctions about what exactly is happening with the ocean currents and how fast these changes are occurring.