Quantcast

Scientists Solve Ocean 'Carbon Sink' Puzzle

Science
iStock

By Robert McSweeney

The oceans are a hugely important "carbon sink," helping absorb CO2 emissions from human activities. Without them, CO2 would accumulate more quickly in the atmosphere, raising temperatures more quickly.

A new study, published in Nature, finds that recent changes in circulation patterns in the world's oceans are playing a key role in how much CO2 they take up.

Weakening circulation patterns have boosted how much CO2 the oceans absorb since the 2000s, the researchers said, but there's no guarantee that this will continue into the future.

Circulation Patterns

The Earth's oceans have absorbed about a third of the CO2 that humans have emitted into the atmosphere since the beginning of the Industrial Revolution.

But the amount of CO2 that the oceans absorb isn't constant. In the 1990s, ocean CO2 uptake dropped off, before increasing again in the 2000s. Recent research shows that the Southern Ocean was central to these changes.

The Southern Ocean is the most prolific of the oceans for carbon storage—accounting for approximately 40 percent of the global ocean CO2 uptake. In the 1990s, strengthening winds circulating around Antarctica affected ocean currents and brought carbon-rich water to the surface. This meant the ocean was less able to absorb CO2 from the atmosphere.

In the 2000s, the winds continued to strengthen, yet the CO2 uptake in the Southern Ocean rebounded. This, combined with increasing CO2 uptake in other oceans, suggested to scientists that there was, ultimately, another factor affecting the ocean carbon sink.

The new study says the reason lies in circulation patterns in the top 1,000m of the world's oceans.

Missing Piece of the Puzzle

The water in our oceans is constantly on the move. In the upper layers of the ocean there are several driving forces responsible, explains lead author Dr. Tim DeVries, an assistant professor in oceanography at the University of California. He tells Carbon Brief:

"The [circulation patterns] are driven by winds and by 'buoyancy forcing'—which means changes in the density of surface waters due to changes in their temperature (heating/cooling) or salinity (adding/removing freshwater)."

Using observed data, the researchers built a computer model to simulate these circulation patterns in the upper ocean. They ran their model to analyze the exchange of CO2 between the ocean and atmosphere over recent decades.

They found that in the 1990s, the ocean circulation patterns were "more vigorous" and coincided with a big dip in CO2 uptake. From around 2000, the circulation patterns then weakened, bringing a rebound in CO2 uptake.

The simplified diagram below illustrates the effect these "overturning" circulation patterns have.

Stronger ocean overturning—as seen during the 1990s—brings more carbon-rich water up from the deeper ocean, the researchers said. When this water reaches the surface it releases CO2 into the atmosphere (see a). More vigorous overturning also means the ocean takes up more CO2 from the atmosphere (b), but not as much as the extra CO2 released.

As the bottom half of the diagram shows, weaker overturning in the 2000s reduces both the amount of CO2 released to the atmosphere (c) and what is absorbed again (d). Overall, this increases how much CO2 the ocean takes up.

Simplified conceptual diagram illustrating how changes in upper-ocean overturning circulation have affected the ocean CO2 sink. Figure shows the a) increased release and b) increased uptake of CO2 during the 1990s—with an overall reduced CO2 sink, and the opposite in the 2000s (c and d). DeVries et al.

The results show that fluctuations in upper ocean circulations are "absolutely the driving force in the variability of ocean CO2 uptake," said DeVries.

In an accompanying "News & Views" article, Dr. Sara Mikaloff-Fletcher, from the National Institute of Water and Atmospheric Research in New Zealand, agrees. She wrote:

"[The paper] is the first to robustly quantify the role of circulation change in the recent decadal shift in CO2 uptake, providing the missing piece of this puzzle."

Major Advance

The paper is a "major advance" in the understanding of changes in the ocean carbon sink, said Mikaloff-Fletcher, but it isn't able to give us any clues for the future:

"It remains unclear for how long the increased carbon uptake observed during the 2000s will persist."

In general, scientists expect that as CO2 levels increase in the atmosphere, more will dissolve into the ocean. DeVries explained:

"The rate at which CO2 is transferred from the air into seawater depends on the difference in the concentration of CO2 in the air and that in the water. So, as humans put more CO2 in the atmosphere, this concentration difference increases and the ocean absorbs more CO2."

If the weak circulation patterns continue, this "may help to enhance the oceanic CO2 sink for some time," the paper says. But there is also the distinct possibility that the changes we are seeing now are temporary, said DeVries:

"The overturning circulation [could] switch back to a more vigorous state in the next decade. In this case, the changes would be reversed and we would go back to a weaker ocean CO2 sink (like in the 1990s)."

This would lead to a faster accumulation of carbon emissions in the atmosphere—and more rapidly-increasing temperatures.

Human-Caused Warming

The researchers don't yet know whether the recent weakening of the ocean circulation patterns are caused by natural variability or human-caused warming.

Global warming is expected to have a similar weakening effect on the circulation patterns as has been seen since the 2000s, DeVries said:

"Human CO2 emissions cause warming … of the surface ocean and makes it less dense. At the same time, the warming melts glaciers and ice caps, which pour fresh water into the ocean. This also makes the surface waters less dense. As surface waters get lighter, they are less likely to sink. This weakens the overturning circulation."

However, at the moment, it's likely that natural variability in the oceans is the dominant factor, said Prof. Nicolas Gruber, professor of environmental physics at ETH Zürich, who wasn't involved in the study. He tells Carbon Brief:

"My working hypothesis is that it is natural variability, but only time will tell. I say this because model simulations suggest that the point where the human-caused impact on the ocean carbon sink is clearly separable from natural variability is rather distant in the future."

Robert McSweeney covers climate science. He holds an MEng in mechanical engineering from the University of Warwick and an MSc in climate change from the University of East Anglia. He previously spent eight years working on climate change projects at the consultancy firm Atkins. Reposted with permission from our media associate Carbon Brief.

EcoWatch Daily Newsletter

Secretary Carson surveys recovery in Florida Panhandle following Hurricane Michael. U.S. HUD / Flickr / Public Domain

By Eren Erman Ozguven

When Hurricane Michael roared onto northwest Florida's Gulf Coast in October 2018, its 160 mile-per-hour winds made it the strongest storm ever to hit the region. It was only the fourth Category 5 storm on record to make landfall in the U.S.

Read More Show Less
Pexels

By Ketura Persellin

Global consumption of beef, lamb and goat is expected to rise by almost 90 percent between 2010 and 2050. But that doesn't mean you need to eat more meat. In fact, recent news from Washington gives you even less confidence in your meat: Pork inspections may be taken over by the industry itself, if a Trump administration proposal goes into effect, putting tests for deadly pathogens into the hands of line workers.

Read More Show Less
Sponsored
Pexels

By Kaitlyn Berkheiser

While enjoying an occasional alcoholic beverage is unlikely to harm your health, drinking in excess can have substantial negative effects on your body and well-being.

Read More Show Less
MStudioImages / E+ / Getty Images

By Jillian Kubala, MS, RD

Backpacking is an exciting way to explore the wilderness or travel to foreign countries on a budget.

Read More Show Less
Tim P. Whitby / 21st Century Fox / Getty Images

The beauty products we put on our skin can have important consequences for our health. Just this March, the U.S. Food and Drug Administration (FDA) warned that some Claire's cosmetics had tested positive for asbestos. But the FDA could only issue a warning, not a recall, because current law does not empower the agency to do so.

Michelle Pfeiffer wants to change that.

The actress and Environmental Working Group (EWG) board member was spotted on Capitol Hill Thursday lobbying lawmakers on behalf of a bill that would increase oversight of the cosmetics industry, The Washington Post reported.

Read More Show Less
Sponsored
A protest march against the Line 3 pipeline in St. Paul, Minnesota on May 18, 2018. Fibonacci Blue / CC BY 2.0

By Collin Rees

We know that people power can stop dangerous fossil fuel projects like the proposed Line 3 tar sands oil pipeline in Minnesota, because we've proved it over and over again — and recently we've had two more big wins.

Read More Show Less
Scientists released a study showing that a million species are at risk for extinction, but it was largely ignored by the corporate news media. Danny Perez Photography / Flickr / CC

By Julia Conley

Scientists at the United Nations' intergovernmental body focusing on biodiversity sounded alarms earlier this month with its report on the looming potential extinction of one million species — but few heard their calls, according to a German newspaper report.

Read More Show Less
DoneGood

By Cullen Schwarz

Ethical shopping is a somewhat new phenomenon. We're far more familiar with the "tried and tested" methods of doing good, like donating our money or time.

Read More Show Less