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Scientists Solve Ocean 'Carbon Sink' Puzzle

Climate
Scientists Solve Ocean 'Carbon Sink' Puzzle
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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.

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In early October, Britain's Prince William teamed up with conservationist David Attenborough to launch the Earthshot Prize, a new award for environmentalist innovation. The Earthshot brands itself the "most prestigious global environment prize in history."

The world-famous wildlife broadcaster and his royal sidekick appear to have played an active role in the prize's inception, and media coverage has focused largely on them as the faces of the campaign.

But the pair are only the frontmen of a much larger movement which has been in development for several years. In addition to a panel of experts who will decide on the winners, the prize's formation took advice from the World Wildlife Fund, Greenpeace and the Jack Ma Foundation.

With more and more global attention on the climate crisis, celebrity endorsement of environmental causes has become more common. But why do environmental causes recruit famous faces for their campaigns? And what difference can it make?

'Count Me In'

"We need celebrities to reach those people who we cannot reach ourselves," says Sarah Marchildon from the United Nations Climate Change secretariat (UNFCCC) in Bonn, Germany.

Marchildon is a proponent of the use of celebrities to raise awareness of environmental causes. In addition to promoting a selection of climate ambassadors who represent the UN on sustainability issues, Marchildon's team has produced videos with well-known narrators from the entertainment world: among them, Morgan Freeman and Mark Ruffalo.

"We choose celebrities who have a lifestyle where they are already talking about these issues," Marchildon explains.

"Sometimes they reach out to us themselves, as David Attenborough did recently. And then they can promote the videos on their own social channels which reach more people than we do — for example, if they have 20 million followers and we have 750,000."

Environmental groups focused on their own domestic markets are also taking this approach. One Germany-based organization that uses celebrities in campaigns is the German Zero NGO. Set up in 2019, it advocates for a climate-neutral Germany by 2035.

German Zero produced a video in March 2020 introducing the campaign with "66 celebrities" that supported the campaign, among them Deutschland 83 actor Jonas Nay and former professional footballer Andre Schürrle. They solicit support as well as financial contributions from viewers.

"Count me in," they say, pointing toward the camera. "You too?"

"We are incredibly grateful for the VIPs in our videos," says German Zero spokeswoman Eva-Maria McCormack.

Assessing Success Is Complex

But quantifying the effectiveness of celebrity endorsement of campaigns is not a straightforward process.

"In order to measure effectiveness, first of all you need to define what is meant by success," says Alegria Olmedo, a researcher at the Zoology Department at the University of Oxford.

Olmedo is the author of a study looking at a range of campaigns concerning pangolin consumption, fronted by local and Western celebrities, in Vietnam and China. But she says her biggest stumbling block was knowing how to measure a campaign's success.

"You need a clear theory of change," explains Olmedo. "Have the celebrities actually helped in achieving the campaign's goals? And how do you quantify these goals? Maybe it is increased donations or higher engagement with a cause."

A popular campaign in China in recent years saw famous chefs Zhao Danian and Shu Yi pledge to abstain from cooking endangered wildlife. While the pledge achieved widespread recognition, both Olmedo and Marchildon say it's difficult to know whether it made any difference to people's actions.

"In life we see a thousand messages every day, and it is very hard to pinpoint whether one campaign has actually made a difference in people's behavior," she explains.

Awareness Is Not Enough

Many campaigns that feature celebrities focus on raising awareness rather than on concrete action — which, for researcher Olmedo, raises a further problem in identifying effectiveness.

"Reach should never be a success outcome," she says. "Many campaigns say they reached a certain number of people on social media. But there has been a lot of research that shows that simply giving people information does not mean they are actually going to remember it or act upon it."

But anecdotal evidence from campaigns may suggest reach can make an active difference.

"Our VIP video is by far the most watched on our social media channels," McCormack from German Zero says. "People respond to it very directly. A lot of volunteers of all ages heard about us through that video."

However, some marketing studies have shown that celebrity endorsement of a cause or product can distract from the issue itself, as people only remember the person, not the content of what they were saying.

Choosing the Right Celebrity

Celebrity choice is also very important. Campaigns that use famous faces are often aiming to appeal to members of the public who do not necessarily follow green issues.

For certain campaigns with clear target audiences, choosing a climate scientist or well-known environmentalist rather than a celebrity could be more appealing — Attenborough is a classic example. For others, images and videos involving cute animals may be more likely to get a message heard than attaching a famous face.

"We choose celebrities who have a lifestyle where they are already talking about these issues," says Marchildon from the UN. "You need figures with credibility."

McCormack cites the example of Katharine Hayhoe, an environmental scientist who is also an evangelical Christian. In the southern United States, Hayhoe has become a celebrity in her own right, appealing to an audience that might not normally be interested in the messages of climate scientists.

But as soon as you get a celebrity involved, campaigns also put themselves at risk of the whims of that celebrity. Prince William and younger members of the royal family have come under fire in recent years for alleged hypocrisy for their backing of environmental campaigns while simultaneously using private jets to fly around the world.

But Does It Really Work?

While environmental campaigns hope that endorsement from well-known figures can boost a campaign, there is little research to back this up.

"The biggest finding [from my study] was that we were unable to produce any evidence that shows that celebrity endorsement of environmental causes makes any difference," says Olmedo.

This will come as a blow to many campaigns that have invested time and effort into relationships with celebrity ambassadors. But for many, the personal message that many celebrities offer in videos like that produced by German Zero and campaigns like the Earthshot Prize are what counts.

The research may not prove this conclusively — but if the public believes a person they respect deeply personally cares about an important issue, they are perhaps more likely to care too.

"I personally believe in the power this can have," says Marchildon. "And if having a celebrity involved can get a single 16-year-old future leader thinking about environmentalist issues — that is enough."

Reposted with permission from DW.

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