Why are oceans absorbing more CO2 from the 
atmosphere than scientists initially determined 
to be occurring via the well-known process of diffusion?

Below is a very interesting and important 
dialectical overview (interactive, complex, 
holistic) of ecology, CO2 emissions, the world's 
oceans, and global climate change.

The feedback loops are way more complicated and 
counterintuitive than the relatively 
straightforward physics of diffusion, which is 
what many scientists initially attributed the 
oceans' CO2 uptake to. The dialectical process 
described (without the author using that term), 
impacts on water density and is unpredictably accelerating the CO2 uptake.

One result (though this article doesn't mention 
it) is the accelerating acidification of the 
world's oceans, with disastrous consequences for complex life.

Mitchel Cohen

Fluctuations in upper ocean circulations are the 
driving force in the variability of ocean CO2 
uptake (and what's causing them to fluctuate?)


Scientists solve ocean ‘carbon sink’ puzzle

<https://www.carbonbrief.org/author/robertmcsweeney>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 
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 say, but there’s no guarantee that 
this will continue into the future.

Circulation patterns

The Earth’s oceans have 
about a third of the CO2 that humans have emitted 
into the atmosphere since the beginning of the 

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 
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 
40% 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 
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 
Tim DeVries, an assistant professor in 
oceanography at the 
<http://www.ucsb.edu/>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 analyse 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 say. 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

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). Data show uptake/release of carbon (black) 
and the difference caused by the circulation 
change (red/blue), in billion tonnes of carbon. Source: DeVries et al. (2017)

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

In an accompanying “News & Views” article, 
Sara Mikaloff-Fletcher, from the 
<https://www.niwa.co.nz/>National Institute of 
Water and Atmospheric Research in New Zealand, agrees. She writes:

“[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, says 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 explains:

“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, says 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 says:

“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, 
Nicolas Gruber, professor of environmental 
physics at <https://www.ethz.ch/en.html>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.”

DeVries, T. et al. (2017) Recent increase in 
oceanic carbon uptake driven by weaker 
upper-ocean overturning, Nature, 
Mikaloff-Fletcher, S.E. (2017) Ocean circulation 
drove increase in CO2 uptake, Nature