The Southern Ocean has been slowing
Global Warming by Absorbing Carbon, But that Could Change
During a research expedition in 1994 in the Southern Ocean it was
noticed that the surface waters were low in oxygen, had a high carbon
content and were extremely acidic – which were surprising signs that
deep ocean waters rich in nutrients, that are typically found in the
deep ocean, were at the surface. The water the research vessel was
passing through were actually ancient water that had been in the deep
ocean for centuries.
The upwelling in the Southern Ocean was controversial in 1994 but has
now been recognised as a hallmark of the Southern Ocean which is driven
by the strongest sustained winds in the world to move continuously
around the landmass of Antarctica. Large amounts of heat and carbon
dioxide transfers from the atmosphere to the waters of the Southern
Ocean, and this has slowed the progress of global warming. Much of the
global circulation of the global ocean is driven by the powerful
currents of the Southern Ocean.
It has been difficult for scientists to carry out research in the ocean
around Antarctica for decades but at last it has become possible for the
research to get under way. It has become possible for the research to be
carried out with floats, moorings, ships, gliders, satellites, computer
models and sensors that are attached to seals. This research is
increasing understanding of Southern Ocean and the global climate by
supplying data to fill very large data gaps. According to Tollefson this
research could be key to improving estimates of how quickly the world
will warm, the length of time the Antarctic ice will survive and the
rate of sea level rise that could be expected.
It is suggested by early data from an array of floats that the amount of
CO2 that could be absorbed per year could be limited by the
upwelling waters. This data is raising new questions about how effective
these waters will be in the future in slowing global warming.
Tollefson says the unique geography of the Antarctic region makes it a
perilous place for ships. There are no landmasses to slow the winds and
waves that circle Antarctica at high speed at around the latitude of 60oS.
And there is a history of ships being engulfed by Antarctic ice, which
includes Endurance, the ship
of Ernest Shackleton in 1915.
The importance of the Antarctic region in controlling global climate was
began to be realised in the 1980s when several groups were looking for
an explanation of what had caused the atmospheric concentration of CO2
to decrease by about 33 % during the last ice age and rise again
following the close of the ice age. It was realised several decades
later that circulation and biology changes in the Southern Ocean could
help to cool and warm the Earth (Sarmiento & Toggweller, 1984).
Some of the first data that has resulted from the new research indicate
that CO2 fluxes into the atmosphere are much higher than
expected, especially in winter. It has been suggested that these results
would imply that the Southern Ocean is a much weaker sink than has been
The unpublished data that has been analysed was from 13 floats that have
been in the water for at least 1 year, now it needs to be determined if
the high CO2 emissions from the Southern Ocean are
representative of larger trends across the entire Southern Ocean
Previously there have been hints of something similar, such as a study
that was published in2007 in
Science (Le Quéré et al., 2007). This study found that the rate at
which the Southern Ocean took up CO2 decreased between 1981
and 2004. The authors of that study blamed the changes on the winds that
encircle Antarctica. Over that period of time the speed of the winds had
increased, probably as a result of the ozone hole in the stratosphere
above Antarctica and possibly because of global warming. Stronger winds
are better able to drag up the deep, ancient water, which releases CO2
as it reaches the surface, which would cause a net weakening of the
Atmospheric CO2 levels would rise even faster in the future
if that trend were to continue. Though a study published in
Science (Landschützer et
al., 2015) found that the
carbon sink began to strengthen in the early 2000s (see “The unreliable
It is not clear if this CO2 increased absorption is a return
to normal or a deviation from the long-term weakening of the sink.
Whichever it is it does show that the Southern Ocean may be much more
fickle as a sink than has been believed.
Carbon is not the only factor affecting the Southern Ocean. The fate of
all the heat that is absorbed by the Southern Ocean is beginning to be
A network of currents carrying water, heat and nutrients throughout the
ocean basins has its starting point in the Southern Ocean. Surface
waters near Antarctica usually become cold and dense enough to sink to
the ocean floor where they form abyssal currents that stay close to the
ocean floor as they flow to the north into all the other major oceans,
The Pacific, Atlantic and the Indian Oceans.
Ship surveys that have been conducted every decade or so since the early
1990s are sources of much of the knowledge that has been accumulated.
When data from the surveys were analysed in 2010 it was found that there
was a pronounced warming trend in abyssal waters, which were thus shown
to be absorbing about 10 % of the excess heat generated by global
warming (Purkey & Johnson, 2010).
Several proposals have been suggested to explain the level of warming in
the deep ocean that was unexpected, centring on the Southern Ocean. The
decrease in salt content of surface waters around Antarctica partially
results from increased rainfall over the ocean during summer. The supply
of cold water to the sea floor to feed the bottom currents could be
reduced as a result of change in the fresher surface waters which have
become less dense. As a result of not getting as much replenishment by
cold water could lead to warming of the deep water.
According to an analysis that is yet to be published, that was based on
the data from the 3rd round of ship surveys, similar trends
were found, though for a fuller picture requires more frequent
Elsewhere moorings have been used to monitor deep water flows. Since
1999 an array of moorings have been maintained in the Weddell Sea which
is one of the main areas where cold surface waters sink to form the
ocean bottom currents. These have recorded a decrease in the salt
content of the water in some areas, though the trend in the long term is
not clear (Gordon, Huber, McKee & Visbeck, 2010).
Along the edge
In January 2015 the Australian icebreaker
Aurora Australis followed a
crack in the ice off Antarctica and reached the edge of the Totten
Glacier, which is one of the biggest drainage points of the East
Antarctic ice sheet. Previously other expeditions had not been able to
get to closer than 50 km of this glacier.
Floats and gliders were deployed into the waters around and beneath the
glacier, the front edge of which is 200 m thick. They made the shocking
discovery that the water at the front of the glacier was 3oC
warmer than the freezing point at the base of the glacier. Prior to this
discovery it had been believed that the glacier was too far from warm
water to be susceptible, but their instruments had reported that the
water was warm all over the shelf in their study area.
It had already been found (Joughin, Smith & Medley, 2014; Rignot et
al., 2014) that warm water
currents are in the process of undercutting the West Antarctic ice sheet
in many areas along the Antarctic Peninsula where the glaciers extend
into the ocean. This expedition found some of the first evidence that
the East Antarctic ice sheet was being affected by the same processes,
which raises new questions about long the ice sheets covering the
Antarctic continent will remain.
At present there is no clear answer for what is driving the warming of
these currents that are near the surface. Changes in the winds over the
Southern Ocean and the upwelling of warm waters have been invoked by
some explanations. The focus of other explanations is on fresher surface
waters and expanding sea ice in some areas. A kind of cap on the ocean
formed by a combination of extra sea ice and fresher surface waters
which funnels some of the warmer upwelling waters towards the coast.
There are plans for the team at the British Antarctic Survey to attach
sensors to Weddell seals to collect water measurements as they forage
beneath the sea ice along the continental shelf. As this zone is where
cold water begins its descent into the abyss it is a particularly
As well as the Weddell seals autonomous gliders will be sent under the
ice on pre-programmed routes in order to collect temperature and
salinity data to depths of 1,000 m. There will also be measurements
taken from ships to make the picture more complete of what is happening
in this crucial region around Antarctica, and its relationship with the
remainder of the circulation of the global ocean.
It has been found by analysing data from ship surveys that upwelling
ocean water does not rise in a simple pattern near Antarctica. Once the
water of the upwelling begins its journey to the surface it only reaches
the surface after swirling around Antarctica 1.5 times. It has been
found that high resolution models are required to capture accurately
Sources & Further reading
Tollefson, Jeff, 2016,
How much longer can Antarctica’s hostile ocean delay global warming?
539, 346–348 (17 November 2016)