Benefits hidden deep in ice bergs

January 13, 2016 12:00 pm
Minerals released by giant icebergs capturing carbon, slowing rate of global warming.

 By using satellites, genealogists can observe icebergs like B31 as they travel across the Southern Ocean. Photo / Getty Images

In November of 2013, a mind-bogglingly large iceberg split off of the
front of Pine Island Glacier in West Antarctica – one of the world’s
fastest flowing glaciers.
Dubbed B31, the iceberg was “roughly the size of Singapore”, according to Nasa.
At
the time, the massive iceberg was mainly viewed as yet another global
warming sign – after all, the melting of West Antarctica, due to warm
ocean waters that are reaching glaciers like Pine Island and melting
them from below, is perhaps the world’s No 1 sea-level threat.
New
research, however, suggests that while global warming is probably
leading to more gigantic icebergs breaking off of Antarctica (and more
icebergs in general), there could be a silver lining.
A
fascinating and unexpected process occurs as these city- or
small-country-sized masses travel across the ocean, one that spurs the
growth of tiny marine organisms and actually stores carbon in the deep
sea, blunting the strength of global warming – a little bit, anyway.

“It is a negative feedback, it is going to take carbon out of
[the] system, it is going to slow down the rate at which carbon dioxide
is increasing,” says Grant Bigg of the University of Sheffield in
Britain, a co-author of the new study just out in Nature Geoscience.
Bigg conducted the work with two university colleagues.
By
using satellites to observe icebergs like B31 as they travel across the
Southern Ocean, Bigg and his colleagues found that the ocean surface
actually turns greener as the icebergs go by.
The reason is that
these huge icebergs are not just made up of ice – frozen in the ice are
minerals like iron, and as the iceberg slowly melts, those too flow into
the ocean.
The Southern Ocean is actually an iron-poor
, so that means that infusions of iron lead to more growth of
tiny marine organisms, called phytoplankton, which float in the upper
layer of the ocean and engage in photosynthesis, pulling carbon out of
the air.
When these organisms then die and fall to the bottom of the ocean, that carbon goes with them – and is sequestered there.
In
other words, huge icebergs are performing a task similar to what some
would-be planetary “geoengineers” have proposed – fertilising key parts
of the ocean with iron in order to enhance photosynthesis and store
carbon.

Iceberg B31 was  said to be roughly the size of Singapore. Photo / Nasa
Iceberg B31 was said to be roughly the size of Singapore. Photo / Nasa

Iron is found in these huge bergs, Bigg says, because they
scrape across land at some point on their way to the ocean, and gather
it up in the process.
“It’s basically from erosion of the underlying surface, or rubbing against mountainsides as the ice goes down,” he says.
This
isn’t the first time that iron from melting glaciers has been found to
enhance Antarctic marine life. Previous research has shown that in
Antarctic coastal “polynyas” – open water areas ringed with sea ice –
more oceanic life blooms in areas where melting glaciers are pouring
more fresh water, and iron, into the sea.
Scientists have often
found so-called “positive feedbacks” that will make global warming
worse, with a classic example being Arctic permafrost. As the planet
warms, the permafrost thaws and emits more carbon dioxide and methane to
the atmosphere – causing the planet to warm still more.
The new
iceberg process, however, would appear to be a negative feedback that
would enhance the so-called carbon “sink” of the Southern Ocean,
allowing it to pull in more carbon than before.
And the loss of
ice from Antarctica into the Southern Ocean (in icebergs of all sizes)
has already increased by 5 per cent over the last 20 years, the new
paper asserts. That trend should continue under climate change, Bigg
suggests.
However, there is no salvation here – although it seems
to be real, the iceberg feedback is relatively small in the grand
scheme of things.
The Southern Ocean overall sequesters about 0.2
gigatons of carbon every year, according to Bigg, out of about 10
gigatons emitted annually. And the large iceberg process may enhance
this sequestration by 10 to 20 per cent, the research suggested.
“The
future may therefore see an increase in Southern Ocean carbon
sequestration through this iceberg fertilisation mechanism, acting as a
secondary negative feedback on climate change,” the study says. But the
key word here is “secondary” – not big enough to really offset that much
warming.
“I think it’s always going to be a secondary influence under likely scenarios,” says Bigg.
By
contrast, the positive feedback involving permafrost appears
considerably larger. It could be as big as 1.4 gigatons per year, recent
research suggests.
So we shouldn’t expect giant icebergs to
rescue us – and we should bear in mind that any speeding up of Antarctic
melting is also a major sea-level rise risk. Nonetheless, the
surprising story of how huge icebergs can lead to more carbon sinking
into the ocean is a stark reminder of the intricacy and complexity of
planetary change.
And it suggests, above all, that as we continue
to dramatically alter the planet, there will be plenty of surprises –
some of them good , perhaps, but many of them the opposite.
 

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