In November of 2013, a mindbogglingly 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 number-one 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 my system, it is going to slow down the rate at which carbon dioxide is increasing,” says Grant Bigg of the University of Sheffield in the UK, 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 environment, 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 — fertilizing key parts of the ocean with iron in order to enhance photosynthesis and store carbon.
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 percent 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 percent, the research suggested.
“The future may therefore see an increase in Southern Ocean carbon sequestration through this iceberg fertilization 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 news, perhaps, but many of them the opposite.