In November of 2013, a mind-bogglingly large iceberg split off from the front of Pine Island Glacier in West Antarctica.

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, from 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 from Antarctica, 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 such as 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.

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