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After examining nearly 200 photos of giant icebergs measuring more than 11 miles in length, researchers have determined that the meltwater they leave behind as they float out to sea could actually help mitigate climate change.

Conducted by scientists at the University of Sheffield in the U.K., and published in the journal Nature Geoscience, the study found that blooms of phytoplankton—tiny plant-like organisms that thrive on the iron and other nutrients shed by the icebergs as they melt—are trapping millions of tons of carbon dioxide each year. After these tiny creatures die or get eaten they eventually end up on the bottom of the ocean floor as decaying organic matter. As part of this process they take with them the carbon dioxide they collected through photosynthesis, thus sequestering it far beneath the already overloaded atmosphere. Each giant iceberg can leave a trail of phytoplankton hundreds of miles long that can last for at least a month.

The study, the first of its kind of this scale, looked at satellite images over the Southern Ocean surrounding Antarctica. The researchers ultimately found that phytoplankton productivity associated with the giant icebergs is responsible for storing up to 20% of all the carbon in the Southern Ocean. The Southern Ocean accounts for around 10% of all the CO2 absorbed by the world’s oceans.

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Giant icebergs—those more than 11 miles in length—make up about half the ice floating in the Southern Ocean, with dozens of them occupying the sea at any given moment. Each iceberg can exist for up to a decade, and their annual flow is equivalent to the flow of the Congo River.

Satellite measurements of enhanced chlorophyll levels around the ‘C16’ giant iceberg in the Southern Ocean, taken on 12 January 2013. Red and yellow areas show high levels spreading out from the iceberg. Grey shading shows cloud cover.
Source: Duprat et al. (2016)

If iceberg calving continues to increase throughout this century as climate models predict, this unaccounted for form of carbon sequestration could become more important than previously thought.

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According to professor Grant Bigg at the University of Sheffield, who led the study, the findings indicate that the size of the carbon sink of the Southern Ocean shown to be associated with the phytoplankton could impact the rate of exchange of CO2 between the ocean and atmosphere. This is an important part of the carbon cycle that acts to determine the rate at which greenhouse gases accumulate in the atmosphere, thus warming the planet.

So how exactly are these giant bergs the source of unanticipated carbon storage?

As Bigg explains in a post about the study on The Conversation, "as Antarctic ice sheets slowly slide towards the ocean they bump along the continent’s bedrock, picking up iron and other nutrients which become imprisoned within the ice."

As these icebergs melt, they release the chemicals near the ocean's surface, where the phytoplankton—otherwise starved for dissolved iron—can grow through photosynthesis, thus utilizing carbon dioxide that would otherwise remain in the atmosphere.

Previous studies of smaller icebergs had shown a much smaller "fertilization effect."

The Antarctic Ice Sheet covers nearly 5.4 million square miles, around the area of the contiguous United States and Mexico combined. If the entire Antarctic Ice Sheet melted, sea level would rise by around 200 feet. According to the National Snow and Ice Data Center, while most of Antarctica is yet to see dramatic warming, an area of the region that juts out into warmer waters north of Antarctica has warmed by 4.5 degrees Fahrenheit since 1950.

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Giant icebergs are confined to the Southern Ocean, as those calving from the smaller Greenland ice sheet are rarely more than half a mile long. The North Atlantic also already has plenty of iron, mostly due to the Saharan dust blown to that area.

"The Arctic and North Atlantic are therefore missing key ingredients for a strong biological response to iceberg meltwater," wrote Bigg. "So if any giant icebergs are going to help slow climate change they will likely come from Antarctica, one of global warming’s most symbolic places."