Antarctica's giant icebergs are helping fight climate change

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Antarctica's Supersized Icebergs Shut Down Currents
In a rare spot of good climate change news, warming temperatures around Antarctica may increase the amount of carbon being absorbed from the atmosphere into the Southern Ocean, according to a new study.

Theoretically, that could help stabilize the climate a bit over the course of the next several decades.

The reason, according to research published Monday in the journal Nature Geoscience, is that the meltwater flowing off the region's giant icebergs—floating hunks of ice that can range in size from 12 to 186 miles long, and up to 1,968 feet deep—has a much bigger impact on marine life than scientists previously thought.

These icebergs come from ice that flows over Antarctica and breaks off at the continent's edge. As the ice moves over land, it picks up iron and other nutrients that, once shed along with the meltwater, nourish enormous blooms of phytoplankton: microscopic plants at the base of the marine food web.

As the phytoplankton die, they sink towards the sea floor, taking with them the carbon they've stored in their cells. Once "sunk" near or at the ocean bottom, that carbon can stay put for years, possibly decades or centuries.

The Southern Ocean already contains about 10 percent of all the carbon stored in the world's oceans, and the calving of icebergs off Antarctica is expected to increase as global temperatures continue to warm. More giant icebergs, therefore, could lead to even more sequestered carbon.

"The role of giant icebergs in climate is rather different than we'd expected," said Grant R. Bigg, the study's co-author and an Earth scientist at the University of Sheffield in the United Kingdom. "In the past everyone thought about changes due to meltwater just in terms of physical impact on the ocean," such as rising sea levels or changes in ocean currents.

"Here we see that there's a really big biological impact, and also the negative feedback on the carbon cycle," Bigg said. "The carbon storage comes from the detritus from dead phytoplankton, and also other species that feed on them."

Bigg and his colleagues concluded that the biological impact of giant iceberg meltwater may account for up to 20 percent of the carbon storage in the Southern Ocean, much more than previously thought.

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Using 175 satellite images to track the changes in the water around 17 giant icebergs between 2003 and 2013, Bigg and his colleagues found that increases in phytoplankton activity extended into waters up to 10 times the iceberg's length, and could last for up to a month after the iceberg passed. This impact was much bigger and lasted longer than similar upticks recorded around small icebergs.

Scientists have grown increasingly confident that warming global temperatures have destabilized some parts of the Antarctic ice sheet, and that the changes may be irreversible.

"As the warming progresses, and this leads to more calving of icebergs, and particularly if the ice shelves become unstable and lose a lot of mass, you will end up with more phytoplankton, and more carbon being drawn out of the system," said Bigg.

But it's not yet clear whether warming to date has created more giant icebergs than in the past, he said, because satellite monitoring of giant icebergs began only about 35 years ago.

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