The calving of Petermann glacier and cliamte change

Forget That Big Iceberg--A Smaller One in the Arctic Is More Troubling

A chunk of ice the size of three Manhattans just broke free in the Arctic, and it has a much clearer link to climate change

Scientific American,

31 July, 2017

The world saw headlines about one of the largest icebergs ever calved a few weeks ago. But a smaller one on the other end of the globe might have bigger consequences.

The chunk of ice, which broke free in the Arctic last week, is more worrisome to climate scientists who are watching one of Earth's largest glaciers shed pieces in a way that stands to raise sea levels.

Compared with the Delaware-sized iceberg that split off of West Antarctica earlier this month, this one is almost paltry — the size of three Manhattans or so. It came off the ice shelf that buttresses the Petermann Glacier at the height of seasonal warming in the Arctic region.

By contrast, the recent Antarctic iceberg, while massive, did not have a clear connection to climate. Even if it foreshadows the split-up of the ice sheet to which it was attached, it would not raise sea levels noticeably. The Arctic calving has a much clearer link to climate change.

Movement of the Petermann Glacier has sped up in recent years, dumping land-based ice into the ocean at a faster rate and drawing more ice down from the center of Greenland, said Laurence Dyke, a researcher at the Geological Survey of Denmark and Greenland. Meanwhile, the ice shelf that braces it and slows the rate of flow is disintegrating as climate change transforms the region.

“You could call it the canary in the coal mine. If that big glacier there is changing quickly, and it is, it's a worrying sign for what's happening in the rest of Greenland,” he said.

The iceberg itself is not particularly notable, according to Dyke. But it could lead to an expansion of major cracks upstream in the ice shelf, causing it to break up more quickly. Most troubling to researchers is a crack at the center of the shelf. It's an unusual place for cracks to form, and it could connect to separate cracks forming at the sides.

The loss of the iceberg also brings the shelf to a state not observed in the 150 years of tracking the glacier — and potentially much longer, Dyke said. The ice shelf bracing the glacier lost major pieces in 2010 and 2012. Both those icebergs were the size of several Manhattans.

Land-based glaciers in Greenland are a primary contributor to global sea-level rise, and they're expected to increasingly lose their mass in the future. Petermann accounts for almost 10 percent of the Greenland ice sheet; it alone could raise sea levels by a foot.

As Petermann retreats, it will draw down ice from the center of Greenland, all of which will have a direct effect on sea-level increase. Researchers have cautioned that sea levels could rise by 3 feet at the end of the century, but a more rapid disintegration of Arctic glaciers would make that number larger. A study published earlier this year in Nature showed that the rate of melting in Greenland has increased fivefold in the last 25 years.

Surface temperatures in the region have risen, and that has spurred a greater rate of melting. There are other links to climate change. As the seas warm, they erode ice shelves from underneath.

“When you've got this double whammy effect of the ocean warming from beneath and atmosphere warming from the surface, it's really chewing at this glacier,” Dyke said. “Ultimately, it's why it's having this big retreat.”

Greenland has lost two ice shelves in the last few decades, and it only has two left, said Andreas Muenchow, a researcher at the University of Delaware. 

He said the drama of a new iceberg is the culmination of years of melting.

Muenchow, who gets daily updates from equipment underneath the ice shelf, said ocean temperatures are steadily rising. As sea ice melts, warm water is flowing in from the Arctic ocean. That increases circulation as wind moves the open water around more freely.

“It's a small, steady, measurable drip, and it's this drip, drip, drip that is eroding these glaciers,” Muenchow said. “It's not always what you see and what's a dramatic event, like an iceberg breaking up. It's this year-round drip, drip, drip by the ocean underneath that really determines what's happening by weakening the ice shelf from below.”