Rising seas: Ice sheets pose uncertain, startling risks for the planet

Beneath the ocean waters off Antartica, massive buried shelves of ice function like buttresses, supporting the continent’s massive ice sheets.

If those buttresses fail, Richard Alley told science writers at a recent conference in State College, Pa., global sea levels will not rise by inches, as predicted by recent climate reports—but instead by as much as 186 feet.

Alley, professor of geosciences at Penn State University, discussed the perilous consequences of rising sea levels, and society’s options in the face of an uncertain future, on Oct. 27 during the New Horizons in Science briefing organized by the Council for the Advancement of Science Writing, part of the ScienceWriters2019 conference.

“We can either treat climate change science like a tweet—pretending like it’s an evil liar—or we can use knowledge,” Alley said.

As climate changes, rising sea levels are a concern for countries that border the world’s oceans. According to the Intergovernmental Panel on Climate Change—also known as the IPCC—sea levels will continue to rise for the foreseeable future.

Coastal cities under threat

Seas are currently rising at a rate of one foot in 90 years, Alley said, pointing out the dramatic consequences already observed within U.S communities. He shared an image of an octopus that swam into a parking garage during an enhanced tidal flood in Miami, and another showing water levels just inches from overtopping a levee during Hurricane Gustav in New Orleans in 2008. Although the levee held, Alley noted the city of New Orleans would have been flooded had sea levels been just a few inches higher.

IPCC currently projects another three feet of sea-level rise by the year 2100. Some critics argue that the IPCC relies on erroneous sea level models, but Alley warned that this prediction may be optimistic. Other respected models suggest significantly greater sea-level rise by 2100.

“If there is any bias in the IPCC sea level projections, it’s low,” he said. “Not high.”

About 70% of the Earth’s fresh water, Alley explained, is stored as ice within glaciers or ice sheets. Glaciers form when annual snowfall freezes and accumulates over thousands of years. Ice sheets are massive glacial bodies that span at least 20,000 square miles—the size of West Virginia or larger. The melting of glaciers and ice sheets under warming conditions is one of the greatest contributors to sea-level rise.

If Greenland’s ice sheets melted, sea levels would rise by 24 feet globally, Alley said. And if all of Antarctica’s ice sheets melted, seas would rise another 186 feet. According to the NASA Jet Propulsion Laboratory’s Visual Earth System Laboratory interactive tool, the southern tip of Florida would be submerged if the Greenland ice sheets melted. If all of the Antarctic ice sheets melted, the entire state of Florida would be underwater.

In Antarctica, ice cliffs imperiled by warming

Alley said there are 39 ice shelves surrounding the continent of Antarctica. Normally, when the ocean and air temperatures are cold enough, these provide structural support for grounded ice on land.

If the ocean near an ice shelf is just 1 or 2 degrees warmer than normal, parts of the ice will melt quickly, weakening or fully detaching as an iceberg. This process, Alley explained, creates exposed ice cliffs that lack structural integrity. As these newly formed cliffs have no structural support for their massive weight, they rapidly begin to collapse. Alley showed an example of a 330-foot ice cliff from Greenland that collapsed because local sea temperatures warmed by just 1 degree.

“We see that the ice breaks, it waits, and then it breaks again,” Alley said. “This 330-foot cliff is not a worst-case scenario. We will see larger cliffs collapsing in the future.”

Loss of ice shelves has already resulted in rapid ice loss. Alley showed images documenting the Alaskan Muir Glacier, which has retreated 7 miles from 1941 to 2004. Observing ice loss at Muir Glacier provides a hint for what could happen with ice cliffs elsewhere. The Aurora Subglacial Basin, for example, has the largest ice thinning rates in East Antarctica. Loss of ice shelves within the Aurora Subglacial Basin—resulting in rapid ice loss from cliffs—is predicted to raise sea levels by 11.5 feet.

Alley noted that current IPCC models do not include the effect that the loss of ice cliffs would have on sea levels. Accounting for that factor requires modeling a complex system that depends on ocean currents, wind currents, cracks present within the cliffs, and more. If breaking ice cliffs were incorporated into models, he said, the projections of sea-level rise might be significantly worse.

In Alley’s view, preventing the kind of sea-level rise that might happen when ice sheets fail is analogous to preventing highway accidents. Society implements strict driving laws to prevent accidents that are catastrophic, though rare. Why, he asked, aren’t we doing the same for climate change?

“The idea that using our knowledge makes us better shows that we can do this,” he said, sounding an optimistic note. “We can solve this problem.”