Sea levels are rising as the Earth grows warmer, and the best-case scenario is that this rise will take centuries rather than decades to swamp the coastlines, Richard Alley said at the AAAS Annual Meeting.

But the Penn State geologist said governments should be worrying about the worst-case scenario—a rapid disintegration of major ice shelves in places like western Antarctica—because researchers can’t rule out that possibility and the impact would be devastating.

Policy-makers should take precautions akin to those in place for the morning commute, Alley said, where the most likely scenario is just slight traffic snarls and bad radio songs like “Muskrat Love.” “What we expect as commuters is a world that is almost as good as it can be,” he said. “But the most likely, the most expected, is way out on the good end of outcomes.” 

Because we know this, cars are equipped with seat belts, children’s car seats and airbags as precautions against the worst commuting scenario of being killed by a drunk driver.

“We put a lot of our travel budget into some things that we do not expect to happen, because it’s such a big deal, and we can’t really predict it,” Alley said, suggesting that a similar logic should apply to the possibility of a major ice sheet collapse.

Alley participated in the Intergovernmental Panel on Climate Change (IPCC), a co-recipient of the 2007 Nobel Peace Prize. As he has talked with policymakers and economists about the IPCC’s findings, he has found that many of them think sea-level rise will be “slow, small and expected” and not overly disruptive to the economy and society.

An iceberg in or just outside the Ilulissat fjord that likely calved from Jakobshavn Isbrae, the fastest flowing glacier in west Greenland. Courtesy of Ian Joughin

There is some scientific evidence to support this view, Alley said. Estimates of major ice-sheet melt come out to 0.6 millimeters per year, contributing about 20% to observed sea-level rise. At this rate, it might take more than 100,000 years for the ice sheets to disappear. 

Alley compared this decrease to going on a diet but only omitting the calories from one-third of potato chip in a year. “The worry is about what if the ice sheets get serious about dieting?” he said.

As might be expected from the 2012 winner of the AAAS Public Engagement with Science Award, Alley outlined this danger with a few instantly grasped analogies.

Ice sheets are big piles, he explained, which spread underneath their own considerable weight, slide into the sea, melt and contribute to sea level rise.

Much attention has been paid attention to how melting may lubricate the bases of ice sheets and speed up this slide. But, the bottom of the Greenland ice sheet, at least, is more like a greased waffle iron than a greased pancake griddle, Alley said. The meltwater does contribute to some sliding, but the rough, corrugated ground under the ice sheet grips the ice tighter than a flat surface might. 

It’s the ice shelves at the edges of these ice sheets, which act like the supportive flying buttresses in a Gothic cathedral, that are Alley’s greater concern. These shelves butt up against adjoining land, creating friction and holding the ice pile in place. When these shelves collapse—in some cases disintegrating within a few months—ice sheets become much more vulnerable to rapid melting.

Researchers are particularly concerned about this possibility occurring in western Antarctica, where the collapse of ice shelves could unleash ice flow that that has been building up for thousands of years behind “bumps” in the land.

Even if the “drunk driver” of ice-sheet collapse doesn’t arrive, Alley said economists should be wary of predictions that people will respond to sea level rise in a highly efficient manner.

He pointed out that government researchers had known about sea level rise, erosion in the Mississippi River delta and stronger Atlantic Ocean storms for 20 years before Hurricane Katrina drowned New Orleans in 2005.

Even under the best-case scenario, Alley said, “we are better off if we respond to climate change. Ignoring it is costly compared to dealing with it.”