Seas could rise as fast as three centimeters a year if fossil fuel consumption continues at its present rate. Such increases would amount to ten times the current rise of roughly three millimeters annually. But Antarctica's vast ice sheets may substantially melt and accelerate the rise of seawaters should the burning of fossil fuel continue unabated, according to new computer simulations of climate change’s future impact.

Scientists had previously thought that East Antarctica's massive ice sheets were relatively safe, requiring thousands of years to pass before warming global temperatures would begin to melt them. But the new simulations, published in Science Advances on September 11, suggest Antarctica's ice is much more vulnerable—and thus sea level rise could be a lot worse.

"Humanity can indeed melt all of Antarctica's ice, if we were to burn all of the fossil fuels," says Ricarda Winkelmann, a physicist by training who now works on computer models at the Potsdam Institute for Climate Impact Research. "What we do today by emitting greenhouse gases within just a few decades triggers changes that will be felt by many, many generations to come."

Courtesy of Ken Caldeira and Ricarda Winkelmann

A trip to Antarctica inspired Winkelmann’s interest in the longevity of the ice on the iciest continent as carbon dioxide continues to accumulate in the atmosphere. "It was really impressive to personally see the ice—its incredible beauty and its sheer mass," she recalls of her time on the research vessel Polarstern.

To explore the long-term implications of global warming for Antarctica, Winkelmann teamed up with climate scientist Ken Caldeira of the Carnegie Institution for Science at Stanford University, who attributes his interest in climate science in part to a 1979 New York Times article warning of "widespread floods" caused by the loss of South Pole ice. Using a computer model developed by Andy Ridgwell of the University of Bristol that simulates how the atmosphere and ocean respond to increasing levels of greenhouse gases, the team fed rising temperatures from various amounts of total pollution into an ice sheet model developed by Anders Levermann of Potsdam University. The simulation suggests how ice will flow and dwindle as it melts in response to temperature changes in the atmosphere and ocean, whether increased snowfall as a result of warming or the additional melting as a glacier loses height. The researchers modeled carbon increases ranging from an additional 93 gigatons (representing another decade of fossil fuel consumption at the present rate) to as much as 12,000 gigatons (the total amount of carbon available from already discovered and recoverable deposits of coal, oil and natural gas) over the next few centuries. A gigaton is equal to a billion metric tons of carbon, and current fossil fuel burning results in about 10 billion metric tons of carbon—10 gigatons—entering the atmosphere each year.

The simulations suggest that if another 500 gigatons of carbon end up in the atmosphere—an amount that would require a transition off fossil fuels by the end of this century—seas would rise by more than a meter within a thousand years. In the worst-case scenario, wherein all the fossil fuels are burned over the next few centuries, the seas could rise as fast as three meters per century, and as much as 50 meters within 10,000 years. Not only is that unprecedented in the 10,000-odd years that human civilization has flourished, but it would doom coastal cities such as New York, Hamburg, Lagos, Shanghai, Sydney and Rio de Janeiro, where more than a billion people currently live. "Each ten gigatons of carbon leads to more or less three centimeters of sea level rise in 1,000 years," Caldeira notes.

Like their human counterparts on coasts around the world, the inhabitants of Antarctica—seabirds, penguins and seals among them—would lose the ice that provides their only home. While some of these animals might find homes in zoos, the majority of ice-dependent species would face potential extinction in the wild in the next millennium. And speculative techniques to preserve the ice, such as lacing the stratosphere with sulfuric acid to mimic the cooling effect of a volcano, most likely will not help. "Ice sheets, once they go, are hard to get back," says Caldeira, who has also studied such climate interventions, sometimes called geoengineering.

It could already be too late to save portions of the ice in West Antarctica. Recent research suggests that the ice sheets of the Amundsen Basin may have passed the point of no return as warmer ocean waters slip up under the vast glaciers. Thus, even if no more excess carbon dioxide were to build up in the atmosphere, that ice would continue to melt. As a result of that meltdown, along with the dwindling of Greenland's ice sheet and mountain glaciers and the expansion of warming seawater, rising sea levels—fast or slow—will be with us for millennia to come.

The simulation further revealed that if more than one trillion metric tons of carbon are dumped into the atmosphere, East Antarctica could face the same fate. "What I was startled by was the speed at which the East Antarctic ice sheet could melt," Caldeira says. "It took around 10,000 years for the big northern hemisphere ice sheet to melt at the end of the last ice age, so I assumed it would take 10,000 years to get substantial melt out of East Antarctica." Instead, extensive melting could take place within 200 years, depending on how much carbon is ultimately emitted.

However, if temperature increases can be held to no more than 2 degrees Celsius—roughly equivalent to another 500 gigatons of carbon, or one trillion metric tons in total—then sea level rise could top out at about two meters.

How these scenarios play out in the future will be profoundly affected by concrete regional infrastructure choices being made today. The decision of which type of power plant to build today in Florida, for instance, could determine whether the majority of the state disappears underwater in a matter of centuries. "Avoiding emissions is really the only practical path," Caldeira says.

By Mapping Specialists. Originally produced for "The Unquiet Ice," By Robin E. Bell, in Scientific American, February 2008.

The simulations have practical implications for billions of coastal residents as well. If the seas rise only slightly this century, one effective form of adaptation would be to build seawalls. However, should sea levels swell at a rate of three centimeters a year, no seawall will suffice and coastal retreat becomes the more viable option.

An unstoppable meltdown could be in store for Antarctica's ice—and all of the other ice sheets around the globe—unless people stop dumping CO2 in the atmosphere like a sewer. Carbon dioxide, once emitted, can remain in the atmosphere for thousands of years, trapping extra heat like a smothering blanket. "It is much easier to know that an ice cube in a warming room is going to melt eventually than it is to say precisely how quickly it will vanish," Winkelmann says. "I certainly hope that mankind will not choose to burn all fossil fuels, simply because I know how enormous the consequences will be."