Rising Seas Look Inevitable

It may be too late to stop the seas from eventually rising and flooding Earth's coastlines. Even if humans manage to eliminate carbon dioxide emissions completely by the year 2100, ocean warming set in motion by the end of this millennium could trigger the collapse of the West Antarctic Ice Sheet and flood New York City, Hong Kong, and other coastal cities, a new study suggests.

Sea level rises when meltwater from land-based masses of ice, such as glaciers, flows into the ocean. But sea level also increases when heat from the atmosphere gets mixed into the upper layers of the ocean, causing that water to expand. In recent decades, this thermal expansion has provided, on average, only about one-quarter of the 1.8 millimeters of sea level rise seen each year, but its contribution is increasing, studies suggest.

Now researchers point to an even bigger threat from warm ocean waters. The floating ice shelves that ring Antarctica could melt. So could the seaward end of land-based ice streams. That would lead to a long-term, catastrophic rise in sea level.

The new analysis, conducted by Nathan Gillett, a climate scientist at the University of Victoria in Canada and his colleagues, considers a rosy scenario. The team assumes that carbon dioxide emissions will rise at moderate rates from now until 2100, when people will switch to renewable energy sources and stop producing carbon dioxide. In this scenario, atmospheric concentrations of the greenhouse gas peak at about 770 parts per million (approximately twice today's level of approximately 390 ppm), Gillett says. Even though no new humanmade carbon dioxide emissions are produced after 2100 and terrestrial and marine ecosystems continue soaking it up, carbon dioxide levels remain above 550 ppm for the next 9 centuries. Oceans will be slow to soak up the atmospheric carbon dioxide, and terrestrial ecosystems—many of which have been storing carbon gradually for centuries—will begin to release some of that carbon after the year 2200, the model suggests. As a result, ocean warming persists throughout the millennium, the researchers reported online yesterday in Nature Geoscience.

Much of that warmth is mixed directly into surface seas by wave action. But some is injected into deeper ocean layers by the thermohaline circulation, a pattern of ocean currents that carries warm, salty water from the North Atlantic southward to the Antarctic. Overall, the team's model suggests that the temperature of waters surrounding the icy continent at depths between 500 and 1500 meters will rise approximately 3˚C between the years 2105 and 2995. Add that to an Antarctic surface warming of as much as 9˚C since the mid-1800s, and that's a recipe for melting ice. At particular risk is the West Antarctic Ice Sheet, a 2.2-million-cubic-kilometer, potentially unstable ice mass that sits on the sea floor at depths where Antarctic waters are warming the most.

Ocean warming alone will result in 25 centimeters of sea-level rise by 2100 and 1 meter by 3000, the researchers estimate. But if warming waters melt the major ice shelves of western Antarctica, which act like dams to hold immense quantities of ice on shore, the entire western portion of the Antarctic ice sheet could melt away. Previous studies hint that such a collapse could boost sea level as much as 4 meters, swamping coasts worldwide.

The team's analysis "looks like a solid study, and the most interesting new result is the tie to the West Antarctic Ice Sheet," says Richard Alley, a climate scientist at Pennsylvania State University, University Park. Human-caused warming, he says, could influence Antarctica's land ice many centuries after we stop burning fossil fuels.