April 15 (Bloomberg) -- Fossilized coral reefs formed the last time the Earth was warmer than today show sea levels could rise rapidly by the end of the century if global warming triggers a collapse of ice sheets in Greenland and Antarctica.

A “catastrophic” rise in the ocean of 4 meters to 6 meters (13 feet to 19.6 feet) is possible, said Paul Blanchon, a scientist at the National University of Marine Sciences in Cancun, Mexico, whose team studied the fossilized reefs. The death and re-emergence on higher elevation of reefs 121,000 years ago could only result from a rapid increase in ocean levels caused by the breakdown of ice sheets, he said.

Ice caps like those that cover the Antarctic and Greenland are thought to be melting at a rate that could raise sea levels by about 12 meters over the next millennium, Blanchon said. His report, to be published in the April 16 issue of Nature, says global warming caused by burning fossil fuels could shorten that time frame to several hundred years.

“What we’re saying is ice sheets may not just melt due to global warming,” Blanchon said. “That’s the real problem that we face, can these ice sheets basically slide into the ocean?”

The complete melting of the Greenland Ice Sheet would add about 7 meters to sea levels and endanger low-lying coastal cities, the UN Intergovernmental Panel on Climate Change said in 2007.

New York, Boston and Washington are among the largest U.S. cities facing flood risk over the next century from rising sea levels, Florida State University-led scientists said in a March 16 study.

From Maldives to U.S.

Low-lying nations such as the Maldives and Bangladesh are also at risk from rising seas caused by global warming while weaker Atlantic currents and the expansion of water as it warms may raise sea levels by up to 51 centimeters (20 inches) on the U.S. East Coast, the Florida State report said.

By the 2080s, New York could experience a rise in sea levels of 15.7 inches to 17.7 inches, the Columbia University Center for Climate Systems Research says. Sea-level rise threatens the city’s storm water system and wastewater treatment plants, the New York Department of Environmental Protection said last year.

Wilkins Ice Shelf

A bridge of ice connecting two Antarctic islands broke up earlier this month, putting the Wilkins Shelf behind it at risk.

Ice shelves, such as the 11,800-square-kilometer (4,600- square-mile) Wilkins Ice Shelf in West Antarctica, are attached to land but rest on the sea so when they melt, they don’t add to sea-level rise. Even so, their loss can accelerate the flow of land-based ice to the ocean, which then can raise water levels.

Current computer models project that the Antarctic Ice Sheet will remain too cold for widespread surface melting and will gain in mass from increased snowfall.

Last week, the Boulder, Colorado-based National Snow and Ice Data Center said older, thicker Arctic sea ice that is better able to survive the summer shrank to the thinnest level ever recorded this winter. Older ice, which is generally thicker, is now about a third of the 1981 through 2000 average.

The UN climate panel predicts sea levels will rise by 18 to 59 centimeters (7 to 23 inches) this century through the melting of glaciers and ice sheets and the natural expansion of water in the ocean as it warms.

Yucatan Evidence

Blanchon’s report documents a study of coral reefs excavated on the northeast coast of the Yucatan peninsula for a tourist theme park. The research establishes a physical connection between reefs that are set apart and at different elevations, offering “compelling evidence for a sea level jump,” Blanchon said in the report.

The observations suggest a sea-level rise of 15 meters in 300 years, or 5 centimeters per year, faster than could occur by the melting of ice sheets, Blanchon said.

“I wouldn’t jump about and say people are going to drown,” Blanchon said yesterday in an interview. “Five centimeters a year is not an awful lot and that’s the maximum. Still, if that were to start happening, it would certainly focus the politicians on getting something done.”

The loss of Arctic sea ice and ocean-level rise could be partially avoided if emissions of greenhouse gases are cut by 70 percent this century, according to a new study by the National Center for Atmospheric Research. Researchers used supercomputer simulations to model the impact of cuts in carbon-dioxide emissions beginning in about a decade.

Delegates from 192 countries are working to reach an agreement on a new climate treaty at a UN-sponsored summit in Copenhagen in December.

To contact the reporter on this story: Jim Efstathiou Jr. in New York at jefstathiou@bloomberg.net

Last Updated: April 15, 2009 14:30 EDT