Society faces collision with rising seas

BALTIMORE, June 24 (UPI) -- Global sea level has risen at the rate of 1.8 millimeters per year over the past 50 years and the rate is likely to increase over the next century to 5 millimeters per year, climate scientists predict.

In addition, the Greenland Ice Sheet has begun to melt more rapidly in the past 25 years, said paleoclimatologist Jonathan Overpeck of the University of Arizona. This could mean a sea level rise of as much as 6 meters -- about 20 feet.

"People around the world are living ever closer to the oceans," said John Church, an oceanographer with Australia's Commonwealth Scientific & Industrial Research Organization Marine Research in Hobart, Tasmania.

"Megacities are developing there with populations of 5 million or more. Society is approaching a collision course with sea level rise," Church told United Press International.

Church, Overpeck and others spoke this week at CLIVAR 2004, a conference on climate change.

Some climate skeptics have argued the rate of sea level rise has not changed in the past century, and therefore it is more likely related to a natural occurrence and not to human activity. Church agreed -- to a point.

"To date we haven't detected an increase in the rate of sea level rise during the 20th century," he said. "But if you look at longer times scales, they do indicate an increase in the rate."

Sea level rose much more slowly in the 19th century than in the 20th, he said. Paleological data prior to then suggest "the rate of rise ... is on the order of one-tenth what we've observed over the 20th century."

Scientists do not yet understand the process enough yet to attribute it to anthropogenic climate change," Church said, "but warming during the 20th century has to be implicated to explain what the observations show. The new results that we have will make that conclusion even stronger."

Rising seas will put some coastal areas underwater, but those areas also face a more immediate threat from the impact of "extreme events." Based on records from Australia, Church said, which compare the beginning and end of the 20th century, maximum sea surges occurred three times more often. A study of England, he said, indicated extreme-event surges will occur 10 times more often due to sea-level change.

"A once-in-100-year event becomes a once-in-10-year event," he said.

Overpeck said the melting rate of the Greenland Ice Sheet increased 16 percent between 1979 and 2002 and the ice is melting up to an elevation of about 6,500 feet or 2,000 meters.

"The last interglacial -- 130,000 years ago -- was the last time (carbon dioxide) CO2 levels were about the same as the pre-industrial, was also the last time that sea level was higher than it is today," Overpeck said.

"That's a very interesting clue," he said. "It wasn't just a little higher than today, but 3 meters to 6 meters higher. Lots of islands -- Key West, the Seychelles -- have fossil coral 3 meters to 6 meters higher, and we can date them very well to this period."

Scientists are reasonably sure the additional water came from Greenland, Overpeck said. During this period, 130,000 years ago, the average temperature was 5.4 degrees to 9 degrees Fahrenheit higher -- 3 degrees to 5 degrees Celsius -- than during pre-industrial times, he said, which means the amount of warming that caused the melting was similar to what is expected in 2100.

"Sea level can rise faster than previously estimated," he said. "In the last deglaciation, 13,000 years to 7,000 years ago, sea level rise was 1.1 meters (about 3 1/2 feet) per century. That's inevitable unless we reduce trace gas emissions."

Church said there are some areas of the oceans where sea level is rising faster than the global average, and some areas where it is rising slower.

The equatorial western Pacific Ocean, the eastern Indian Ocean and the northwest coast of Australia are seeing slightly smaller increases, while the northern and southern Pacific basins are rising a little more than average. These regional differences "explain the contradicting studies that have found varying rates," he said. "Global sea level is definitely rising and will continue to have impact in the future."

Three major factors affect sea level, Church said. The most important is the thermal expansion of the oceans. Second is the melting of the non-polar glaciers. Third is the melting of the Greenland Ice Sheet.

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Dan Whipple covers the environment for UPI Science News. E-mail sciencemail@upi.com