GAINESVILLE, Florida, July 12, 2015 (ENS) – Global sea levels have risen at least six meters, or about 20 feet, above present levels many times over the past three million years, finds a new review analyzing three decades of research on melting polar ice sheets.

What worries scientists the most is the fact that amount of melting was caused by an increase of only one to two degrees Celsius in global mean temperatures, the increase that Earth is experiencing now.

Antarctica

Icy spires, arches, and blocks drift through a sunset in Antarctica’s narrow Lemaire Channel. Jan. 5, 2013 (Photo by Christopher Michel)

When past temperatures were similar to or slightly higher than the present global average, sea levels rose at least 20 feet, suggesting the same outcome if current climate trends continue.

Findings published in the journal “Science” showed that the seas rose in response to melting ice sheets in Greenland and Antarctica, said lead author Andrea Dutton, a University of Florida geochemist.

“This evidence leads us to conclude that the polar ice sheets are out of equilibrium with the present climate,” she said.

Dutton and an international team of scientists assessed evidence of higher sea levels during several periods to understand how polar ice sheets respond to warming.

Combining computer models and observations from the geologic record, they found that during past periods with average temperatures 1 to 3 °C (1.8 to 5.4 °F) warmer than preindustrial levels, sea level peaked at least 20 feet higher than today.

“As the planet warms, the poles warm even faster, raising important questions about how ice sheets in Greenland and Antarctica will respond,” she said. “While this amount of sea-level rise will not happen overnight, it is sobering to realize how sensitive the polar ice sheets are to temperatures that we are on path to reach within decades.”

“Studies have shown that both the Greenland and Antarctic ice sheets contributed significantly to this sea level rise above modern levels,” said co-author Anders Carlson, an Oregon State University glacial geologist and paleoclimatologist.

“Modern atmospheric carbon dioxide levels are today equivalent to those about three million years ago, when sea level was at least six meters higher because the ice sheets were greatly reduced,” Carlson said.

“It takes time for the warming to whittle down the ice sheets,” added Carlson, who is in OSU’s College of Earth, Ocean and Atmospheric Sciences, “but it doesn’t take forever. There is evidence that we are likely seeing that transformation begin to take place now.”

melting ice

Broken ice sheet at the North Pole as seen from a passenger aircraft, Oct. 22, 2014 (Photo by pranav)

Twenty feet of sea level rise may not sound catastrophic, but coastal cities worldwide have exploded in both population and infrastructure over the past 200 years. A global mean sea level rise of 20 feet could spell disaster to the hundreds of millions of people living in these coastal zones.

Much of the state of Florida, has an elevation of 50 feet or less, and the city of Miami has an average elevation of six feet.

Parts of New Orleans and other areas of Louisiana were overcome by Hurricane Katrina – by a surging Gulf of Mexico that could be 10 to 20 feet higher in the future.

Dhaka in Bangladesh is one of the world’s 10 most populous cities with 14.4 million inhabitants, all living in low-lying areas.

The low-lying cities of Tokyo and Singapore also are vulnerable to sea level rise.

Co-author Peter Clark, an OSU paleoclimatologist, said that because current carbon dioxide, CO2, levels are as high as they were three million years ago, “we are already committed to a certain amount of sea level rise.”

“The influence of rising oceans is even greater than the overall amount of sea level rise because of storm surge, erosion and inundation,” said Carlson, who studies the interaction of ice sheets, oceans and the climate system on centennial time scales. “The impact could be enormous.”

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