Evidence from fossil coral reefs in Mexico underlines the potential for a sudden jump in sea levels because of global warming, scientists report in a new study.
The potential for future rapid sea-level rise is perhaps the greatest threat from global warming. But the question of whether recent ice loss from Greenland and Antarctica is the first indication of such a rise is difficult to answer given the limited duration of the instrumental record. New evidence from an exceptionally exposed fossil reef in the Xcaret theme park in Mexico (about 35 miles south of Cancun on the east coast of the Yucatan Peninsula) provides a detailed picture of the development of reef terraces, erosion surfaces and sea-level excursions in the region during the last interglacial.
A combination of precise uranium-series dating and stratigraphic analysis, together with comparison with coral ages elsewhere, suggests that a sea-level jump of 6.5 to 10-feett occurred about 121,000 years ago, consistent with an episode of ice-sheet instability towards the end of the last interglacial. On that evidence, sustained rapid ice loss and sea-level rise in the near future are possible.
Paul Blanchon, the lead author of the study, said he sought a position as a research scientist at the National Autonomous University of Mexico's Institute of Marine Sciences in Puerto Morelos so he could focus on the unusual fossil reefs, visible for hundreds of yards where canals were cut into the rocky ground.
With three co-authors from Germany, Dr. Blanchon calculated the ages of coral samples by measuring isotopes of thorium in the fossils. The team then confirmed the ages by comparing the Mexican reefs with coral reefs in the Bahamas whose ages had been thoroughly studied. The team says it found that two Mexican reefs grew during the last "interglacial," or warm interval between ice ages.
To determine the pace of sea-level rise in that period, Dr. Blanchon charted patterns of coral revealed in excavations at the resort. He said his work revealed a clear point where an existing reef died as the sea rose too quickly for coral organisms to build their foundation up toward the sea surface. Once the sea level stabilized again, the same group of corals grew once more, but farther inshore and up to 10 feet higher in elevation, a process known to geologists as backstepping.
Such an abrupt change from stable coral growth to death and a sudden upward and inshore shift of a reef could happen only because of a sudden change in sea level, he said. "The potential for sustained rapid ice loss and catastrophic sea-level rise in the near future is confirmed by our discovery of sea-level instability" in that period, the authors write.
Other experts on corals and climate fault the work saying that big questions about coastal risks in a warming world remain unresolved. Among the most momentous and enduring questions related to human-caused global warming are how fast and how high seas may rise. Studies of past climate shifts, particularly warm-ups at the ends of ice ages, show that fast-melting ice sheets have sometimes raised sea levels worldwide in bursts of up to several yards in a century.
A question facing scientists is whether such a rise can occur when the world has less polar ice and is already warm, as it is now, and getting warmer. Citing the evidence from fossil coral reefs, the authors of the new study say with conviction that the answer is yes.
But in interviews and e-mail messages, several researchers who focus on coral and climate said that although such a rapid rise in seas in that era could not be ruled out, the paper did not prove its case.
Daniel R. Muhs, a United States Geological Survey scientist who studies coasts for clues to past sea level, cited a lack of precise dating of the two reef sections. William Thompson, a coral specialist at the Woods Hole Oceanographic Institution, agreed, saying that given the importance of the conclusion, Dr. Blanchon interpreted the physical features without enough corroborating evidence.
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