canal of fossil coral
© Paul BlanchonCoral fossils in canal walls at a Mexican resort show evidence of a rapid increase in sea level 121,000 years ago, researchers say. Other experts on corals and climate are not convinced.
The study, being published Thursday in the journal Nature, suggests that a sudden rise of 6.5 feet to 10 feet occurred within a span of 50 to 100 years about 121,000 years ago, at the end of the last warm interval between ice ages.

"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.

Yet other experts on corals and climate are faulting 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.

The study focuses on a set of fossil reef remains exposed in excavations for channels at a resort and water park, Xcaret, about 35 miles south of Cancún on the east coast of the Yucatán Peninsula.

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.

"I spent the last four years looking at those cross sections and piecing the story of those reefs together bit by bit like a jigsaw," he said in a telephone interview.

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.

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.

But Dr. Blanchon maintains that the work will hold up, saying the signs of abrupt change are etched in the rock for everyone to examine