Marlowe Hood Yahoo News 15 Apr 09;
PARIS (AFP) – A breakthrough study of fluctuations in sea levels the last time Earth was between ice ages, as it is now, shows that oceans rose some three meters in only decades due to collapsing ice sheets.
The findings suggest that such an scenario -- which would redraw coastlines worldwide and unleash colossal human misery -- is "now a distinct possibility within the next 100 years," said lead researcher Paul Blanchon, a geoscientist at Mexico's National University.
The study, published by the science journal Nature, will appear in print Thursday.
Rising ocean water marks are seen by many scientists as the most serious likely consequence of global warming.
The UN Intergovernmental Panel on Climate Change (IPCC) predicted in 2007 that sea levels will rise by up to 59 centimeters (23 inches) before 2100 due simply to the expansion of warmer ocean waters.
This relatively modest increase is already enough to render several small island nations uninhabitable, and to disrupt the lives of tens of millions of people living in low-lying deltas, especially in Asia and Africa.
But more recent studies have sounded alarms about the potential impact of crumbling ice sheets in western Antarctica and Greenland, which together contain enough frozen water to boost average global sea levels by at least 13 metres (42 feet).
A rapid three-meter rise would devastate dozens of major cities around the globe, including Shanghai, Calcutta, New Orleans, Miami and Dhaka.
"Scientists have tended to assume that sea level reached a maximum during the last interglacial" -- some 120,000 years ago -- "very slowly, over several millennia," Blanchon told AFP by phone.
"What we are saying is 'no, they didn't'."
The new evidence of sudden jumps in ocean water marks was uncovered almost by accident at a site in Mexico's Yucatan peninsula that had been excavated for a theme park.
Blanchon and three colleagues from the Leibniz Institute of Marine Science in Germany discovered the remains of coral reefs that made it possible to measure with great precision changes in sea level.
Using contiguous reef crests -- the part of the reef closest to the surface of the water -- as benchmarks, the researchers pinpointed a dramatic jump in sea levels that occurred 121,000 years ago.
"We are looking at a three-metre rise in 50 years," Banchon said. "This is the first evidence that we have for rapid change in sea level during that time."
Only collapsing ice sheets could account for such an abrupt increase, he added.
The last interglacial period, when sea levels peaked six metres higher than current levels, was warmer than the world is today.
But as manmade climate change kicks in, scientists worry that rising temperatures could create a similar environment, triggering a runaway disintegration of the continent-sized ice blocks that are already showing signs of distress.
The recent breakaway of the Wilkins Ice Shelf from the Antarctic peninsula, for example, while not adding itself to sea levels, makes it easier for the glaciers that feed it to flow straight out to sea.
It is still unclear whether this and other dramatic changes seen in ice sheets recently are signs of imminent collapse, or natural processes that have not been observed before.
The Yucatan peninsula is one of only a few regions in the world where the virtual absence of seismic activity over the last several hundred thousand years makes accurate measurements of sea level rise during the last interglacial possible.
"What we have to do now is look at other stable areas, such as western Australia, and confirm the same reef back-jumping signature we found in the Yucatan," said Blanchon.
"Once we have done that, we can say our findings are rock solid."
Coral Fossils Suggest That Sea Level Can Rise Rapidly
Andrew C. Revkin, New York Times 15 Apr 09
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 study, 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.
One of the most momentous and enduring questions related to human-caused global warming is how fast and 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 in Mexico.
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.
Fossil Corals Show Catastrophic Sea-Level Rise?
Brian Handwerk, National Geographic News 15 Apr 09;
Fossil coral reefs at a Mexican theme park "confirm" that sea levels rose rapidly about 121,000 years ago, according to a controversial new study.
Previous research on fossil reefs had shown that sea levels surged by 13 to 19 feet (4 to 6 meters) near the end of the last time period between ice ages, known as an interglacial period. But researchers have been unsure whether this sea-level rise happened quickly or gradually.
By mapping the ages and locations of ancient corals at Xcaret, an eco-park in the Yucatán Peninsula, Paul Blanchon of the National Autonomous University of Mexico, and colleagues, were able to chart when the reefs died and were replaced by others on higher ground.
Their data suggest that sea levels rose by about 10 feet (3 meters) in 50 years—much faster than the current annual rate of 0.08 to 0.1 inch (2 to 3 millimeters).
Because this event happened during an interglacial period—similar to the one we're in currently—the find boosts the chances that today's melting ice sheets could trigger rapid sea-level rise, the study authors say.
But not all experts on corals and climate are convinced by the new study.
Tad Pfeffer, of the University of Colorado at Boulder, noted that Blanchon's team couldn't directly measure the rate of sea level change around the Mexican corals, because the age estimates aren't accurate enough.
Instead the study authors compared changes seen in Xcaret to those seen in reefs with well-established ages in the Bahamas.
"It's an interesting idea, but one that for me is only suggestive and not compelling," Pfeffer said.
"I'd want to see something more solid than this if I'm going to buy the idea of such rapid sea level rise at the time [of the last interglacial]."
Even if the new study is confirmed, Pfeffer added, more research would be needed to determine if rapid sea-level rise 121,000 years ago provides evidence that similar changes can happen now.
"And of course, when would 'now' be?" he asked.
"'In the next few decades' vs. 'the next few thousand years' are both 'now' on the time scales at which glacial and interglacial periods are defined, but are very different situations in terms of how we determine responses."
Mike Kearney, of the University of Maryland, said it's "within the realm of possibility" that global warming will trigger a sudden collapse of the West Antarctic ice sheet, which could lead to a rapid increase in sea levels like that predicted by the new study.
(Related: "PHOTOS: Jamaica-Size Ice Shelf Breaks Free")
"But the big unknown is whether any of the things we think we know about the Antarctic ice sheet prove to be true," Kearney cautioned.
"One camp says [rapid sea-level rise] could happen, another camp says it would take thousands of years. I'm not sure what the conventional wisdom is right now. It depends on who you talk to."
Findings appear in this week's issue of the journal Nature.
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