Summit County Citizens Voice

Sea level rise is a serious concern for low-lying countries in coastal areas.

Findings will help model future sea-level changes

By Summit Voice

SUMMIT COUNTY — New evidence gathered from coral reef fossils suggests that sea levels fluctuated by 13 to 20 feet within the span of just a few thousands of years during a warm interglacial period known as the Eemian Age, about 125,000 years ago.

“This was the last time that climate was as warm as — or warmer than — today,” said William G. Thompson, a geochronologist with the Woods Hole Oceanographic Institution. “If today’s ice sheets continue to melt, we may be headed for a period of ice sheet and sea-level change that is more dynamic than current observations of ice sheets suggest.”

The researchers used data from an improved method of dating fossil coral reef skeletons in the Bahamas, finding that sea levels were considerably less stable than earlier believed.

Because coral reefs grow near the sea surface, they are accurate markers of former sea levels. At the study area in the Bahamas, two fossil reefs are separated by an erosional surface that was cut by wave action. The first reef grew when sea levels were about 13 feet higher than today.

“The fall of sea-level is indicated by the wave-cut erosion of this first reef,” said Wilson, “and the second sea-level rise was recorded by the growth of new corals on this eroded surface. The dating of fossil corals below and above this erosional surface, using our new methods, reveals important details about the timing of sea-level change that were previously obscured.”

Thompson teamed up with colleagues H. Allen Curran and Brian White of Smith College, and Mark A. Wilson of the College of Wooster, experts on the key Bahamas fossil coral sites.

“The geologic evidence for sea-level change at these sites is convincing,” said Curran, “but we couldn’t absolutely prove sea-level oscillation without more precise dating.”

A better understanding of sea-level change in the past can help to inform predictions for the future. Historical records such as those from tide gauges extend back only a century or so.

“The geological record offers a longer perspective on rates of change,” Thompson said. “And sea-level changes during previous warm intervals are especially relevant to today’s situation.” Sea levels during the Last Interglacial are known to have been about 20 feet higher, on average, than they are today.

“The real surprise is that sea levels were oscillating during this period … How much sea level will rise over the next century or two is a crucial question for the significant part of the world’s population that lives in coastal zones,” Thompson said.

The finding of a significant sea-level oscillation 120,000 years ago is in sharp contrast to the last 5,000 years, where sea level has been relatively stable. “It appears that the smaller ice sheets of the Last Interglacial were significantly less stable than today’s ice sheets,” Thompson said.

Should the current warming trend continue, Thompson said, a scenario similar to that of the Last Interglacial could result. “Variable sea level during the Last Interglacial points to instability in the polar ice sheets, which were somewhat smaller than today. If changing climate leads to smaller ice sheets in the future, this may provoke similar instability.”

The paper was published online in the Sept. 11 edition of Nature Geoscience.

Filed under: climate and weather, coral reefs, Environment, global warming Tagged: | Environment, global warming, Interglacial, sea level, sea level rise, Summit County News, Woods Hole Oceanographic Institution