A new method to determine the age of fossilised coral reef skeletons has provided evidence that the sea level may not remain as stable in a warming world as had been previously estimated.
The research – which dated fossil coral reef skeletons to a period 125,000 years ago during the last warm period where the ice sheets were as small or smaller than they are today – found sea-level oscillations during a period of time which was assumed to be relatively stable.
Instead of stable sea levels, the researchers estimated that the sea level oscillated up and down by 4 to 6 metres (13 to 20 feet) over a few thousand years, approximately 120,000 years ago during a period of time known as the Last Interglacial.
“This was the last time that climate was as warm as—or warmer than—today,” said WHOI geochronologist William G. Thompson, lead author of the study. “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.”
“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.
Unsurprisingly, historical records only date back a century or so, meaning that the really long term records have to be garnered from geological evidence.
“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.”
The sea levels during the Last Interglacial are known to have been around 6 metres (20 feet) on average higher than they are today, but what scientists did not know was that they were oscillating during this period instead of remaining steady.
To acquire a more accurate estimate from the geological record, Thomson developed a new way of interpreting the uranium and thorium isotope rations that have been traditionally used to date coral. Along with colleagues H. Allen Curran and Brian White of Smith College, and Mark A. Wilson of the College of Wooster, Thompson studied two coral reefs in the Bahamas.
Coral reefs form near the surface, and therefore provide a relatively accurate estimation of former sea levels.
“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.”
“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.”
Source: Woods Hole Oceanographic Institution
Image Source: H. A. Curran, Smith College