Reef Warns Of Sea Level Rise

A fossil coral reef, lying several metres above today’s high tide mark at Foul Bay near Margaret River, points to the high point of the last major sea level rise.

Investigators from the ARC Centre of Excellence for Coral Reef Studies (CoECRS) consider the reef – the most southerly coral reef yet known – is a harbinger of what could happen again as global CO₂ levels and temperatures rise during the 21st century.

“We’ve dated the reef to about 128-125,000 years ago, right in the middle of the last interglacial, or the last period of global warming before our most recent ice age,” says Professor Malcolm McCulloch, deputy director of CoECRS and an earth scientist at The Australian National University.

“The reef lies about 2.5 metres above the current high tide zone, which means that for it to survive and grow, sea levels would have had to be at least 3 to 4 metres higher than at present.

“There is some evidence – still controversial – that sea levels may briefly have been as much as 6 metres higher.”

The coastline of WA, being geologically stable, has a number of both living and fossil coral reefs along it, the results of the Leeuwin current bringing coral larvae down from Indonesia and northern Australia over many tens of thousands of years. Together these reefs indicate what occurred during the last big sea level rise.

“At the time when this reef grew we know that atmospheric CO₂ levels were high, climate had warmed, that the northern icesheets had melted significantly and that sea levels rose - before dropping by around 130 metres again as the ice-age returned and locked up water,” Prof. McCulloch says.

“Water temperatures off Margaret River would then have been more like water temperatures off Geraldton today, allowing the corals to flourish and reefs to form. The discovery is of particular importance as it shows that sea levels rose not only because of the expansion of the oceans due to warming, which can account for ~1/2 metre of sea level rise, but also because it indicates that relatively large scale melting of landbased icesheets occurred in Greenland or the Antarctic.  It is the very rapid rise in sea level from catastrophic melting of landbased icesheets where there the greatest uncertainty and concern lies. Now from the Margaret River corals we have evidence of not only unusually warm ocean temperatures, but that that this was associated with rapid melting of icesheets contributing to an additional 3 to 4 metres of sea level rise.  ”

The cause of the sudden global warming leading to the end of the ice-ages is thought to be due to changes in the elliptical nature of the Earth’s orbit combining with other factors, such as variations in its tilt or wobble.  During the ice ages atmospheric CO₂ was around 180 parts per million, compared to 280 parts per million during interglacials. We don’t know quite why CO₂ levels rose so much during the last interglacial, but it may have been partly due to a reduction in the ocean’s ability to take up atmospheric carbon, as the seas warmed.

Today CO₂  levels are even higher, at 380 parts per million with the additional 100 ppm being mainly due to human activities.  Prof. McCulloch says the oceans currently removed around 40 per cent of the anthropogenic CO₂ from the atmosphere and it would be a matter of concern if their ability to do this decreased under global warming.  If levels continue to increase, to above  500 – 600 parts per million as many scientists expect to be the case by 2050, then the climate shifts and warming effects and will become even more dramatic and surpass those of the Last Interglacial warm period”

“Sea level rises and falls have occurred throughout geological history and are all slightly different from one another.  However what took place in the last period of global warming 125,000 years ago gives us an idea of what to expect under the current phase,” he said.

“We should certainly be paying attention to what the corals are telling us.”