Foul Bay in Western Australia, where fossilised coral remains have indicated polar ice-caps have melted in the past.

Credit: CoECRS

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SYDNEY: A fossilised coral reef in Western Australia has raised fears that climate change may threaten coastal Australian cities, scientists have said.

The reef – the most southerly coral reef discovered – lies in Foul Bay, near the Margaret River. Lying above the high tide mark, the 125,000-year-old reef indicates the sea level in the previous period of warmer global weather, called an interglacial period, according to Malcolm McCulloch, deputy director of ARC Centre of Excellence for Coral Reef Studies (CoECRS).

“The coral reef exists a lot farther south than they are currently now found,” said McCulloch. “In fact, we’ve shown that the water temperatures in the southern part of Australia were warm enough throughout that whole [interglacial] period to enable coral reefs to grow and survive. It’s a combination of showing there were both warmer waters and higher sea levels. It also means there was much more extensive melting of ice sheets.”

McCulloch explained there are two reasons for increases in sea levels. Thermal expansion increases the water volume, yielding a half-metre increase in sea level for every 2-to-3-degree Celsius hike in temperature. The other source of sea level rise comes from the melting of the massive ice sheets that are sitting on continents, he said.

Thermal expansion was figured into the calculations, said McCulloch, but calculating the effects of ice sheet melting are much harder. “What this work shows is that if we had sea level [rise] of 3 to 4 metres, at least the majority of the rise must be due to the more extensive melting of the ice sheets. It does show that the ice sheets that we presently have can certainly readily melt more. … It wouldn’t be unexpected because that’s what happened the last time the Earth was going through a warming period.”

During the previous interglacial period, the Earth was warmer, even though the carbon dioxide (CO2) levels were much lower. Back then, the CO2 levels were 280 parts per million (ppm), said McCulloch. However, the temperature was warmer because the “heat received from the sun in the northern hemisphere was a little more than today,” he said.

These days, human production of greenhouse gases, one of which is CO2, has warmed the Earth in a different way, according to McCulloch, with the extra CO2 trapping the heat that arrives on Earth.

“With global warming, the inference is that we warm our planet by adding CO2,” he said. “The potential to get another 3 to 4 metres of sea level rise is pretty straightforward. The Earth has done that in the past. People speculate there’s even much more sea level rise than 3 to 4 metres, but that’s a bit more controversial.”

Since the last interglacial period, the levels of CO2 have risen about 100 ppm, bringing the amount of CO2 to 380 ppm, said McCulloch. If CO2 levels reach 500 ppm and above, the temperature would increase by 2 to 3 degrees Celsius, making it just as hot as the previous warming period.

The most alarming aspect of the research is that scientists have no way of predicting when ice sheets will collapse, causing a surge in sea levels.

“The problem with melting ice sheets is that it doesn’t happen gradually or steadily. It can be catastrophic and … the concern is the rate of sea level rise,” said McCulloch. “There may not be much time. These things could happen quite rapidly, like tens of years.”

Beyond the environmental fears, McCulloch also foresees problems for Australia’s cities, many of which lie on the coast.

“Sea level rise would change quite a bit,” he said. “Things like Sydney Airport would get wiped out as well as things like housing and a lot of the major infrastructure we have in big cities, that we have on our coast, around Melbourne, Sydney and Brisbane.”