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Warmer seas and coral: a bit good; mostly bad

  • 10 Feb 2012, 11:43
  • Verity Payne

Coral reefs growing on the West Coast of Australia have thrived throughout the 20th Century despite rising atmospheric carbon dioxide levels and warming seawater, according to a new study. But does this study mean that the concern over how man-made climate change might affect coral reefs is unfounded? As is so often the case in science, it's just not that straightforward.

The report's findings came as a surprise to the study's lead author Dr Timothy Cooper, of the University of Western Australia Oceans Institute, as he explains in this Science podcast:

"We were expecting in recent times that there would be a decline in coral growth rate for this particular coral."

The research paper, published in the journal Science last week, received a fair bit of coverage - particularly in Australia, with most articles declaring it good news.

The Herald Sun, for example, writes that the

"findings undermine predictions that global warming will devastate coral reefs."

Fox 9 describes the findings under the headline:

"Research: Ocean Warming Good For Coral"

Meanwhile the New York Times Green blog asks:

"Are other reefs winning rather than losing from the changes in the climate seen so far?"

Corals and temperature - a complex picture

Unfortunately, as newspapers often like to ignore, several things can be true at the same time. So while the results may point to growth for some corals, climate change is still causing others to change and, in some cases, die back.

The rising seawater temperature caused by global warming has generally been thought to be bad news for corals. This is because it can cause so-called coral bleaching - where corals expel the microscopic algae living in their tissues. The algae are crucial for the corals to survive, so if coral bleaching events are numerous or sustained entire reefs can die. 1998 saw the most extensive mass bleaching events due to the unusually warm seawater caused by strong El Niño conditions.

Scientists studying coral reefs have found that coral cover has declined in some areas over the last thirty years; in the Caribbean ocean, the Indo-Pacific ocean and on Australia's Great Barrier Reef for example.

Yet this study saw coral growth in the studied species - Porites, a reef-building coral - increase as the sea got warmer. And the effect seems to be greater in the more southerly reefs, where the increase in sea surface temperature has been the greatest. So the corals, particularly those in cooler water, are taking advantage of warming waters. This finding is similar to that of research published last year, which reports that some tropical coral reef species around Japan have advanced northwards since the 1930s, taking advantage of the spread of warmer waters.

Cooper and his colleagues say their study shows that the news for Western Australian corals is good, but only up to a point. They suggest that while these corals have thrived in the recent warmer seawater they might not continue to do so as temperatures rise further. This is because corals have an optimum temperature range for growing their shells. As seawater continues to warm the temperature could exceed the optimum temperature for the corals.

A new research paper - also published last week - highlights this. It finds that corals grown in the laboratory can be damaged by both cold and warm waters. The corals can, however, adapt better to prolonged cold temperature events than warm events.

Ocean acidification: still bad news for corals

And the story doesn't end with temperature. The world's oceans absorb carbon dioxide gas from the atmosphere that interacts with seawater, altering its chemistry. This is a process known as ocean acidification, as it makes the naturally alkaline seawater less alkaline (thus more acidic). You can find more details on ocean acidification here.

Scientists suggest that ocean acidification and the changes to ocean chemistry that go along with it bleach reef-building corals such as Porites, and diminishes their growth rates.

Since the start of the industrial revolution the oceans have absorbed around a third of the carbon dioxide emitted due to human activity. According to recent reports, this has caused a change in ocean acidity a hundred times greater than the natural rate of change over the last 20,000 years.

Ocean acidification is projected to increase as atmospheric carbon dioxide continues to rise. The video below shows how ocean acidity has changed, and is projected to change over the coming century:

The animation shows how aragonite saturation (a measure of ocean acidity) at the ocean's surface is projected to decrease towards the end of the 21st century as man-made carbon dioxide accumulation in the atmosphere continues to rise. Source: Tobias Friedrich, SOEST Hawaii.

Cooper and his team suggest that in the future Western Australian coral growth might no longer by controlled by temperature, but by ocean acidification. If ocean chemistry changes as it is projected to it seems unlikely that Western Australian corals will continue to do so well.

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