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