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February 19, 2010 – Original Source: New Scientist

Rising ocean temperatures might leave coral reefs in seriously hot water – without clouds for protection.

Five years ago Graham Jones and his team at Southern Cross University in Lismore, New South Wales, Australia, demonstrated that algae living in coral tissue produce a gas called dimethyl sulphide (DMS). When released into the atmosphere, DMS helps clouds form over coral reefs. Jones says that the clouds block sunlight and cool the sea.

His team have now discovered that a rise in ocean temperature of only 2 °C causes some algae to stop producing DMS. As a result, fewer clouds will form over the coral, thinks Jones, allowing more sunlight to shine on the water, warming it still more.

Heat treatment

Jones and his colleague Esther Fischer studied staghorn corals taken from Heron Island in Queensland, Australia, by subjecting them to different water temperatures in the lab while recording the amount of DMS released into the atmosphere.

When the water temperature rose from the annual mean of 24 °C to 26 °C, no DMS was released, says Jones.

Normally, DMS exuded by coral algae is picked up by the wind and carried up into the atmosphere. The gas is oxidised and forms small sulphur aerosol particles that attract water vapour and produce clouds.

The findings support Jones’s past work, which found that extreme warming of water around the Great Barrier Reef in the early 1990s led to lower DMS levels in the water. But he says this is the first study to measure the effect of water temperature on the amount of DMS entering the atmosphere.

From coral to rainforest

Jones also suspects that DMS may have a significant role in the regional climate of north Queensland. He says that in winter, south-easterly trade winds may carry the DMS aerosol particles into rainforests, producing rain; in the monsoon period, north-easterly winds are responsible for the rainfall.

“We believe it is no coincidence that much of Australia’s rainforest lies adjacent to the northernmost reefs,” says Jones. If this is true, he says, lower levels of DMS over coral reefs could dry out north Queensland’s rainforests.

But so far, Jones has only suspicions: there is a loose trend worldwide between the locations of coral reefs and rainforests, but much more research is needed, he says.

Other explanations

David Bourne (PDF) of Australia’s Institute of Marine Sciences in Townsville, Queensland, says it’s likely that rising ocean temperatures could stop the production of DMS. At high temperatures coral expel their algae: “If you lose the algae, it makes sense that you see the loss of DMS,” he says.

There are other explanations for the forests’ locations, however, says Penny Whetton of CSIRO Marine and Atmospheric Research in Aspendale, Victoria. These include atmospheric circulation – the movement of air worldwide – and the altitude of the Queensland rainforests.

Jones’s findings will be presented at an international symposium on DMS at the National Institute of Oceanography in Goa, India, in October.

Journal reference: Environmental Chemistry, DOI: 10.1071/en06065 (in press)

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