Warmer seas may rob corals and rainforests of clouds

Just 2 °C more and reefs stop producing a cloud-seeding gas, which could leave corals hotter still and rainforests drier

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. 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. 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. …