The European Space Agency's Envisat satellite could be used to keep tabs on the state of the oceans' coral reefs, Australian researchers say.

Scientists from Australia's Commonwealth Scientific and Industrial Research Organisation's (CSIRO) Wealth from Oceans Flagship program say that the satellite's MERIS sensor can detect coral bleaching down to a depth of ten metres. In theory, the satellite could scan reefs worldwide on a fortnightly basis.

The MERIS sensor takes multispectral optical pictures of Earth's oceans, land and atmosphere.

Normally the observations are made from boats, planes or helicopters, but this is a slow process, and many reefs are not easily accessible. Regularly collected data would be invaluable to researchers because bleaching events can happen quickly, and bleached coral is soon colonised by blue-green algae. This makes it hard to spot, the researchers say.

CSIRO's Dr. Arnold Dekker notes: "Coral bleaching needs to be mapped at the global scale. High-spatial resolution satellites can only do it on a few reefs due to cost and coverage constraints. We need a system that has appropriate coverage and revisit frequency, with a sufficient amount of spectral bands and sensitivity."

He argues that there is no system better suited to the task than the MERIS instrument.

When scanning in full resolution mode, the instrument is so sensitive that for each complete 300-metre pixel of coral under one metre of water it should be possible to detect a two per cent bleaching of live coral. Under ten metres of water, a bleaching of seven to eight per cent should still be detectable.

Coral bleaching is likely to be one of the first tangible effects of global warming, as it is closely linked to unseasonably high summer sea temperatures, and to solar radiation. Extensive bleaching events in 1998 and 2002 have been linked to the shifting El Niño current.

"The concern is that coral reefs might pass a critical bleaching threshold beyond which they are unable to regenerate," Dekker concludes. ®