Possibly even more serious is the problem of coral bleaching, the process in which coral literally loses its colour and turns a pallid white. The stresses can include pollution, changes in salinity levels, or silt. And rising water temperatures compound the other factors. Wider implications Corals usually live at the upper edge of their temperature tolerance, and 1998 was the hottest year since records began. An increase of even one degree above the usual maximum temperature at a site can be fatal.
Many corals usually recover from a short spell of bleaching.
But longer periods, or recurrent bouts, can leave them severely weakened and more susceptible to disease and damage. Coral reefs have a wide ecological value NOAA found that a rise in water temperature of one degree for more than a month was often enough to cause extensive bleaching. It is not clear what role the El Nino phenomenon in the Pacific played in raising ocean warmth last year. But climatologists believe global water temperatures could rise by about two degrees over the next half century as climate change takes hold. So John Prescott's dives may not have been very happy ones.