Coral bleaching, Great Barrier Reef, Australia, satellite image. Colours show chlorophyll concentrations in the waters around Heron Island (centre), ranging from pink (low) through blue, turquoise, green and yellow to red (high). Coral reefs, seen as the offshore areas of green and yellow (centre and upper centre) expel their chlorophyll-containing algae when sea temperatures rise. This causes them to lose their colour and die, and is known as coral bleaching. Red spots show areas of severe coral bleaching. North is at top. The Heron Island inset can be seen in image E690/056. The image area is around 600 kilometres across. The data was obtained in 2005 and 2006 by the MODIS instrument on the Aqua satellite.