Coral bleaching is a condition that can seriously damage or kill entire reef systems. Corals contain microscopic plants called zooxanthellae that colour the corals' tissue and provide them with food by photosynthesis - the process by which plants manufacture food from light.

Without these tiny plants corals cannot survive or deposit the huge amounts of limestone in their skeletons. When corals become stressed, the zooxanthellae are the first to go. Stressed corals expel the zooxanthellae turning white or "bleaching". If zooxanthellae do not re-colonise the coral's tissue, the coral will die.

Mass bleaching is considered by most reef scientists to be a serious challenge to the health of the world's coral reefs. The worst coral bleaching ever was recorded in 1998. Every reef system in the world's tropical oceans was affected. In some places, such as the Indian Ocean, entire reef systems died.

What causes coral bleaching?

Increases in ocean temperatures are causing the rise in the intensity, frequency and extent of coral bleaching. The six major episodes of coral bleaching over the past 20 years were caused by periods of increased water temperature. Corals are highly sensitive and can only live in water between 18 to 30 degrees Celsius. Most bleaching events are attributed to a 1.0 degree Celsius increase in temperature above the summer maximum water temperature.

Tropical sea temperatures have increased by 1.0 degree Celsius over the past 100 years and are currently increasing at a rate of 2.0 degrees C per century. Mass death of corals frequently and increasingly follows bleaching events. In addition to killing corals, increased temperature has recently been found to inhibit coral rehabilitation by reducing its abilities to reproduce and grow.

Increasing levels of greenhouse pollution from the burning of coal, oil and gas are pushing up average global temperatures. This is recognised by the majority of scientists, the United Nations and governments around the world.

Can't coral adapt to warmer temperatures?

Our climate is changing at a very fast rate. Corals do not appear to show any sign that they are able to adapt fast enough to keep pace with an accelerating increase in ocean temperatures. Arguments that corals will adjust to predicted patterns of temperature change are unsubstantiated and most evidence shows that bleaching events are signs that the ability of corals to acclimatise is currently being exceeded. Corals may adapt in evolutionary time, but such changes are expected to take hundreds of years, suggesting that the quality of the world's reefs are declining rapidly.

This has enormous implications for the health and wealth of tropical and subtropical societies.