How can half a degree of average temperature result in such devastating impact on coral reefs?

Question

Yesterday I read a number of news stories about the new IPCC's (International Panel on Climate Change) report regarding the current state of global warming. It reported that we are on track for hitting their safety threshold for global warming, namely an increase of 1.5 °C average temperatures since pre-industrial times. A number of reports are made, such as for example:

• Maintaining the goal of 1.5 degrees instead of 2 degrees will result in 10cm lower sea levels by the year 2100.

However it's the report on the effect on coral reefs that really shocked me. At the moment our level is 1 degree above pre-industrial levels. The report predicts that if the level is allowed to rise to 1.5 degrees (which is supposedly going to happen in 2030 at this rate), that 70 to 90 per cent of coral reefs will be destroyed. Further, if this metric rises to 2 degrees, that we can expect over 99% of coral reefs to be destroyed. This pretty much sounds to me that they will all but be destroyed if we allow up to a 2 degree increase.

I'm pretty naive on the subject, so I'm wondering why such a temperature increase of half a degree can have such a marked effect on coral reefs in particular.

Answer 1

Coral die because of coral bleaching, a process in which the symbiotic algae living in the coral (which give the coral its colour), leave the coral host.

Bleaching can occur for a variety of reasons, but the ones that relate to a warming climate are:

1. Ocean acidification caused by dissolution of carbon dioxide in the water.
2. Thermal stress. According to this website, "Corals do best when ocean water is between 73 degrees F and 84 degrees F." (22–29 °C). Long durations of heated sea surface water exceeding that would cause bleaching and coral death. For example, a recent bleaching event in the Great Barrier Reef had temperatures of 4 °C above average, where the average is around 27 to 28. This puts it above the comfort zone of the coral.

Answer 2

Mysterious glowing coral reefs are fighting to recover

A new study by the University of Southampton has revealed why some corals exhibit a dazzling colourful display, instead of turning white, when they suffer 'coral bleaching' -- a condition which can devastate reefs and is caused by ocean warming. The scientists behind the research think this phenomenon is a sign that corals are fighting to survive.

Many coral animals live in a fragile, mutually beneficial relationship, a 'symbiosis' with tiny algae embedded in their cells. The algae gain shelter, carbon dioxide and nutrients, while the corals receive photosynthetic products to fulfil their energy needs. If temperatures rise just 1?C above the usual summer maximum, this symbiosis breaks down; the algae are lost, the coral's white limestone skeleton shines through its transparent tissue and a damaging process known as 'coral bleaching' occurs.