Climate change: what impact on the survival of the world’s coral reefs?

Talk by Nick Graham

The coral reefs are not necessarily well known by the general public. They are built by animals; corals are in fact tiny coral animals called polyps that grow in colonies and are distinguished by the diversity of their colours and structures. A food base for entire populations in the tropical regions, they are also the lifeblood of a major economic activity: tourism. 

Sadly, this environment is affected by climate change as illustrated first and foremost by coral bleaching. A healthy coral is characterised by the symbiotic relationship between the coral animals and the nourishing algae in their tissue. The coral’s energy, which gives it its bright colour, occurs through photosynthesis. But it is a delicate balance, and when the coral is affected this relationship breaks down. The coral animal rejects the algae which can no longer provide it with its energy and colour. Once the algae have been ejected from the tissue, the coral animal becomes translucent, hence the bleaching effect. This coral “stress” is largely due to heat. Coral can only grow within a limited thermal tolerance range; all it takes is a 1°C increase in temperature for bleaching to occur. Without algae to feed it, coral can live no more than five or six weeks before starving to death. 

The thermal enemy named El Niño

Over the last few years there has been a rise in coral bleaching. Its devastating effects are can be confirmed within a short amount of time as show by a photo of a reef in December 2014, bleached two months later and dead in August 2015. There has been much discussion in the media about the fate of the Great Barrier Reef. But this is not the only reef to fall prey to these waves of bleaching: many countries in the Pacific Ocean have lost 80% or more of their coral. 

Coral bleaching is linked both to rising temperatures and temperature variations. In addition to the steady rise in oceanic temperature over the years, there are the effects of the transient and extremely intense warming of phenomena such as El Niño, the most widespread.

This type of event often occurs when warm water is concentrated to the east and the centre of the Pacific Ocean. El Niño could change climate models across the globe. The drought that is affecting certain parts of Amazonia has led to forest fires, while in Peru we observe massive rainfall. 

There has been a sharp rise in the water temperature of the Caribbean Sea and the Indian Ocean. El Niño is therefore taking place at a time when ocean water temperatures are gradually increasing, leading to coral bleaching which only recently has been observed on such a large scale. The phenomenon was first observed in 1983, and in 1998 widespread bleaching was found in the Tropics, linked to an extremely intense El Niño event. An estimated 16% of coral life has perished this year alone, especially in the western part of the Indian Ocean which has lost 50% of its coral. In the same period, over 90% of the coral in the territorial waters of the Seychelles perished. The reef has just about been wiped out. 

The variable resistance of the reefs

Needless to say, the extinction of the coral reef has consequences on ecosystems. Coral is home to many different types of fish that live off the reef so long as it remains intact. In the Seychelles, it is estimated that half the fish species have disappeared with the extinction of the coral. The more vulnerable fish are of course the corallivore species, i.e. those that feed off the coral, although they are not all affected to the same extent. It has been established that four species of fish whose lives depend on the corals have disappeared from the waters of the Seychelles.

After the big wave of coral bleaching in 1998, the condition of 21 coral reefs is monitored every three years since 2005 in the Seychelles. The aim is to establish whether the coral recovers over time, or whether a new ecosystem takes its place. We observe that sea grass often grows in these areas, and that these large algae prevent the coral from growing back. Yet 12 out of the 21 monitored reefs have recovered their structure over the last 15 years. 

The coral reef currently covers less than a quarter of the total ocean surface

The coral reef currently covers less than a quarter of the total ocean surface and represents up to 33% of marine biodiversity. It also provides food security to millions of people. The contribution of this natural heritage to nutrition and medical progress is estimated at 350,000 dollars per hectare, making its preservation critical. It future partly depends on our commitment to honouring the Paris Agreement. Not doing so will spell the end of massive surfaces of coral.