The causes of reefs bleaching

12 June 2023 | Protect the environment

Coral reefs are majestic ecosystems but fragile. The bleaching of reefs isn’t an unknown and misunderstood phenomenon anymore. Close to 90% of corals might disappear by 2050. Only, if we know the causes, it will be possible to act. After our first article presenting the coral reefs operation, let’s follow this series by analyzing a few causes, of human and non-human origin. 

Environmental factors 

 A major part of builder corals of reefs lives in symbiose with a seaweed, the zooxanthellae. It provides carbohydrates to the coral by the realization of the photosynthesis. Thus, the coral can grow and build its calcium carbonate skeleton. We say that this relation is symbiotic because, in exchange of this service given by the seaweed, the coral offers to its zooxanthellae essential nutrients for its survival thus a protected environment. 

 This symbiotic relation can be disturbed by a few factors, provoking a stress or a coral fracture. Entraining thus a fragilization or even a mortality. 

 Meteorological conditions and extreme climatic events such as storms, tsunamis, hurricanes or cyclones can deteriorate coral reefs by many ways:

  •     Heavy rainfall can lower the localized and of a punctual way the water salinity. Even a small change of life conditions of corals cause them a stress that fragilize them. 
  •     Strong waves formed by the swell and wind generate instant damages by breaking corals. Especially damaging in shallow water, up to 20 meters.

They fall and then can’t bounce back alone, get silted up and can’t go out of the sand to feed them and carry out a photosynthesis through the zooxanthellae. Moreover, when a coral fall, it can lead other corals into its fall, causing more damages again; 

  •     Runoff water, following the torrential rains that fell on soils, runoff water is now charged in sediments and flows to the sea. 

 Some of these factors are nevertheless influenced by actions from human origins. Indeed, cyclones are chronic climatic events. They however touch localized places, the Great barrier reef in Australia has for example known 55 episodes between 1969 and 1997.

But, the warming and the acidification of some lands of ocean have increased the frequency of these climatic upheavals, making corals more fragile and more vulnerable. 

Factors of anthropogenic origin

 Indeed, global warming accelerates the formation and the creation of weather conditions, and becomes more and more violent. Storms, cyclones, earthquakes, are part of the life of coral reefs as we just see it. 

 The destruction isn’t a fatality for reefs. Some fall from their feet and are reformed. Those that fall but don’t survive will be excellent substrates to new colonies. 

In contrast, repeated disasters and in fact, the repeated aggression prevent corals to grow and survive to the many stresses involved. Aggressions of a human origin, we can talk about the overfishing, pollution (carbon emissions more and more strong, plastic, spills) and mass tourism. 

The overfishing

 Needs in the fishing sector always grow. This caused the overfishing of many species and also reef species. 

As an extreme action, the overfishing affects the balance and the fragile harmony of the reef life. 

For example, fishing a lot of the herbivore fishes harms sustainably at life and at coral survival. The herbivore fishes aren’t enough anymore to ensure their roles: graze the grass present in the reef. Thus, leading to the increase of proliferating and asphyxiating algae on the corals.

Indeed, according to a study of 2017 published in Nature, it is shown that in the Caribbean, the parrot fishes play a determining role by grazing the seaweeds in competition with the corals. Moreover, the massive fishing of herbivorous fish populations is now becoming a determining factor into the increase of seaweeds causing the death of corals. 

Weight the fishery is essential for the life and the survival of coral reefs.

Acidification of the ocean and increase of temperatures 

The acidification of the ocean can be defined by a diminution of Ph, caused mostly by the CO2 absorption present in the atmosphere. We note at the same time, a considerable increase of carbon dioxide present in the atmosphere since the industrial revolution. 

As explained in the article on the role of coral reefs, the ocean is a wonderful carbon sink. They absorb a fourth of the CO2 produced by humanity. More important than forests, that are in the second position. 

Yet, this absorption reaches its limits: the too much fill of CO2 absorbed provokes modifications of the chemistry of ocean carbonates, which prevents plants and marine animals from building their shells or other limestone habitats. For this reason, struggling against the carbon emissions of websites allow to protect the oceans and their ecosystem, and a natural major carbon sink. 

Increasing temperatures 

 According to a recent study published the 1st of February 2022 in the review PLOS Climate, even if the announced objectives of the Paris Agreement are respected, coral reefs are probably condemned in reason of global warming. 

Indeed, if the ambitious objective of an average increase of 1.5°C compared to the pre-industrial era, still according to the Paris Agreement, is reached, more than 90% of corals aren’t capable to adapt against the repeated heat waves.

 Furthermore, with a warming of +2°Cn the mortality would reach 100% according to the study published in the revue PLOS Climate cited above. 

Massive tourism

The massive tourism has an instant impact on coral reefs:

  •     Wild mooring of touristic boats ;
  •     Solar cream ; 
  •     Trampling ; 
  •     Palm strokes ; 
  •     Water charged in sediments and/or polluted by the fact of the construction of new touristic infrastructures ; 
  •     Etc. 

 The site of Maya Bay in Thailand is now the example most cited to show the impact of the massive tourism on corals. Closed until 2021 to allow the biodiversity to regenerate, associations of coral reefs rehabilitation have also played a role in the coral regeneration. For example, Ocean Quest Global, that has planted more than 20 000 corals in the bay to allow reefs to renew itself more quickly. 


The bleaching causes have two origins:

  •     Naturals: like cyclones, storms and hurricanes ;
  •     Anthropogenic: the acidification and the increase of temperatures, both caused by an increasing too much important of carbon emissions compared to the pre-industrial era, the massive tourism, the overfishing, etc. 

 The coral reefs, like the ocean in a general way, are resilient, as long as they are given time. If nothing is made, almost 90% of corals are likely to disappear by 2050. 

 In fact, it’s nowadays essential more than ever to reduce our carbon emissions. 

Thus, Greenoco contributes by helping his clients to reduce the carbon footprint of their websites. The digital pollution can be reduced by actions easily feasible, in order to know which ones. Every website doesn’t emit the same quantity of carbon dioxide. That’s why we’re making personalized audits to accompany you in this approach. 

 To realize a first measure of your website, don’t hesitate to use our online simulator of the carbon footprint of website. 

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