Predicting local coral reef responses to global warming in the Anthropocene

By Dr Sean Porter and Tanja Hanekom

South Africa has exceptionally rich marine biodiversity around its entire coastline. In the north-eastern tropical parts of the country, this is exemplified by Africa’s southern-most coral reefs, located in the iSimangaliso Wetland Park World Heritage Site. These beautiful coral reefs are rich natural assets and support the livelihoods of many people in the area, as they are world class diving destinations (Figure 1). Although they are well protected from local threats, they are vulnerable to global-scale threats such as climate change – especially global warming.

Anthropogenically induced climate change is one of the key defining characteristics that typifies the Anthropocene epoch we find ourselves in – a period during which human activity has been the dominant influence on climate and the environment. Unfortunately, the marine environment and coral reefs in particular, have not been spared from the negative impacts of human activities. Ocean warming and marine heat waves have become more intense and regular since the industrial era and have been directly linked to greenhouse gas emissions from the combustion of fossil fuels. Air pollution in Europe is equivalent to smoking two cigarettes per day, and in parts of India and China, it is equivalent to smoking a pack a day (Figure 2)!

Coral reefs are very sensitive to abnormally warm temperatures, which are predicted to occur more frequently in the next hundred years due to climate change. In fact, 2017 was the third warmest year on record, 2019 was the second-most and 2016 was the warmest year ever recorded. 

 

Coral reefs act like the caged canaries miners used to carry with them to warn them of noxious gases; corals similarly act as an early warning sign of adverse conditions – in this case global warming. Abnormally warm water temperatures cause corals to lose their colour, bleach and die, a phenomenon known as coral bleaching (Figure 3). This has devastating socio-economic impacts and directly affects people’s livelihoods (e.g. fisheries, ecotourism, coastal erosion, etc.).

Baseline information on reef growth (biogenic accretion and dissolution of coral reefs) in South Africa and in the western Indian Ocean does not exist, and the effects of climate change on these processes at the individual coral colony level and at the community level are largely unknown. Therefore, laboratory experiments were conducted on typical South African corals and coral communities with the aim of measuring calcification (accretion and dissolution), growth and the ability to acquire energy (photosynthetic efficiency) under current and future global warming temperatures predicted to occur with regularity within the next 80 years. See the SAAMBR coral experimental setup.

The results showed suppressed growth in hard coral and especially soft coral under future predicted temperatures by as much as 20-85% respectively, over a month-long period. Concomitantly, photosynthetic efficiency also declined by approximately 20%, in both hard and soft coral. At the coral reef community level, net community calcification also decreased, by 40%. Hard corals began to bleach and die after a month of being exposed to temperatures predicted to occur within the next 80 years and soft corals also showed obvious signs of thermal stress with increased mucous secretion.

The results indicated that predicted future increases in temperatures within the next 80 years will severely affect our local coral reefs in South Africa, despite their high-latitude location where temperatures are relatively cooler compared to the tropics (Figure 4). This will negatively impact coastal livelihoods dependent on these reefs for ecotourism and will impoverish our natural heritage. We have already started seeing some of the effects of global warming on our local reefs, with cases of mild coral bleaching already having been documented.

The Paris Agreement on climate change aims to keep global temperature rise to below 2°C. At the current rate of global warming, the Earth’s long-term average temperature will reach 1.5°C above the 1850-1900 average by 2035 and 2°C by 2065. Significant action is therefore required to reduce human activities that contribute to greenhouse gas emissions soon if the goal of the Paris Agreement is to be realised and the future of coral reefs safe guarded from global warming.

Further reading:

Obura et al 2017: https://gcrmn.net/wp-content/uploads/2019/03/COI-REEF-LR-F2.compressed.pdf

Gudka et al. 2018: https://drive.google.com/file/d/1vook-hHa3A9cuY0WZV0KmGqE768LT_Ws/view

Porter & Schleyer 2017: https://link.springer.com/article/10.1007/s00338-016-1531-z

Schleyer et al. 2018: https://www.sciencedirect.com/science/article/abs/pii/S0025326X18306532

Berkeley Earth:  http://berkeleyearth.org/