Even the Best Heat-Adapted Corals in the World are Doomed by Climate Crisis

Isabel Williams
Nov 4 · 4 min read

Coral reefs are the most significant marine ecosystems in the world. They cover just 0.1% of the ocean floor and yet support more than 25% of all marine fish species. The services they provide are worth over $1 trillion, including coastal protection, tourism and fishing. It is the intricate three dimensional structure of coral reefs that makes it so significant and able to provide these services.

Corals can dissipate wave energy and reduce the impact of storm surges on coasts, without which protection millions of dollars would have to be spent on building flood defences. The beauty of the diverse shapes and colours of the tangled metropolis of the reefs draws millions of people every year to experience it for themselves, generating significant income for local residents. Tourists also come to admire the fish diversity supported by the corals, and it’s this fish diversity and abundance which further helps local communities via sustainable fishing activities.

Corals are animals, small polyps residing in the calcium carbonate skeleton that is so important as a habitat for all the other life on the coral reef. But they form a symbiotic relationship with photosynthetic algae. These algae provide the coral with up to 90% of its energy, without which it wouldn’t be able to build all the incredible structures seen on the reef.

When temperatures get too hot, the coral expel these symbiotic algae in a process known as coral bleaching. The once rainbow of the reef fades to an eerie ghostly white. If the increased temperature persists then the polyps starve and erosion of the calcium carbonate skeletons proceeds, reducing the integrity of the reef structure.

Coral reefs are astonishingly beautiful and incredibly valuable to human well-being. They have also been doomed by human activity to destruction. The IPCC’s 2018 special report on 1.5°C of warming predicts with high confidence that above 1.2°C “coral-dominated ecosystems will be non-existent.” Global warming induced by anthropogenic greenhouse gas emissions has increased the length and frequency of dangerously warm days in reefs, resulting in greater mass bleaching events. With predictions based on current emissions trends suggesting warming of between 1°C — 3°C, the wholesale destruction of reefs looks to be truly catastrophic.

Some corals, such as those in the Arabian/Persian Gulf, are situated in naturally warmer waters than others and so have a higher tolerance to heat, suggesting that they will be more resilient in the face of global warming. So have these corals really managed to escape the climate change gauntlet?

Unfortunately, despite being more resilient to higher water temperatures, there is another destructive force looming up on reefs: ocean acidification. Carbon dioxide dissolves in water to make it slightly acidic, and this disturbs the carbonate equilibrium in the sea. The Gulf coral polyps can only colonise seafloor that is hard and cement-like, and the formation of this kind of ground is disrupted by the higher acidity of the sea. Scientists predict that within 300 years increased carbon dioxide will significantly degrade the substrate requirements for these unique corals. It seems even the best adapted reefs in the world cannot cope with what we’re throwing at them.

Bibliography

Heron et al. 2017. Impacts of Climate Change on World Heritage Coral Reefs : A First Global Scientific Assessment. Paris, UNESCO World Heritage Centre.

WWF. 2018. Living Planet Report — 2018: Aiming Higher. Grooten, M. and Almond, R.E.A.(Eds). WWF, Gland, Switzerland.

Saving the World’s Coral Reefs, Coral Reef Alliance, URL: https://coral.org/coral-reefs-101/why-care-about-reefs/food/ last accessed: 03.11.19

Purkis, S.J., Renegar, D.A. & Riegl, B.M. (2010) The most temperature adapted corals have an Achille’s Heel Marine Pollution Bulletin, 2011 pp 246–250

Riegel B.M., Purkis, S.J., Al-Cibahy, A.S., Abdel-Moati, M.A. & Hoegh-Guldberg, O. (2011) Present Limits to Heat-Adaptability in Corals and Population-Level Responses to Climate Extremes, PLOS One 2011

Hoegh-Guldberg, O., D. Jacob, M. Taylor, M. Bindi, S. Brown, I. Camilloni, A. Diedhiou, R. Djalante, K.L. Ebi, F. Engelbrecht, J. Guiot, Y. Hijioka, S. Mehrotra, A. Payne, S.I. Seneviratne, A. Thomas, R. Warren, and G. Zhou, 2018: Impacts of 1.5ºC Global Warming on Natural and Human Systems. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)].

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