Endangered coral reefs

27/07/2024

5 minutes

oceans and climate

Coral reefs around the world are facing an unprecedented crisis due to global warming. A recent study by the Global Coral Reef Alliance, published in the Oxford Open Climate Change journal, shows that 2023 was the hottest year on record, with record ocean temperatures. This critical situation reveals major changes in ocean circulation that threaten the survival of reefs, and calls for urgent action to mitigate these impacts. The researchers say that if current trends continue, reefs, which are vital to marine life and the global economy, could disappear.

By Laurie Henry

Coral reefs, often referred to as the rainforests of the ocean, play a vital role in marine biodiversity and coastal protection. However, these ecosystems are currently threatened by global warming, which is responsible for massive coral bleaching.

RECORD TEMPERATURES IN 2023

The year 2023 was marked by the highest temperatures ever recorded on land and at sea. Satellite data shows that sea surface temperatures in the northern hemisphere have reached critical levels. In particular, a study by the Global Coral Reef Alliance (Goreau and Hayes, 2024) highlighted massive coral bleaching around Jamaica. This temperature rise has been confirmed by measurements taken in 175 countries, revealing unprecedented thermal anomalies that are one degree Celsius warmer than the warmest monthly averages previously observed.

2023 is confirmed as the hottest year on record. BBC illustration produced with ERA5, C3S/ECMWF data © Goreau et al., 2024

Researchers used hotspot analyses to identify areas where surface water temperatures exceeded seasonal averages, thereby predicting bleaching events. Forecasts for 2024 indicate that reefs in the southern hemisphere could also be seriously affected.

CORAL BLEACHING IN 2023

Coral bleaching is a phenomenon in which corals lose their colour and food source as a result of thermal stress. The phenomenon is caused by the expulsion of zooxanthellae, symbiotic algae that live in coral tissue and provide both their colour and much of their food through photosynthesis. When water temperatures rise significantly, stressed corals expel these algae, leading to bleaching. Areas such as the Caribbean, the east and west coasts of Mexico and Central America, and Pacific islands such as Kiribati and Fiji have reported cases of bleaching and near-total coral reef die-offs. Around Jamaica, sea surface temperatures were 1.5°C above historical averages, causing massive bleaching.

Map of hotspots of maximum coral reef bleaching in 2023 produced by NOAA. © Goreau et al., 2024

Satellite data and local observations have shown that almost 63% of the world’s coral reefs were affected by bleaching in 2023. In Jamaica, almost all the “hard” corals have completely bleached and many have died. “Hard” corals or “hermatypic” corals are those that build reefs by secreting calcium carbonate to form solid structures.

This phenomenon has a profound effect on the marine ecosystem, as bleached corals are more susceptible to disease, predation and pollution. Even if they survive the initial bleaching, their growth and reproduction are severely impaired.

MAJOR ENVIRONMENTAL AND ECONOMIC IMPACTS 

The disappearance of coral reefs would have major ecological and economic consequences. They provide a habitat for around 25% of all marine species, although they cover less than 1% of the seabed. Coral reefs also play a vital role in protecting coastlines from storms and erosion, absorbing wave energy and sheltering coastal communities.

Map of maximum warming degrees per week for coral reef bleaching, produced by NOAA. © Goreau et al., 2024

Their decline would directly affect the fishing and tourism industries that are vital to millions of people around the world, particularly in developing countries. Commercial and artisanal fisheries depend on reefs for the reproduction and feeding of many fish species.

In addition, coral reef tourism, including diving and snorkelling, generates billions of dollars each year. The loss of these ecosystems would not only reduce marine biodiversity, but also the economic income and livelihoods of local communities.

CHANGES IN OCEAN CIRCULATION 

As well as bleaching, this record year has had another impact on the oceans. The authors show that global warming has caused significant changes in ocean circulation. Warm currents such as the Gulf Stream and the Kuroshio have shown accelerated warming, transporting tropical heat towards the polar regions. Specifically, in 2023, warm currents transported heat towards the poles at a rate 15% faster than in previous decades.

This accelerated transport has intensified warming in the subpolar oceans, contributing to accelerated melting of polar ice and more pronounced ocean stratification. This increased stratification prevents vertical mixing of surface and deep waters, limiting the exchange of heat and carbon dioxide between different layers of the ocean.

The main warm currents are represented by white arrows and the cold currents by black arrows. All major surface currents and upwelling areas are warming faster than average and show strong hot spots in 2023  © Goreau et al., 2024These changes amplify the effects of global warming, a crucial factor not included in the IPCC models. Cold currents, such as the upwelling areas off the west coast of South America, have also warmed significantly. By 2023, these currents show a 20% decrease in upwelling activity, reducing primary productivity and severely impacting local fisheries. The reduction in upwelling means that fewer nutrients are being transported from deep waters to the surface, reducing the food available to marine ecosystems.

These altered dynamics are not only contributing to the melting of polar ice, but also to rising sea levels, which directly threaten coastal ecosystems and the human communities that depend on them. The researchers stress the urgent need for global climate action to mitigate these impacts and protect coral reefs, which are vital for marine biodiversity and coastal protection.


Source : Thomas J F Goreau, Raymond L Hayes, « 2023 Record marine heat waves: coral reef bleaching HotSpot maps reveal global sea surface temperature extremes, coral mortality, and ocean circulation changes », Oxford Open Climate Change, Volume 4, Issue 1, 2024, kgae005

Cover Photo : Bleached area of Acropora palmata photographed in Jamaica in October 2023  © Sabine Hossenfelder/Oxford Open Climate Change

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