Coral reefs, often referred to as the “rainforests of the sea,” are among the most biodiverse and crucial ecosystems on Earth. They sustain around 830,000 species^[1] - over 33% of all known marine life - and deliver nearly USD 10 trillion in value through food, livelihoods, and coastal protection, directly supporting close to a billion people worldwide.^[2] However, these critical ecosystems are under severe threat. Since 2009, 14% of the world’s coral has disappeared, and the reefs that remain now face an unprecedented fight for survival.

A report released on April 23 by the International Coral Reef Initiative (ICRI) - a global partnership dedicated to reef preservation - underscored the severity of the crisis. It confirmed that the world is currently experiencing its fourth global coral bleaching event, which began on January 1, 2023. To date, the event has affected 84% of coral reefs across at least 82 countries and territories, making it the most widespread and prolonged bleaching episode ever recorded. ^[3] In comparison, previous global bleaching events impacted 21% of reefs in 1998, 37% in 2010, and 68% between 2014 and 2017. Bleaching is triggered when ocean temperatures rise 1°C to 2°C above normal, causing corals to expel the symbiotic algae that both give them their vibrant colour and supply vital nutrients. Deprived of these algae, corals turn pale, become more susceptible to disease, and risk death. Although corals can recover from brief periods of heat stress, prolonged or repeated exposure - as seen in recent decades - has severely weakened reef systems and drastically reduced their capacity to regenerate.

The ongoing global coral bleaching event is one of the clearest and most alarming signs of climate change’s impacts on the world’s oceans. Covering around 70% of the planet’s surface, the oceans absorb more than 90% of the excess heat trapped by greenhouse gases emitted through fossil fuel combustion, deforestation, and other human activities. And that heat gain is accelerating. The Intergovernmental Panel on Climate Change (IPCC) estimated that between 1971 and 2018, the oceans absorbed approximately 396 zettajoules of heat or 91% of the Earth's total heat uptake during that period. ^[4] This amount of energy is equivalent to the detonation of more than 25 billion Hiroshima atomic bombs. It is also equivalent to the detonation of one Hiroshima atomic bomb in the ocean every second for nearly 50 years. The oceans further absorbed an additional 16 zettajoules of heat between 2023 and 2024 - an amount equivalent to 140 times the world’s total electricity generation in 2023. ^[5]

The majority of the heat absorbed by the oceans is concentrated within the upper 1 kilometer of the water column, acting as a buffer that has, until now, shielded humanity from the most severe impacts of climate change. However, this heat retention has also caused a significant rise in ocean surface temperatures. For instance, in 2024, the average surface temperature of non-polar oceans reached an unprecedented 20.87°C, surpassing the previous records of 2023 (20.80°C) and 2016 (20.61°C). ^[6] This rising temperature trend has led to a marked increase in marine heatwaves. Today, the ocean surface experiences approximately 50 extreme heat days per year, a dramatic rise from just 15 extreme heat days annually in the 1940s. ^[7] In some regions - such as the Indian Ocean, tropical Atlantic, and western Pacific - marine heatwaves occur up to 80 days each year, or roughly one in every five days. That has adverse impacts on sea life including coral reefs. Scientists compared the impacts that marine heatwaves had on sea life to “wildfires that take out huge areas of forest.” ^[8] The resultant loss of coral reefs threatens marine ecosystems, food security, economies, and storm resilience of coastal communities that depend on healthy reefs for their survival.

In addition to harming coral reefs, rising ocean temperatures contribute to the intensification of storms. The excess heat serves as fuel for hurricanes, cyclones, and typhoons, making them more powerful and destructive - particularly for coastal and even inland communities. Warmer oceans also cause seawater to expand, a process known as thermal expansion, which is a major driver of global sea level rise. This warming further accelerates the melting of glaciers and polar ice sheets, compounding the problem. The resulting sea level rise sets off a chain reaction of consequences, including the loss of coastal ecosystems and the services they provide, salinization of groundwater, increased flooding, and damage to infrastructure along coastlines.

Coral reefs are also increasingly threatened by plastic pollution. Each year, an estimated 11 million tonnes of plastic enter the oceans, with this number projected to rise to 29 million tonnes by 2040. ^[9] Once in the ocean, plastic debris can smother reefs, blocking the sunlight needed for photosynthesis and creating anoxic conditions that suffocate coral ecosystems. As plastics degrade, they break into sharp fragments that damage coral tissues, making corals up to 20 times more vulnerable to disease. Another major threat is the ingestion of microplastics, which corals mistake for food. This leads to a false sense of fullness, depriving them of essential nutrients and hindering their growth, structure, and reproductive success, endangering entire reef systems. Additionally, plastics leach toxic chemicals like bisphenol A (BPA) and phthalates, which disrupt the hormonal systems of marine life and contribute to reef die-offs. These toxins accumulate in marine organisms, posing significant risks to human health through seafood consumption.

The ongoing global coral bleaching event serves as a stark reminder of the escalating crisis in our oceans, underscoring the urgent need for immediate and comprehensive global climate action, as well as sustained marine conservation efforts. Without swift and coordinated intervention, these ecosystems risk crossing a critical tipping point, triggering cascading effects that could destabilize marine food webs and undermine the livelihoods of countless communities dependent on ocean resources. The window for meaningful action is closing rapidly, and the decisions we make today will have far-reaching consequences. Whether it is accelerating climate mitigation to curb further warming, strengthening adaptation measures to protect vulnerable marine ecosystems, curbing plastic pollution, or investing in the resilience and restoration of coral reefs, the steps we take now will determine the future health of our oceans. Protecting these ecosystems is no longer just an environmental priority - it is a moral, economic, and existential necessity. Our actions, or inaction, will shape the legacy we leave for future generations and the survival of life on Earth.

End Notes

^[1] Fisher, Rebecca et al. “Species Richness on Coral Reefs and the Pursuit of Convergent Global Estimates.” Volume 25, Issue 4, 16 February 2015, Pages 500-505. https://www.sciencedirect.com/science/article/pii/S0960982214016236
^[2] Costanza, Robert et al. 2014. “Changes in the global value of ecosystem services.” Global Environmental Change. Volume 26. May. Pages 152-158. https://www.sciencedirect.com/science/article/abs/pii/S0959378014000685
^[3] “84% of the world’s coral reefs impacted in the most intense global coral bleaching event ever.” International Coral Reef Initiative (ICRI). April 23, 2025. https://icriforum.org/4gbe-2025/
^[4] Forster, P. et al. 2021. “The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity.” In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, 923 Cambridge, United Kingdom and New York, NY, USA, pp. 923-1054. https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf
^[5] Cheng, L., Abraham, J., Trenberth, K.E. et al. 2025. “Record High Temperatures in the Ocean in 2024.” Adv. Atmos. Sci. 42, 1092–1109. https://link.springer.com/article/10.1007/s00376-025-4541-3
^[6] “Global Climate Highlights 2024.” The 2024 Annual Climate Summary. Copernicus Climate Change Service. January 10, 2025. https://climate.copernicus.eu/global-climate-highlights-2024
^[7] Marcos, Marta et al. 2025. “Global warming drives a threefold increase in persistence and 1°C rise in intensity of marine heatwaves.” Proceedings of the National Academy of Sciences. Volume 122. No. 16. https://www.pnas.org/doi/abs/10.1073/pnas.2413505122
^[8] Carrington, Damien. “Heatwaves sweeping oceans ‘like wildfires’, scientists reveal.” The Guardian. March 4, 2019. https://www.theguardian.com/environment/2019/mar/04/heatwaves-sweeping-oceans-like-wildfires-scientists-reveal
^[9] Xia, Zhu et al. “Plastics in the deep sea – A global estimate of the ocean floor reservoir.” Deep Sea Research Part I: Oceanographic Research Papers. Volume 206. 104266. April. https://www.sciencedirect.com/science/article/abs/pii/S0967063724000360

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