The reef surrounding Namotu Island, Fiji, has experienced serious coral bleaching caused by increasing ocean temperatures. Credit: Beau Pilgrim / Climate Visuals

By Kizito Makoye
DAR ES SALAAM, Tanzania, Jul 23 2026 (IPS)

Every morning, as the Indian Ocean retreats from the reef off Zanzibar’s Jambiani village, Chiku Chande steps into the knee-deep lagoon carrying bundles of seaweed seedlings. Years of farming these waters have taught her about every channel and coral outcrop. But lately, the corals that once glowed in brown and gold have become chalky white.

“You don’t need anyone to tell you these changes,” Chande tells IPS. “We see it with our own eyes. The coral has lost its colour.”

Like many families in Jambiani, the 48-year-old grandmother depends on the reef. It shelters her seaweed farm from crashing Indian Ocean waves, supports fish stocks and brings tourists to the village. As the corals bleach and slowly die, she worries about the future.

Chande has watched the reef change year after year. Scientists say her observations mirror what is happening across the world’s tropical oceans.

Scientists long regarded El Niño – the natural warming of parts of the Pacific Ocean that disrupts weather patterns globally – as the main driver of mass coral bleaching

A new study led by Climate Central and published in Oceanography concludes that human-induced climate change has driven every global coral bleaching event over the past four decades. The researchers found greenhouse gas emissions have warmed the oceans so much that El Niño now acts mainly as a trigger rather than the root cause.

“Our study shows that without climate change, coral bleaching would be a rare and isolated event, and global mass coral bleaching simply would not occur,” says Andrew Pershing, Climate Central’s chief programme officer and the study’s lead author.

2026-2027 projected bleaching risk map. A report published in Oceanography argues that nearly every global coral bleaching event over the past 40 years would not have occurred
without human-caused climate change. Credit: Oceanography

Corals bleach when unusually warm water forces them to expel the microscopic algae that provide most of their food and give them their vibrant colours. If temperatures quickly return to normal, corals can recover. But prolonged heat starves them and can kill entire reef ecosystems.

To determine how much of that warming came from human activity, researchers compared today’s sea surface temperatures with computer simulations of a world unaffected by greenhouse gas emissions. They then assessed whether bleaching would still have occurred under those cooler conditions.

Their analysis showed that every global bleaching event since 1998 – including those in 1998, 2010, 2014-2017 and the ongoing event that began in 2023 – required human-caused warming to push ocean temperatures beyond bleaching thresholds. Although El Niño coincided with many of those events, the study found they would not have become global bleaching episodes without decades of warming driven by fossil fuel emissions.

Women seaweed farmers in Zanzibar’s Jambiani village receive guidance while tending seaweed farms in the shallow lagoon, where warming seas have also left nearby coral reefs increasingly bleached. Scientists say human-caused climate change, rather than natural climate cycles, has driven every global mass coral bleaching event on record. Credit: Muhidin Michuzi

The findings have particular implications for Tanzania.

The country’s 1,400-kilometre coastline, which includes the reefs surrounding Zanzibar, Pemba and Mafia islands, supports fisheries, tourism and thousands of coastal livelihoods. Healthy reefs also protect beaches and coastal settlements by absorbing up to 97 percent of incoming wave energy, reducing erosion and storm damage.

As reefs deteriorate, fish populations decline, tourism suffers and coastlines become increasingly vulnerable to flooding and erosion.

Similar pressures are happening across tropical reefs worldwide. Although coral reefs cover less than one percent of the ocean floor, they support roughly a quarter of all marine species and generate an estimated US$2.7 trillion annually in ecosystem services through fisheries, tourism and coastal protection. The Global Coral Reef Monitoring Network estimates the world lost about 14 percent of its coral between 2009 and 2018 as repeated marine heatwaves left reefs too little time to recover.

Recent ocean temperature records suggest the pressure is intensifying.

According to the Copernicus Marine Service, June 2026 was the warmest June on record for global sea surface temperatures, while marine heatwaves covered about 82 percent of the world’s oceans by the end of the month. Because the oceans absorb more than 90 percent of the excess heat trapped by greenhouse gases, marine heatwaves are becoming more frequent and severe, increasing the likelihood of coral bleaching.

The Climate Central researchers project that by 2028 human-caused warming will outweigh El Niño’s influence in every coral reef region worldwide. Bleaching, they say, will increasingly reflect persistently warmer oceans rather than unusually strong natural climate cycles.

Using the same attribution methods, the researchers also assessed bleaching risk during the developing 2026-2027 El Niño. They found that while the climate pattern could still trigger bleaching in some regions, human-caused warming would account for most of the risk, particularly in the southern Caribbean, the Pacific coasts of Central and South America, and parts of East and Southeast Asia.

Globally, the monthly average sea-surface temperature for the extra-polar ocean (60°S–60°N) was the highest for June, exceeding the previous record set in June 2024 by just 0.01ºC. This partly reflected the development of strong El Niño conditions in the equatorial Pacific, according to the Copernicus Climate Change Service. Credit: ECMWF/Copernicus Climate Change Service

The study challenges the long-held assumption that bleaching is mainly a response to natural climate cycles. Researchers say bleaching can no longer be viewed as an occasional event linked to natural climate cycles; scientists say it is increasingly becoming a recurring consequence of steadily warming oceans.

“By isolating the effect of climate change, our study shows that the fate of reefs is closely tied to how much carbon pollution goes into the atmosphere. Protecting reefs from overfishing and pollution is very important. We need as many thriving reef areas as possible. Even during severe events, there are often pockets of coral that persist. These areas can be the source for recovery in the short and long term,” Pershing tells IPS

The Intergovernmental Panel on Climate Change warns that even if global warming is limited to 1.5 degrees Celsius above pre-industrial levels, up to 90 percent of the world’s coral reefs could disappear by mid-century. At 2 degrees Celsius of warming, losses could reach 99 percent.

Chande does not speak in terms of climate attribution. She only knows the reef looks different from when she began farming seaweed. Researchers say the bleaching she has witnessed reflects a much deeper shift: oceans that have steadily warmed over decades because of greenhouse gas emissions.

“When I was younger, the corals were full of colour and fish,” she said. “Now everything looks white. I worry about what is happening to the sea,” she says.

IPS UN Bureau Report

Follow @IPSNewsUNBureau