How 100 days of record ocean heat is reshaping life in and around the seas

Fish and other marine organisms are highly sensitive to even small changes in temperature. If they shift from one area to another – or disappear altogether – it is a sign of warming waters.

Scientists are also tracking a range of other changes in the oceans to understand how the climate is changing.

Oceans logged their 100th straight day of record average heat on Thursday, said Climate Central, a US-based research organisation, citing data from the US National Oceanic and Atmospheric Administration (NOAA).

A 100-day streak is not unprecedented, but temperatures have remained at record levels since 2 June. On Wednesday, the average ocean temperature stood at 21.17C – 0.79C above the 1991-2020 average used by climate scientists and 0.95C above the 1982-2010 average.

The figures still need to be confirmed but are “very unlikely to change”, said Zachary Labe, a climatologist at Climate Central and former senior NOAA scientist. Labe described the 100-day milestone as “a symbolic marker”.

“A run of 99 days compared with one of 102 days will not make much difference in terms of impact,” he said. “The real determining factor is the prolonged period of record heat that we continue to see this year.”

In other words, the individual readings form part of a much wider series of records seen over recent years. Changes in average ocean surface water temperature.

Records piling up

The streak also included the highest average ocean temperature recorded on a single day.

Two recognised datasets give slightly different results. The European Union's Copernicus ocean monitoring service recorded 21.10C on 22 August using its ERA5 dataset. Climate Reanalyzer at the University of Maine recorded 21.24C on 23 August using NOAA's OISST dataset – 0.82C above average.

Measurements come from thousands of buoys scattered across the world's oceans, as well as ships and satellites.

The pace of ocean warming is also accelerating. It doubled between the periods 1960-2005 and 2005-2025.

Greenhouse gas emissions are the main driver of ocean warming, with oceans absorbing more than 90 percent of excess heat in the atmosphere, Labe said.

Reference temperatures are measured in surface waters down to a depth of 200 metres, a sunlit zone where photosynthesis is possible and which contains most visible marine life and plants.

Three Pacific hurricanes gather strength as El Nino intensifies

Nearly half of marine heatwaves since 2000 would not have happened without climate change, which makes them longer and more intense, a study published in March 2025 found. More than 80 percent of the ocean experienced a heatwave during the first half of 2026.

Current El Nino conditions, which are particularly severe, “amplify human-caused climate warming”, said marine biologist Tom Pickerell, director of the Ocean Program at the World Resources Institute, an environmental research organisation.

But El Nino does not directly warm every ocean hotspot.

Areas including the North Pacific and Mediterranean are also experiencing unusually high temperatures. On Monday, Mediterranean waters were 3.2C above the seasonal average, while the Gulf of Mexico has also been breaking records.

Ocean and atmospheric temperatures remained close to record levels in 2024 and 2025 despite the return of La Nina, which cools ocean waters. Temperatures were even higher than during previous El Nino years.

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Heat sinking deeper

Fractions of a degree may sound small, but the amount of heat accumulating in the ocean is enormous and has reached record levels for nine consecutive years.

In 2025, the third-hottest year on record, oceans absorbed another 23 zettajoules of heat.

That was equivalent to “12 Hiroshima atomic bombs exploding every second for a year”, Lijing Cheng, a professor at the Institute of Atmospheric Physics at the Chinese Academy of Sciences and lead author of a study co-signed by 50 international scientists, said earlier this year.

That heat, Labe added, is “going deeper and deeper into the ocean”.

The consequences are already being felt by marine life. Warming is changing the distribution of fish populations on a large scale, while marine heatwaves are killing coral reefs.

“Just one extra degree in the water has a significant effect on marine life,” Claire Bergin, a climate scientist at Maynooth University in Ireland, told RFI. 

“On land, we can install air conditioning to cool ourselves. But the capacity of the seas to adapt is much more limited.”

Climate Central modelling indicates that El Nino has played little role in the deadly coral bleaching linked to marine heatwaves.

“Our research across the Pacific shows that extreme ocean temperatures are no longer isolated events,” said Serge Planes, a director at France's National Centre for Scientific Research (CNRS) in French Polynesia.

“Even a slight increase above the usual summer maximum, when it lasts for several weeks, can disrupt the relationship between corals and their symbiotic algae, causing widespread bleaching and coral mortality.” A simplified projection showing the factors responsible for coral bleaching.

From food to tourism

Warming oceans are not only an environmental threat but an economic one, said Pickerell, who is also secretary of the High Level Panel for a Sustainable Ocean Economy, an international group of world leaders focused on ocean policy.

Fisheries and food supplies, tourism, energy infrastructure and coastal communities are all exposed.

“This is not just an environmental problem, it is an economic shock,” Pickerell said.

“This should be a concern for every finance minister, not simply environment ministers. A warmer ocean disrupts fisheries and food supplies, damages reefs and beaches that are assets for tourism, puts energy infrastructure under strain and fuels extreme weather events that affect coastal communities and businesses.”

If the ocean were a country, its economy would rank seventh in the world, contributing up to $3 trillion and supporting hundreds of millions of jobs, Pickerell added.

“And that is only an underestimate,” he said, because the figure does not include services provided by the ocean such as storing carbon.

“Countries must prepare not for a single risk such as sea-level rise or ocean acidification, but for multiple pressures that reinforce each other.”

Sardine catches in Morocco fell by 46 percent between 2022 and 2024 because of warmer waters. In the Mediterranean, warming intensified Storm Daniel, which killed 6,000 people in Libya. On the other side of the world, Cyclone Gabrielle caused $14 billion in damage in New Zealand.

These effects are “felt even more strongly in countries already struggling with debt and inequality and can lead to people being displaced further afield and to geopolitical instability”, Pickerell said.

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Rising seas, shrinking ice

Ocean warming is also raising global sea levels as warmer water expands and glaciers and polar ice melt. Island communities from Tuvalu in the Pacific to Saint-Pierre-et-Miquelon off Canada's coast and Germany's low-lying Halligen islands are under threat.

By the end of 2025, average global sea levels were about 11 centimetres higher than in January 1993. In island and delta regions including Kiribati and Bangladesh, rising seas are eroding and flooding land, entering freshwater reserves, and damaging agriculture and drinking water supplies.

Energy systems are also vulnerable.
Five of six nuclear reactors at the Gravelines power station in northern France, western Europe's largest nuclear plant, stopped operating this summer because of a “massive influx of jellyfish” into seawater pumping stations. Climate change encourages jellyfish proliferation.

Sea ice is meanwhile retreating at both poles, with its maximum winter and minimum summer extent becoming smaller. As reflective white ice disappears, it exposes darker water that absorbs more heat, accelerating warming and damaging sensitive ecosystems.

Uncertainty remains over possible disruption to the Atlantic Meridional Overturning Circulation, or Amoc, a major system of ocean currents that helps regulate climate and is showing worrying signs of weakening.

“Governments are managing the ocean of the past, not the one we have now or the one we will have,” Pickerell said.

“Adaptation happens through projects in silos that do not interact with each other – the fishing industry, ports, energy...”

Cutting greenhouse gas emissions, he warned, remains the only real way to tackle the underlying problem.

“The ocean is not simply a victim, it is part of the solution. If we manage to decarbonise shipping and offshore renewable energy is deployed on a large scale, we could achieve 35 percent of emissions reductions by 2050.”

This article was adapted from the original version in French by Géraud Bosman-Delzons.