Amazonian lakes are rapidly heating to levels hotter than spa baths as unprecedented heatwaves and severe drought sweep across the region, new research reveals.

Record-Breaking Water Temperatures Exceed 40°C

Scientists recorded one Amazon lake surpassing 40°C (104°F) after prolonged exposure to intense sunlight and cloudless skies. As water levels fell dramatically, shallow lakes such as Lake Tefé — only two metres deep — reached a staggering 41°C, warmer than the typical temperature of a spa bath.

“We couldn’t even put our fingers in the water. It was really hot, not just in the top bit, but right down to the bottom,” said the lead researcher, Ayan Fleischmann, from the Mamirauá Institute for Sustainable Development. “You put your finger in and remove it instantaneously, it’s unbearable.”

These extreme conditions triggered mass mortality events, including widespread deaths of endangered Amazon river dolphins and countless fish unable to withstand such lethal temperatures.

Unprecedented Dolphin Die-Offs Shock Researchers

During a six-week period in September 2023, up to 200 river dolphins were found floating dead in Lake Tefé. According to researcher Fleischmann, nothing similar had been witnessed in the region for at least a century.
“It was completely surreal and really scary,” he said.

The scale of the die-off prompted scientists to investigate additional water bodies across the Amazon.

Half of Studied Lakes Hit Lethal Heat Levels

Of the 10 lakes examined, half experienced daytime temperatures above 37°C, according to the study, published in the journal Science.

Researchers analysed temperature data from central Amazon lakes during the extreme drought of 2023, which was soon followed by another severe drought in late 2024.

Normally, Lake Tefé averages around 30°C during the hottest months, but in 2024 it surged to 40°C, shattering previous records.

Amazon Lakes Warming Faster Than Global Average

Over the past three decades, Amazon lakes have warmed by 0.3°C to 0.8°C per decade, significantly outpacing global water-temperature trends.

At the same time, the lakes are dramatically shrinking.

• Lake Tefé lost about 75% of its surface area during the 2024 drought.

• Lake Badajós shrank by an astonishing 90%.

Adrian Barnett, senior lecturer in behavioural ecology at the University of Greenwich, who was not involved in the research, said: “The paper shows the extraordinary impacts climate change is having, even on such huge ecosystems as the Amazon, and that these are not restricted to the forests, but the aquatic realm as well.”

“A 10C increase in water temperature is unparalleled,” he said. “The volume of energy needed to achieve this in such huge volumes of water is jaw-dropping.”

Most Amazonian fish species — along with river dolphins and manatees — traditionally breed during the low-water season.
Researcher Barnett warned that the extreme heat and shrinking habitats meant 2023 was likely a catastrophic year for reproduction across many species.

There are few local solutions to this problem, according to Barnett. “Something that’s happening at such a huge scale really requires a systems approach and that means attacking the root cause of the problem, which is fossil fuel emissions and the causes of global warming itself,” he said.

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