Lake Tefé reaches extreme temperature

In September 2023, Lake Tefé in the Brazilian Amazon reached a water temperature of 41 °C throughout its entire water column, which was about 2 meters deep. This meant that the water was uniformly hot from the bottom to the surface, leaving no cooler refuge for aquatic life.

Mass dolphin deaths follow

In the weeks following the temperature spike, 209 freshwater dolphins were found dead. On September 28, 2023, 70 dolphin carcasses were discovered, and the total number reached 209 in the following weeks. These dolphins, which must surface to breathe, were unable to find cooler water to escape the extreme heat.

Experts investigate the phenomenon

Hydrologist Ayan Fleischmann from the Mamirauá Institute was sent to the site to investigate the mass dolphin deaths. He described the water as too hot to touch, and noted that the extreme temperature affected not only dolphins but also fish, phytoplankton, and other aquatic organisms. The lake turned red due to changes in algae pigmentation.

Study reveals causes of the heatwave

A study published in Science on November 6, 2025, analyzed the 2023 event using satellite data and hydrodynamic models. It found that an unprecedented drought and heatwave simultaneously affected the Amazon’s waters. Lake Tefé reached 41 °C throughout its 2-meter water column, which is warmer than a typical bathtub and much hotter than most thermal baths.

Factors behind the extreme heat

Jordi Catalán, a specialist consulted by the Spanish Science Media Centre, described the event as unprecedented even in the tropics. The study identified four main factors: strong solar radiation, reduced lake depth, light winds, and turbid water. These conditions caused the heat to be trapped in a shallow layer of water, intensifying the temperature spike.

Lake shrinkage and solar exposure

According to the Anadolu Agency, Lake Tefé’s surface shrank by 75 percent due to the drought. The study found that the lake experienced extremely high solar radiation for 11 cloudless days, functioning like an open-air solar collector. Adrian Barnett, an ecologist not involved in the study, noted that a 10 °C increase in water temperature has no precedent in recorded history.

Long-term warming trend

Satellite data from 1990 to 2023 show a steady increase in lake temperatures at a rate of 0.6 °C per decade. Over 33 years, this trend would have added nearly 2 °C to the temperature if it continued linearly. The 2023 heatwave added to this underlying warming, highlighting the impact of climate change on tropical lakes.

Tropical lakes as climate sentinels

The Science editorial summary describes the Amazonian episode as an extreme example of the effects of climate change on aquatic life. Lakes are often seen as sentinels of climate change, but most research has focused on temperate regions. Fleischmann emphasizes that tropical lakes, which support food security and local livelihoods, have been overlooked in climate studies.

Call for climate action and monitoring

Fleischmann warns that the climate emergency is already here, and he advocates for long-term monitoring of lakes and involving local communities in adaptation efforts. While the study acknowledges the difficulty of linking physical phenomena to biological impacts, it highlights that 2023 was not an isolated event—another record drought followed in the Amazon the next year.