As the consequences of global warming multiply on a global scale, a new alert arises in the heart of the Amazon. A study published in the journal Science, carried out by researchers from the Mamirauá Institute for Sustainable Development in Brazil, highlights a critical situation. Amazonian lakes now reach temperatures above 40°C, exceeding those recommended for thermal baths. This abrupt upheaval is the result of a combination of historic droughts, dramatically low water levels, and extreme weather conditions.
In this context, aquatic ecosystems, already fragile, are pushed to their limit. The study documents the direct impacts on wildlife, in particular freshwater dolphins, but also on local populations, which are highly dependent on these environments. These data offer a concrete and dramatic insight into how climate change is transforming the most remote and vulnerable tropical areas.
Record heat in the waters of the Amazonian heart
The year 2023 marked a turning point in climate monitoring of the Amazon. In September and October, an exceptional heat wave, combined with an intense drought, caused the temperatures of several Amazonian lakes to rise to levels never before measured. The most striking: Lake Tefé, located in the Brazilian state of Amazonas. It reached 41°C across its entire water column, at a depth of just two meters. A temperature higher than that recommended for hot tubs by American health authorities (40°C according to the CDC).
Study scientists used satellite data, field measurements and hydrodynamic modeling to study ten lakes in the region. Result: five of them crossed the critical threshold of 37°C during the day. An exceptional level for bodies of water of this size in a tropical zone.
Unlike temperate lakes, tropical lakes benefit from little nighttime cooling. Here, the absence of wind, constantly clear skies and high turbidity (water very laden with organic matter) allowed massive thermal accumulation. These conditions limited evaporation and prevented any effective heat loss. According to Ayan Fleischmann, lead author, cited by The Guardianthe low wind speed played a determining role. Even more so than the air temperature itself.
These findings show that Amazonian lakes are not simply heated at the surface, but heated at depth. This prevents any refuge area for aquatic species. This is a new dynamic, little observed until now. It then calls into question the supposed thermal resilience of large tropical ecosystems.
Mass death among dolphins and collapse of biodiversity
The consequences of these thermal peaks were immediate and spectacular. In Tefé, more than 200 dead dolphins were found floating on the surface of the lake between the end of September and October 2023. The two species concerned –Inia geoffrensis (Amazon pink dolphin, or boto) and
Sotalia fluviatilis (tucuxi) – are both classified as endangered on the IUCN Red List.
The researchers suspect that the animals were not able to escape from this “thermal trap”. The high temperature extended throughout the water column, preventing the dolphins from seeking cooler areas at depth. Heat stress would have caused neurological and physiological failures. “
It is possible that the heat affected their brains, preventing them from fleeing to the Amazon River », Indicates Fleischmann in his interview with CNN.
© AS Fleischmann et al., 2025
Long-term warming of central Amazon lakes and major warming in 2023.
The fish also suffered massively. In a single monitored fish pond, more than 3,000 individuals died within a few days. Many other losses could not be counted in natural environments. The period was particularly critical. Indeed, it coincided with the breeding season for many aquatic species. According to Adrian Barnett, an ecologist at the University of Greenwich, “ 2023 will undoubtedly have been a catastrophic year for the renewal of aquatic populations “.
These losses affect the entire food chain. A sustained decline in these species would lead to cascading effects on fish-eating birds, reptiles and mammals. But also on the human communities that depend on fishing. This is therefore a systemic upheaval, well beyond a simple one-off phenomenon.
Lakes disappearing and warming at a worrying rate
Alongside extreme temperature increases, Amazonian lakes are undergoing spectacular physical shrinkage. During the drought of 2024, following that of 2023, Lake Tefé lost 75% of its surface area, while Lake Badajós contracted by 90%. This contraction of water masses accentuates their thermal vulnerability. Less water means a greater ability to heat quickly, especially under constant equatorial sun.
Remote sensing data analyzed by Fleischmann's team show that the average temperature of surface waters has increased by 0.3 to 0.8°C per decade over the past three decades in the region. This trend is higher than the global average for lakes. The latter is around 0.2 to 0.6 °C per decade, according to previous publications on temperate lakes.
The combination of prolonged droughts, extreme temperatures and the absence of wind leaves little room for resilience. Especially since tropical lakes are rarely monitored. Unlike temperate zones, the region lacks permanent measuring stations. This greatly complicates long-term modeling. However, scientists believe that tropical lakes remain among the most vulnerable to climate change. In fact, they are directly affected by variations in precipitation, cloud cover and evapotranspiration.
This decline in lake surfaces does not only result in a loss of habitat for wildlife: it also affects local biogeochemical processes, such as carbon storage, the concentration of dissolved oxygen, or even the spread of diseases linked to stagnant water. Thus, the phenomenon also has consequences on water quality and the dynamics of surrounding ecosystems.
Ecological, but also social crisis for Amazonian populations
The effects of lake warming go far beyond the boundaries of biodiversity. During the droughts of 2023 and 2024, thousands of people living in riverside communities found themselves cut off from the rest of the region. In an Amazon where land transport remains rare, river navigation becomes vital. However, the extreme drop in water levels has made many roads impassable, isolating residents from access to food, drinking water and medical care.
These populations are also directly dependent on aquatic ecosystems for their food (fishing), their income (aquaculture, crafts), and their culture. When fish die and dolphins disappear, it is not only an ecological disaster, but a direct attack on their way of life.
Water is also becoming a scarcer resource. During periods of high heat, the concentration of natural pollutants (organic matter, micro-organisms) increases, making the water undrinkable without treatment, which is not accessible for many villages. Outbreaks of diarrhea and water-borne diseases have already been reported in some remote areas, according to local NGOs.
“ When aquatic ecosystems are disrupted, entire Amazonian societies suffer the consequences. », recalls Ayan Fleischmann. In the absence of resilient infrastructure and sufficient institutional support, these populations find themselves on the front lines of climate change, often without means of defense.
This reality highlights the need to integrate the human dimension into climate policies, particularly in tropical regions. Protecting aquatic ecosystems is not just about saving biodiversity. It also means ensuring the survival of millions of people.
Source: Ayan Santos Fleischmann et al., “Extreme warming of Amazon waters in a changing climate”. Science 390, 606-611 (2025).
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
