What drove Amazon lakes to hit their hottest temperatures ever recorded

In 2023, parts of the Amazon got so hot that some lakes hit a temperature of 105.8°F (41°C). Researchers measured the spike across a network of lakes in the central basin during a severe heat wave and drought.

That one number says it all – water turned hot enough to trap fish and cut off entire river communities for weeks.

Villages were left with blocked channels, fuel shortages, and urgent calls for help as the water kept dropping.

How the Amazon lakes overheated

A rare combination lined up: low water, clear skies, light winds, and high turbidity – water clouded by stirred up sediment. Together, they let sunlight drive rapid heating while the usual cooling pathways fell behind.

The work was led by Ayan Santos Fleischmann, a hydrologist at the Mamirauá Institute for Sustainable Development (MISD) in Tefé, Brazil. His research focuses on how rivers, lakes, and climate interact across the Amazon, and how those links shape daily life.

Calm air mattered most. With little wind, the latent heat flux – heat carried away by evaporation – shrank and the lakes kept more heat through day and night.

The team tracked ten lakes and found that five exceeded 98.6°F (37°C) during daytime hours. One lake – Tefé -warmed through roughly 6.6 feet (2 meters) of water with little relief at depth.

When lake beds turn to wide, shallow pans, even small waves stir up bottom mud. That resuspension, the lifting of sediment back into the water, boosts absorption and accelerates heating under clear skies.

Rising lake temperatures in the Amazon

The lakes did not just heat up – in some cases, they also shrank, leaving broad shallows that absorb heat quickly and provide no cool refuge.

Tefé stretches about 37 miles (60 kilometers) across northwestern Brazil and feeds numerous river villages. Its footprint can swing widely with the seasons and with the rise and fall of the main river.

In recent decades, surface temperatures in these waters have risen about 1.1°F (0.6°C) per decade during the dry season. Globally, lake surfaces have warmed about 0.6°F (0.3°C) per decade since the late 1980s, according to an analysis of nearly 300 lakes.

Scientists now consider lake heat waves a distinct hazard. Severe events are far more likely under human influence, as shown by a 2022 study.

During the 2024 drought, Tefé lost about three-quarters of its area. Badajós shrank even more, leaving vast mudflats where boats normally ran.

Wind mattered more than air

A lake loses heat mainly two ways, by evaporation and by infrared radiation to the sky. In 2023, winds fell so low that evaporation slowed, and water shed less heat overnight.

The team used a hydrodynamic model – a physics based lake simulator – to test the role of wind and air temperature. Even when air ran hotter, the model showed daytime peaks surged only when winds weakened.

Daily swings were enormous. The diel (day-to-night) cycle in Tefé ranged up to 23.4°F (13°C) – a stressor for organisms that cannot move to cooler layers.

Clear skies also mattered because they raised the total sunlight striking the water. Lower albedo, reflectivity of a dark, sediment rich surface, amplified absorption and warmed the whole column faster.

The cost of a hot Amazon

The heat hit endangered Amazon river dolphins, Inia geoffrensis, and tucuxi dolphins, Sotalia fluviatilis. In late 2023, more than 200 lake animals died in and near Tefé during the Amazon peak temperatures.

Fish suffered too, both in the wild and in ponds that support local aquaculture. Tropical ectotherms, animals whose body temperature follows the environment, tend to have narrow safe ranges even in normal years.

Warm water also holds less dissolved oxygen, which multiplies the strain. The combined stress of heat and low oxygen pushes many species toward thresholds that sharply cut survival.

Riverside families also felt the heat. Stranded channels delayed shipments of food and medicine, and long detours added time and cost to reach clinics and markets.

The cycle of Amazon heat

Several drivers converged in 2023 and 2024. A strong El Niño – a periodic Pacific warming that shifts rainfall and winds – dominated regional patterns. It combined with unusually warm North Atlantic waters and long-term climate warming over the basin.

Water-level control adds a twist. When connected lakes drop with the main river, backwater effects keep them shallow and slow, priming them for rapid heating under clear skies.

“Although this study presents data from 2023,” said Fleischmann, noting that another extreme drought hit the central Amazon in September and October 2024 – a warning that the pattern is repeating.

Reducing risk in a hotter Amazon

Monitoring is thin across tropical lakes, and that leaves people and wildlife exposed to surprise. More on-the-ground sensors and timely satellite checks would help target rescue, water access, and fishery decisions.

Managers can sometimes create small refuges. Deepened channels, shaded inlets, or temporary barriers can bring cooler river water into a hot lake and give stressed species brief relief.

Over the longer haul, cutting greenhouse gas emissions curbs the odds of severe lake heat waves. That link is clear in attribution research that ties extreme freshwater heat to human-caused warming.

The study is published in the journal Science.

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