An unprecedented heatwave and drought in 2023 turned the Amazon’s lakes into shallow simmering basins, with water temperatures soaring to temperatures above 40 degrees Celsius (ºC) in one case and water levels plunging to record lows, researchers report. The extreme temperatures had impacts ranging from isolating remote riverine communities to driving mass die-offs in fish and endangered Amazon river dolphins. The findings confirm a worrisome warming trend across the Amazon’s poorly monitored lakes and rivers and portend escalating climate impacts on tropical freshwater ecosystems worldwide. “Although this study presents data from 2023, in September and October of 2024, another extreme drought occurred in the central Amazon,” write Ayan Fleischmann and colleagues, “with new record-breaking low water levels and severe lake water heating associated with hydrological and meteorological conditions similar to those observed in 2023.” Aquatic ecosystems worldwide are warming rapidly. This increase is projected to continue under ongoing climate change, resulting in more frequent and severe lake heat waves. Although lakes are considered sentinels of climate change, most research has focused on temperate regions. Tropical lakes, like those in the Amazon, are highly vulnerable to intense warming yet remain understudied and poorly monitored.

Here, Ayan Fleischmann and colleagues present an analysis of water temperature measurements from 10 central Amazonian lakes during the 2023 drought. Supported by satellite data and hydrodynamic modeling, Fleishmann et al. show how intense drought and heat combined to dramatically raise water temperatures. According to the findings, temperatures in 5 of the 10 lakes experienced exceptionally high daytime water temperatures, exceeding 37 ºC. Notably, temperatures in the shallow waters of Lake Tefé soared as high as 41 ºC – hotter than your average spa bath – throughout its 2-meter water column. According to the authors, a combination of extremely low water levels, strong solar heating, calm winds, and high turbidity created ideal conditions for severe lake heat waves in the Amazon. The findings suggest that low wind speeds were likely the most critical driver of extreme warming, more so than air temperature itself. With little wind, less heat was lost through evaporation and nighttime cooling, allowing lakes to grow progressively hotter under intense sunlight and persistent clear skies. What’s more, the authors show that Amazon lakes have been warming rapidly – roughly 0.3 to 0.8 ºC per decade over the last 30 or so years – rates higher than the global average. Many lakes in the region also shrank dramatically during the 2024 drought, with Lake Tefé losing 75% of its area and Badajós Lake shrinking by 90%.