Freshwater lakes are critical ecosystems that regulate regional climates, support biodiversity, and provide essential resources for human societies. However, as global warming accelerates, extreme heatwaves are increasingly impacting these aquatic systems.

A new study led by the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences (NIGLAS) has revealed that lake heatwaves are generally more intense, longer-lasting, and more frequent than their atmospheric counterparts. The study was recently published in Communications Earth & Environment.

To compare heatwave trends, spatial patterns, and underlying drivers between lakes and the overlying atmosphere, the researchers analyzed daily surface-water and near-surface air temperatures from 265 global lakes spanning the period 2000–2022.

The findings showed that when lake and atmospheric heatwaves coincide, the resulting "coupled events" become significantly more extreme, exposing freshwater ecosystems to prolonged and amplified heat stress. Composite metrics that combine intensity and duration further confirmed that lakes accumulate more cumulative thermal exposure than the atmosphere above them.

The team also identified key environmental factors that amplify these differences. "Beyond temperature itself, declining wind speed can significantly exacerbate extreme heat in lakes," said YANG Yifan, the study's first author. "Wind should therefore be recognized as a critical driver when forecasting and adapting to lake heatwaves." Weaker winds, the researchers explained, suppress turbulent mixing and evaporative cooling, strengthen thermal stratification, and allow surface waters to warm more rapidly while retaining heat for longer periods.

Spatially, geographic location emerged as the dominant factor shaping contrasts between lake and atmospheric heatwaves, followed by lake size and depth.

Temporal dynamics also revealed distinct patterns. During coupled lake-atmosphere heatwaves, lake heatwaves typically begin earlier and persist longer. This extends the window of biological exposure and raises ecological risks for aquatic life.

Projections under future emissions scenarios suggest that the gap between lake and atmospheric heatwaves may narrow by the end of the century, as atmospheric temperatures are expected to rise faster than surface water temperatures.

"Freshwater systems experience longer and more cumulative heat stress than the atmosphere alone," said associate Prof. DENG Jianming, the study's corresponding author. "This discrepancy is not trivial, it is a call to action. We need to pay greater attention to the impact of heatwaves on lakes."