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 hitting lakes.

Published in Communications Earth & Environment, a new international study led by the Nanjing Institute of Geography and Limnology, 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 analyzed daily surface-water and near-surface air temperatures from 265 lakes worldwide (2000–2022) to compare heatwave trends, spatial patterns, and underlying drivers in lakes versus the overlying atmosphere.

The researchers found that when lake and atmospheric heatwaves coincide, the resulting coupled events become substantially more extreme, exposing freshwater ecosystems to prolonged and amplified heat stress. Composite metrics that combine intensity and duration further demonstrate that lakes accumulate more cumulative thermal exposure than the atmosphere above.

The team identified key environmental factors intensifying these differences.

“In addition to temperature, decline in wind speed can significantly exacerbate lake heat extremes. Wind should therefore be treated as one of the important drivers in forecasting and adapting to lake heatwaves.” said the first author Yifan Yang.  Weaker winds suppress turbulent mixing and evaporative cooling, enhance thermal stratification, and allow surface waters to warm more rapidly and retain heat longer. 

Spatially, geographic location was the dominant determinant of lake–air contrasts, followed by lake size and depth.

Temporal dynamics also revealed distinct patterns. During coupled lake–atmosphere heatwaves, lake heatwaves begin earlier and persist longer, extending biological exposure windows and increasing ecological risk.

Projections under future emissions scenarios indicate that the gap between lake and air 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, this discrepancy is not trivial, it's a signal for action, more attention should be paid to the impact of heatwaves on lakes.” said the study's corresponding author Jianming Deng. “Our study provides a foundation for improved forecasting and for the development of effective freshwater-ecosystem adaptation strategies.”