In recent years, a pronounced global surge in the frequency and intensity of heat extremes has signaled the integration of climate change into a new normal. This shift poses a serious threat to both economic and societal sustainable development. 

Moreover, lakes display remarkable sensitivity to climate change, serving as pivotal indicators and regulators of global environmental shifts and regional climates. Heat extremes can rapidly elevate the surface water temperature in lakes, swiftly compromising their physical, chemical, and biological attributes. This disruption, in turn, has profound and irreversible consequences for the entire lake ecosystem. 

To quantify the contribution of heat extremes to the lake warming in China, Prof. SHI Kun and other researchers from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, collaborated with Dr. R. Iestyn Woolway from the University of Bangor, UK. They utilized the 1985-2022 ERA5-Land (European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5-Land) air temperature data as the forcing data, Landsat surface temperature products provided by NASA and USGS as the reference for lake surface water temperatures, and combined with the physical/statistical hybrid model Air2Water, to construct a long time-series lake surface water temperature dataset for 2,260 lakes. They conducted a comparative analysis of the patterns of change in summer air temperatures, lake surface air temperatures, and lake surface water temperatures. This analysis was performed both before and after the removal of heat extremes, allowing for the quantification of the contribution of heat extremes to the warming of China's lakes from 1985 to 2022. 

The study was published in Nature Communications on January 2. 

The results show that China experienced an average of about 454 days of extreme heat from 1985 to 2022. Overall, the number of days with heat extremes showed an increasing trend, about 2.08 days/decade, as well as a gradual increase in the intensity of heat extremes, about 0.03°C/day·decade. After removing the heat extremes, the average increase in air temperature in China decreased from 0.32 ± 0.10 °C/decade to 0.17 ± 0.08 °C/decade. 

The research further quantified the effect of heat extremes on long-term changes in lake surface water temperatures. Removal of heat extremes resulted in a decrease in mean cumulative heat from 172.98°C to 66.65°C and a decrease in intensity of heat extremes from 0.43°C/day to 0.19°C/day. In addition, the average change in lake surface water temperature decreased from 0.16°C/decade to 0.13°C/decade, indicating that changes in heat extremes also contributed significantly to the increase in lake surface water temperature. 

Although the number of days of heat extremes in the lake surface air temperature accounted for only 3% of the study period (~459 days), the contribution of heat extremes to the change of surface water temperature of China's lakes was as high as 36.5% from 1985 to 2022. It suggests that the occurrence of short-term heat extremes can have profound impacts on lake systems on seasonal, annual and even longer time scales. 

This study promotes an insightful understanding of the impacts of climate change on lake ecosystems, and is of great scientific and practical significance for the construction of risk assessment and warning systems for heat extremes, and the establishment of new adaptation strategies and coping mechanisms for lakes under the new climate change norm of frequently occurrence of heat extremes.

Contribution of heat extremes to lake surface water temperature (LSWT) (IMAGE by SHI Kun)