Heat waves in lakes are increasing

Mountain lake

Mountain lake in New Zealand. (Image: simonbradfield / iStock)

Climate change is not only causing water temperatures to rise on land, but also in bodies of water, as well as more frequent and more pronounced heat waves. Using satellite images and mathematical models, researchers have now calculated the effects that ongoing climate change will have on temperature in lakes by the end of the century. If the CO2 emissions remain high, the heat waves in lakes will become more intense and last longer. The consequences for the ecosystem in and around the lakes are hardly foreseeable.

Global warming not only slowly increases the mean temperatures of land and oceans, it also promotes extreme climatic events such as heat waves. Numerous studies have already shown that climate change leads to more frequent heat waves on land and on the sea surface, which are also more intense than in previous years. Associated with this is the risk of serious, sometimes irreversible ecological damage, which also entails economic and social problems, for example if there is a shortage of water or certain foods are no longer available. Lakes also presumably play an important role in this context. However, little research has been done to date on how heat waves affect these waters.

Longer and stronger heat waves

Researchers led by R. Iestyn Woolway from the Climate Office of the European Space Agency ESA have now addressed this knowledge gap. They modeled the temperature changes between 1901 and 2099 for over 700 lakes worldwide. To do this, they used satellite images and a mathematical model that takes into account air temperature, wind speed, humidity, solar radiation and air pressure, among other things. They tested this for various scenarios with different levels of CO2 emissions. As defined by Woolway and colleagues, a heatwave is when the surface temperature of a lake is higher for at least five consecutive days than it was for 90 percent of the days between 1970 and 1999.

The result: “Heat waves will be hotter and longer by the end of the twenty-first century,” the researchers say. According to their data, the water temperature during current heat waves is on average 3.7 degrees higher than the mean temperature of lakes worldwide. If greenhouse gas emissions remain high, this could rise to 5.4 degrees, the study found. According to the forecasts, the duration of the heat waves will also increase: While heat waves in lakes have so far usually subsided after about a week, in the scenario with the highest emissions they could last for over three months. For some lakes, the heat could even become a new normal.

Possible feedback effects

The authors took into account lakes with a depth between two and 60 meters. “In deeper lakes, heat waves last longer but are less intense than in shallower waters. This applies to both historical and future periods, ”they report. The reason for this is that deep waters have a higher heat storage capacity and can therefore, to a certain extent, absorb high temperatures from the surface. However, strong temperature differences between the upper and lower water layers mean that the lake does not mix as well.

The environmental scientist Moritz Lehmann from the University of Basel points out that poor mixing increases the oxygen deficiency in deeper water masses. “Under these conditions, the greenhouse gas methane is increasingly formed,” he warns. This in turn leads to further global warming and thus exacerbates the effects. The authors did not include such feedback mechanisms in the calculations.

Effects on ecosystems

Warming can also have serious consequences for lake ecosystems and everyone who depends on them. “Heat waves can lead to certain species being favored while others are disadvantaged or even become extinct. This can lead to shifts in the composition of the species communities, ”explains Lutz Becks, head of the Aquatic Ecology and Evolutionary Biology working group at the University of Konstanz, who was not involved in the study.

How exactly the heat waves will change the ecosystems, however, can hardly be predicted. “Since species always interact with other species, these shifts in species compositions are complex. For example, a species can have a high tolerance to heat waves, but its prey, for example, cannot, which in turn can lead to the predator’s extinction. The measurement of tolerance curves of individual species can only help us to a limited extent, ”says Becks.

Furthermore, the data only provide information about general global relationships. “What that means for a specific lake in Central Europe, for example, and what specific consequences this has on site cannot be derived from the study,” comments climate change expert Diana Rechid from the Helmholtz Center Geesthacht. For this, more detailed, spatially resolved simulations are necessary. The current study clearly shows what influence the future development of greenhouse gas emissions will have on inland waters around the world. In a scenario with lower emissions, heat waves would be stronger and longer than before, but with a temperature increase of 4 degrees and an estimated duration of one month, they would be much milder than in the negative extreme scenario.

Source: R. Iestyn Woolway (European Space Agency Climate Office) et al., Nature, doi: 10.1038 / s41586-020-03119-1

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