Heatwaves do not only occur on dry land: we have known for a few years that the oceans are also increasingly affected by heat waves. And an new study, published January 20 in the journal Nature, has just shown that the lakes are not immune to these hot spells. According to the simulations carried out by the authors, lake heatwaves will gain in length and intensity by the end of the century, and this, on the whole planet. With the risk of upsetting these subtle ecosystems.
For this new study, scientists looked at records of water temperatures collected during the 20th century in some 700 lakes around the world. That is a colossal amount of data. “The authors were able to take advantage of the new possibilities offered by satellite technologies and by those of data processing and simulation, to deliver an unprecedented global vision of the problem of heat waves in lakes”, estimates Stéphane Goyette, climatologist at the University of Geneva.
The authors used these historical data to calibrate computer models of climate change, and thus make projections for the future. Their results give you a cold sweat. While in the years 1970 to 1999, lake heatwaves lasted an average of 7.7 days, with an excess of about 3.7 ° C over the expected temperatures depending on the location and the season, we can expect that, by the end of the century, they will extend over 95.5 days with an excess of around 5.4 ° C, at least in the most pessimistic scenario, where no mitigation measures climate change is taken.
In Lake Neuchâtel, which was part of the analysis, the warming is likely to be even more marked than the average: “The number of heatwave days per year will pass on average from 9 to 212 and their intensity will increase. from 1.7 ° C to 5.2 ° C by the end of the century ”, if nothing is done to contain the warming, details Iestyn Woolway, one of the authors of the study, researcher at the Dundalk Institute of Technology in Ireland. Other scenarios were considered in the study, with reductions in anthropogenic CO2 emissions; they also predict an intensification of lake heat waves, although of lesser magnitude.
The same phenomenon is found all over the world. “One of the interesting aspects of this work is to note that the multiplication of heat waves concerns both high and low latitude lakes”, says Stéphane Goyette. These hot spells will not only occur in summer, but could occur in any season and last for long periods – some small lakes are even at risk of chronic overheating.
What will be the impact of these transformations? For Bastiaan Ibelings, ecologist at the University of Geneva, “these extreme events will add to the gradual warming underway, with potentially significant consequences on ecosystems.” In Switzerland, global warming has already led to changes in the thermal structure of large lakes. The waters are more stratified and their complete mixing no longer occurs in winter, because the temperatures are no longer cold enough. Consequently, the waters of the bottom of the lakes are less well oxygenated.
“We can fear that this promotes the development of toxic microalgae such as Planktothrix rubescens, a species specific to large alpine lakes, with risks for the quality of drinking water withdrawals”, notes Bastiaan Ibelings. Greenhouse gases such as methane could also form in areas with low oxygen at depth and then be released to the surface during degassing. Last but not least, some species may have difficulty adapting to the heat. “This is for example the case of whitefish [ou féras, ndlr] who need cool temperatures for their development, ”says the biologist.
Bastiaan Ibelings therefore calls for caution: “Containing warming to + 2 ° C by the end of the century, as provided for by the Paris Agreement, would give species a better chance of adapting to the changes. of their environment. ” “We can be happy to have a longer period for swimming, but we must not forget that the lakes fulfill other functions outside our leisure time,” emphasizes Stéphane Goyette.