Heatwave season expanding in Europe, warns Swiss climate scientist

MeteoSwiss says this May is the fourth hottest in Switzerland since records began in 1864, at 1.8°C above average. Keystone / Claudio Thoma

Europe is sweltering under unseasonal May heat, with temperatures exceeding 30° degrees Celsius (86° Fahrenheit) in several countries and records tumbling. Swiss climatologist Sonia Seneviratne warns that the heatwave season is expanding, underscoring growing climate risks.

The United Kingdom broke a century-old temperature record twice in 24 hours on Tuesday as a spring heatwave gripped parts of Europe, prompting government warnings about risks to life. A high of 35.1°C was recorded at London’s Kew Gardens, the Met Office said, surpassing the 34.8°C set there a day earlier.

France also saw records fall, with temperatures hitting 36°C in the southwest on Monday and remaining above 20°C overnight in many areas. Météo-France attributed the spike to a “heat dome” – heat held in place by a high-pressure weather front – pushing temperatures more than 10°C above seasonal norms.

Switzerland is also unusually warm. MeteoSwiss says this May is the fourth hottest since records began in 1864, at 1.8°C above average. Temperatures exceeded 30°C in canton Ticino and north of the Alps, reaching 34.2°C in Sion (canton Valais) on Wednesday.

With climate change, heat is arriving earlier, lasting longer, reaching higher altitudes and growing more intense, MeteoSwiss says.

Extension of heatwave period

Sonia Seneviratne, a senior climate scientist at the federal technology institute ETH Zurich and vice-chair of the Intergovernmental Panel on Climate Change (IPCC), says this trend is expected.

“An event that used to occur roughly once every ten years without human influence now happens three or four times per decade,” she told Swiss public radio RTS. “As every month is getting warmer, we are seeing the heatwave period extend from May to the end of summer.”

Warming varies regionally, she notes, but Switzerland is particularly affected, with temperatures rising about 3°C on average – double the global increase of around 1.5°C.

“Europe as a whole is warming faster than most other continents,” she said. “Temperature anomalies compared to historical norms are very significant.”

Many unknowns

Climate change will have a wide range of impacts on Switzerland in future, says the Swiss climatologist: less snow in winter, more extreme temperatures in summer, severe heatwaves, drier summers, more frequent droughts and extreme precipitation, with possible flooding.

Global warming could also bring unexpected shifts. “We don’t know how it will end,” Seneviratne said, warning of possible tipping points that could accelerate change.

And events that were once virtually impossible are becoming more likely as baseline conditions shift, she added.

Consequences for ecosystems

Scientists are closely monitoring the current heatwave in Europe, which is already straining ecosystems.

Serge Zaka, an agricultural engineer and agroclimatology specialist, explains that at this time of year plant roots are not large enough to draw water from deep below the ground.

Higher temperatures and the timing of the current hot weather cause risks to ecosystems, he says: “For cereals, for example, this is the period of grain filling or ear emergence [critical growth stages for crops]. So these unseasonable temperatures cause withering and stress.”

Lake mixing period shortened

Lakes, which remove excess nitrogen from the water, are also being affected. A studyExternal link by an international team of scientists led by the University of Basel and the Swiss Federal Institute of Aquatic Science and Technology (Eawag) finds climate change is disrupting the nitrogen cycle.

Lakes play a key role in the global nitrogen cycle. Microorganisms convert nitrogen compounds such as nitrate or ammonia into dinitrogen gas (N₂), which is released to the atmosphere and effectively removed from the biosphere. This nitrogen removal process is called denitrification.

But this cycle is being disrupted and the vital filtering role of lakes is becoming less effective. Scientists reached this conclusion after taking measurements in Lake Baldegg, canton Lucerne.

Their study published in the journal Nature Microbiology shows that the intensity of denitrification in lakes is seasonal. In winter, the process is more dynamic than in summer. During the cold season, the waters of Lake Baldegg undergo a complete mixing. The warm, oxygenated upper layer sinks, and the oxygen-poor lower layer rises to the surface. With warmer temperatures, the lake’s mixing period is shortened by 27 days. This nitrogen that is not removed by lakes can end up in the sea.

The consequences can be serious, warn researchers. Excessive nitrogen concentrations can cause algal blooms and create oxygen-deprived zones. This excess nitrogen also disrupts ecosystems.

The algae can cause skin irritations or digestive problems in humans and be fatal for animals. It is recommended to check the water quality before swimming and to take a shower after swimming in lakes or rivers, especially in shallow, stagnant water. Several cantons have already issued warnings.

Translated from French by AI/sb/ac