Leipzig study: What would a ‘summer of the century’ look like in the future under current climate conditions?

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The summers of recent years have been hot, but not exceptional from a meteorological point of view. However, according to researchers at the University of Leipzig, the next extreme summer would far exceed anything we have experienced so far. To be prepared for this, it is important to understand what a ‘summer of the century’ might look like in the current climate. This is precisely what scientists at the University of Leipzig have investigated in a new study.

They found that late summer is now warming significantly more than early summer, and that the soil tends to be drier than in summers of previous years.

“As a result, there is now a longer period in late summer during which favourable weather conditions, combined with very dry soil, can lead to extreme and prolonged heat,” says the study’s lead author, Dr Peter Pfleiderer from the Institute of Meteorology at the University of Leipzig, summarising one of the study’s findings. The study has just been published in the specialist journal *Earth’s Future*.

Through their analysis of ‘century summers’, the researchers demonstrate that the changes generally expected in summer are even more pronounced during these rare, extreme summers. At the same time, summers in Europe are warming more rapidly than in other parts of the world. The study illustrates just how hot prolonged heatwaves can become in the current climate and how individuals, local authorities and society as a whole should adapt.

Dry soils and heat extremes

In the pre-industrial climate (1850 to 1900), ‘centuries-in-a-century’ summers were very warm throughout the entire season, as Pfleiderer explains. The researchers found that ‘centuries-in-a-century’ summers in the current climate could start off relatively cool and then remain consistently very hot throughout mid- and late summer.

“We also show that, for summers in general as well as for extreme and rare summers, hot days tend to occur more frequently in the current climate, thereby increasing the likelihood of prolonged heatwaves,” explains the climate researcher.

Extreme heat is strongly linked to dry soil. When favourable weather conditions cause a region to warm up, some of the energy is required for the evaporation of water. However, once the soil is dry, the energy is converted solely into sensible heat and temperatures can rise to extreme levels.

“Until now, it has not been possible to investigate the influence of drier soils on heat during ‘century summers’, as the available simulations were insufficient for the statistical analysis of such rare events. Here we show that changes observed for average summers would be significantly more pronounced during once-in-a-century summers,” explains Junior Professor Dr Sebastian Sippel from the University of Leipzig. The researcher is the corresponding author of the study.

Hot summers in the simulation

The researchers have run climate simulations using a method known as ‘rare-event sampling’. In rare-event sampling, many simulations begin at the start of summer. The simulations are repeatedly checked to see whether they are developing into hot summers. If this is not the case, the simulations are terminated early to save computing time.

“This enabled us to simulate a large number of very hot summers in both the current and pre-industrial climates at a manageable computational cost. Using these simulations, we were then able to investigate how ‘once-in-a-century’ summers have changed beyond general warming trends,” reports Pfleiderer.

Researchers at the University of Leipzig will continue to investigate extreme summers in Europe. They aim to find out how changes in atmospheric circulation have affected extreme summers. They also intend to examine how ‘once-in-a-century’ summers have changed in other regions of Europe. For example, it is expected that factors other than those in Central Europe will be more significant in the Mediterranean region or in Eastern Europe.

Original title of the publication in *Earth’s Future*: “Heat extremes in hot summer seasons intensify more than in average summers under global warming”