Scenarios for a climate-neutral world

Climate modeller Edouard Davin is studying the cooling effects of urban trees. He also wants to use his research to help solve problems like water shortages in developing countries.

OCCR member Edouard Davin and his colleagues weren’t expecting it. The climate model specialist, who has recently started his research at the University of Bern, describes the study he worked on at ETH Zurich as an “interesting surprise”. That study sheds light on the role that trees play during hot spells in European cities, and it shows that the cooling effect varies greatly from region to region. It is most pronounced in central Europe, Scandinavia and the United Kingdom, while in southern Europe the cooling potential decreases – especially when it’s hot. “The cooling effect depends strongly on the context,” says Edouard Davin. “We don’t fully understand why that is yet.”

One hypothesis: the more parched the soil, the less of a cooling effect that urban trees have. In the future, climate change is likely to cause even drier conditions in summer – even in central Europe. “We could therefore see a decrease in cooling in regions that currently benefit from a particularly high cooling effect,” states the study, published recently in the journal Nature Communications. While irrigation would help maintain this cooling, the necessary water could become scarce in future summers.

A treasure trove of data on urban climates

For its study, the research team relied on high-resolution satellite data on 293 European cities – from Tromsø in Norway to Nicosia in Cyprus. Statistical methods were used to compare the surface temperatures of urban areas that are either entirely covered by trees or completely built up.

This is a rather unexpected field for a climate modeller like Edouard Davin. For his doctoral thesis at the Université Pierre & Marie Curie in Paris, he worked on incorporating land use scenarios into a global climate model. Later, at ETH Zurich, he worked with regional modelling and high-resolution climate simulations. He was also the lead author of the 2019 IPCC Special Report on Climate Change and Land, among other projects. “In recent years,” Davin says, “I’ve increasingly linked modelling with statistical analysis of remote sensing data. I’m trying to combine these two worlds.”

Recently, the modeller with the broad research horizon was appointed to the University of Bern and became a member of the Oeschger Centre. He is a professor of climate scenarios for sustainable development at the Wyss Academy for Nature. So what exactly is he working on there? Davin explains that climate science has long focused on high-emission scenarios – the worst case scenarios – so to speak. Now, however, it’s also important to show “how the climate-neutral world of tomorrow could be built.” Climate scenarios for sustainable development should illustrate a successful transformation to a world in line with the Paris climate goals. This includes questions such as: Where are reductions in CO2 emissions possible, and to what extent? Or: What role can so-called negative emission technologies play?

From climate problems to solutions

When it comes to these issues, Edouard Davin says his specific expertise revolves around land use. “It’s about managing land use so that it goes from being a climate problem to a climate solution.” At the Wyss Academy and OCCR, however, Edouard Davin wants to use interdisciplinary projects to answer questions like: Does forest regeneration improve water availability? “Climate science should help solve development problems. This is very worthwhile.”

Land use isn’t just about the impact of agriculture and forestry on the climate; it’s also about urbanization. And conversely, it’s also about the effects of heatwaves and urban heat islands both in the global south as well as in Switzerland. This is precisely what the OCCR’s Urban Climate Bern project at the University of Bern is addressing. Multi-year, high-resolution temperature measurements should provide a detailed picture of urban heat stress. These data will enable the modelling of future developments and serve as a basis for decisions on climate adaptation measures.

Combining measurement technologies

In Edouard Davin’s eyes, it’s a very exciting project. True, it only provides measurements from 65 temperature sensors within a single city – quite a contrast to the ETH Zurich urban tree study involving satellite data from nearly 300 locations across Europe. What interests Davin is the contrast between the two types of measurements. They differ in one key respect: satellites record temperatures of surfaces such as treetops, rooftops and streets. Urban Climate Bern’s sensors, on the other hand, determine the ambient temperature at a standardized height of three metres above the ground. “This method allows much more precise measurements,” says Edouard Davin, “and temperatures measured in the air are much more relevant to people’s well-being than those from the land’s surface.”

The newly appointed professor of sustainable climate scenarios sees numerous connections between his research and OCCR’s work – from collaborating on regional modelling projects to exploring the economic impact of sustainable climate scenarios. In the Urban Climate Bern project of the OCCR Climatology Group, however, his ideas are already very concrete: “I would like to link the results of the two approaches. We could try to develop a model linking the two types of measurements.”

The role of urban trees in reducing land surface temperatures in European cities. Jonas Schwaab, Ronny Meier, Gianluca Mussetti, Sonia Seneviratne, Christine Bürgi & Edouard L. Davin. Nature Communications 12/2021)

(December 2021)