Researchers conclude that the global water cycle is significantly influenced by climate change.

For Christoph Scherr, Professor of Climate and Water Cycles at ETH Zurich, “global warming” is not entirely accurate when it comes to describing the driver of climate change. “A better term is ‘wetting the climate,’” he explains. “Most of the solar energy that reaches the Earth evaporates the water and thus drives the hydrological cycle.” Correctly calculating the implications is the most difficult task of all for climate modellers.

In order to build a global climate model, grid points spaced between 50 and 100 km apart are used. This scale is too coarse to map small-scale local thunderstorm cells. However, it is these thunderstorm cells—and where they occur—that drive atmospheric circulation, especially in the tropics, where solar radiation is highest.

The solution is, for now, to add additional parameters to the model in order to set the draw. “But predicting future climate change is still very inaccurate,” Scherr says. “If we don’t know how many clouds form in the tropics, we don’t know how much sunlight hits the Earth’s surface — and so we don’t know the actual size of the global energy balance.”

Amazing accuracy

Over the next few years, scientists hope to address this mystery. Scheer, for example, is now working with models with much higher resolution—one to two kilometers—that provide a more accurate picture of meteorological activity. To illustrate, he and his group ran a sequence on a supercomputer that simulated weather events in the Tropical Atlantic over a period of years to decades.

The visualization is strikingly similar to the satellite image: rain fronts moving east to west across Africa; Finely organized cloud fields form off the coast of Brazil; Hurricanes develop in the mid-Atlantic Ocean and then head north. “The model knows absolutely nothing about a tropical climate,” Cher exclaims excitedly. “But based on the laws of physics alone, it can still give us a realistic picture of what’s going on.” It’s still not feasible to create long-term scenarios with such high-accuracy models, but they’re working on making existing global models more accurate.

Using the example of southwestern Europe, Schär shows how higher-resolution models are also able to predict extreme weather events more accurately. Current models greatly underestimate the amount of rain that can fall in an hour. By contrast, the high-resolution models generate very realistic distributions and correctly determine that in autumn, for example, there is a high probability of particularly heavy rains and floods on the southern edge of the Alps, along the Ligurian coast and in Provence.

Today’s forecasts for extreme precipitation events align with a law of physics formulated in the 19th century by Rudolf Clausius and Émile Clapeyron. “They were just doing basic research,” Scheer explains. “Practical applications in climate change weren’t even on the radar at the time.” The Clausius-Claperon relationship says that the atmosphere can hold about 6% more water vapor per degree Celsius of warming. In other words, we can expect to see heavier precipitation events in the future. “This will have consequences for flood prevention,” Scherr says. “We will no longer be able to design flood protection based on past events.”

The laws of physics tell us that a warmer atmosphere will absorb more water vapor. Despite this, many areas are expected to suffer from water shortages. Scheer explains the paradox: “The absolute moisture content of the atmosphere is generally rising, but the relative humidity can also decrease regionally. In other words, more water will evaporate from the Earth; but at the same time, cloud formation will decrease.” Also in certain areas, as there will then be less rain.” Scheer says this would have dire consequences not only for southern Europe, but also for the countries of North Africa, which are already suffering from a water shortage.

Floods and forest fires

Too much water, or too little, is similarly a major concern for hydrologist Manuela Brunner. Assistant Professor at ETH, focusing on the impact of extreme weather events on mountain regions. “Mountain water plays a major role in causing floods and droughts,” she explains. “Mountains are particularly affected by climate change because temperatures rise more there than in lowland areas.”

To check whether flooding is likely to become more frequent and severe in the future, Brunner uses a combination of observational data and model-based simulations. “In the Alps, the picture is pretty mixed of the type of moderate flooding that usually happens once every 10 to 20 years,” she explains. “In some areas, that risk has increased; in others, it is already declining.” One of the main factors here is the condition of the soil. “If the soil is dry, it can absorb a lot of water and thus mitigate flooding. But if the ground is already saturated, this effect is lost.”

However, Brunner predicts an increased risk of severe flooding for 100 years throughout the Alpine region. “In this case, there’s so much rain that the state of the ground doesn’t make much of a difference,” she says. She explained that while we know the individual factors that can cause flooding, we still lack an understanding of how they interact. “What happens, for example, when it rains heavily during a thaw?” she asks. “When will this develop into an extreme event? How often will we see this combination?”

Floods are not the only threat facing the Alpine region. “In the future, we will see more frequent droughts on the north side of the Alps and maybe even forest fires,” Brunner says. A number of factors come into play here: first, there is less precipitation in the summer; secondly, soil evaporation is increasing due to higher temperatures; And third, snow levels drop in the spring, which in turn means that vegetation is more likely to dry out.

“Although precipitation in the winter months generally increases, higher temperatures mean that less and less of this is stored as snow,” Brunner explains. “And if, as we enter the warmer months, snow cover is lower in the spring, that could exacerbate water shortages during dry summers.”

Brunner is particularly concerned about the possibility of prolonged droughts for several years. “In the past, we didn’t have to worry after a dry summer in the Alps, because there was always enough rain by the end of the following winter to make up,” she says. “But in the future, the water shortage may worsen during the winter.”

How fast are glaciers melting?

To make matters worse, it is now clear that glaciers will soon stop providing the same amount of meltwater in summer as they did in the past. “In the best-case scenario, Switzerland will still have 40% of the current glacier volume by 2100,” says Daniel Varinotti, professor of glaciology at ETH Zurich. “At worst, there will be only a few percent left.” Whatever the case, he is confident that Switzerland can keep track of these changes. “We know exactly how much ice is still there because we’ve already done radar surveys on most of the glaciers.”

Things are more complicated in the Himalayas, where Varinotti and his team also run a project. There, the glaciers are at a much greater altitude, which makes surveying much more difficult. At the same time, neighboring countries are reluctant to provide data for research for strategic and geopolitical reasons. Thus predictions of when Himalayan glaciers will melt can vary by as much as a decade. “For the lowlands, which are more densely populated, this makes a huge difference,” he adds.

In Switzerland, too, there is an urgent need to see how much water melting glaciers will contribute in the future – not least because concessions for a number of hydroelectric power plants will be renewed in the next few years. Not only do these operators need to know how much water will be available to them in the future; They also require detailed forecasts regarding extreme weather events. “They worry about whether the water intakes are of sufficient capacity,” Farinotti explains.

There is another issue of greater concern: the melting of the polar ice sheets. “In our group, we are currently building a detailed flow model of the Greenland ice sheet based entirely on physical processes,” Varinotti explains. “We are mapping ice masses at a resolution of 25 meters in order to assess what will happen to the ice sheet over the next few decades.” To run this complex simulation, the team is preparing to take advantage of LUMI, Europe’s fastest supercomputer.

Along with other researchers, Varinote’s group is also investigating the Antarctic ice sheet, which faces a number of threats. In particular, there are issues with the West Antarctic Ice Sheet, which sits on bedrock below the ocean’s surface. “The topography of this bedrock plays a major role in how quickly the ice recedes,” he explains.

This is undoubtedly a vital question for a number of coastal regions around the world. “If the West Antarctic ice sheet starts to melt, sea levels could rise by 1 meter by the end of the 21st century,” Varinotti says. With 250 million people living in areas that would then be underwater, there is no need to ask why the future of the polar ice sheets is also important at low latitudes.