What the heat does to us

Since 2014, he has been coordinating the Drought Monitor: An online tool that provides daily updated data on soil moisture in Germany. At the click of a mouse, three maps with colors ranging from blue to dark red show the current situation (or optionally a 14-day retrospective), down to a soil depth of 1.8 meters, and in topsoil down to a depth of 0.25 meters, which becomes moist more quickly after rain and can hold the water better or worse depending on its composition. This so-called plant-available water is shown on the third map of the same name; it thus gives farmers an indication of how thirsty their crops or fruit trees are and whether it makes sense to irrigate.

What role does heat play in this? "Droughts can occur even without heat," Marx explains. "But high temperatures are a major driver because they accelerate evaporation." For example, at 30 degrees air temperature, up to six liters of water evaporate per square meter per day, literally dissolving into thin air and lost to the soil and the plants or trees growing there. Every degree more increases evaporation.

The overallsoil map in mid-July 2022 makes it clear what the combination of a long-lasting lack of rain and many heat days is causing: Only in the Alpine region, the Bavarian Forest, and between the North Sea and western Baltic Sea coasts is there no drought. The Black Forest region, Franconia, and almost all of eastern Germany south of the Elbe River, with the exception of the Harz Mountains and the Thuringian Forest, are colored red to deep red (extreme to exceptional drought). The rest of the country varies from yellow (exceptionally dry) to orange (severe drought). "This situation is now continuing for the third summer in a row," says Andreas Marx. "For Germany, this is completely new."

He also knows the consequences: In 2018 alone, agricultural losses were estimated at 700 million to three billion euros. In addition, there is massive damage to forests and woodlands because the starving spruce trees in particular can no longer defend themselves against bark beetles. Whole swaths of land are falling bare, making it obvious to everyone that the climate crisis has long since taken its toll.

The drought monitor is based on simulations with the so-called mesoscale hydrological model. It was developed at the UFZ and is becoming increasingly popular at other research institutions. Every night it is fed with data from 2500 measuring stations of the German Weather Service as well as from 40 of its own soil moisture measurements, with the modeled results then being prepared as a map.

These are used by a wide variety of interested parties: By farmers and foresters as well as advisory institutions, by seed developers and risk insurers, by river navigation and drinking water associations, which are also increasingly running out of resources, and–this is also new in Germany–by fire departments. "The drier forest soils are, the more flammable they are. Heat dries out the topsoil in particular, so the risk of forest fires grows extremely on hot days. A discarded butt, a misfire on the catalytic converter, and a spark can turn into dangerous flames," Marx knows. "That's why more and more forest areas are being closed off. I'm afraid our forests will burn like in southern Europe in the foreseeable future."

All the more important is the question of what will happen next with the heat? Astrid Kiendler-Scharr, director of the Troposphere Institute for Energy and Climate Research in Jülich, Germany, and lead author of the latest IPCC report, or more precisely, of the chapter on "Short-lived climate pollutants," which was included there for the first time, can answer this question. "The hot temperatures we have been suffering from in this country lately are fueled by the currently measured 1.1 degrees more compared to pre-industrial times," explains the physicist.

"What's clear is that by 2050, the number of heat days will increase, by more than two weeks in particularly affected regions, even if we manage to meet the Paris climate targets." A Hamburg summer will then feel like Milan today, Cologne like Palermo.

The reason: Every gigaton of CO2 that we released into the atmosphere in the past will have an effect for decades to centuries. Kiendler-Scharr is therefore researching climate pollutants that contribute just as much to global warming as CO2, but are much shorter-lived: Methane, for example, ozone, nitrogen oxides or other fine dust aerosols.

"These substances offer us a powerful adjusting screw because they only remain in the atmosphere for a few hours to a maximum of twelve years," says Kiendler-Scharr. "If we reduce them effectively, we can slow warming by up to 0.8 degrees by 2100. And I can't say it often enough: Every hundredth of a degree counts to protect us from even more extreme events like heat waves!"

To collect as much and as accurate data as possible for computational models, her Jülich institute has spent more than 20 years equipping airliners with measuring equipment, building the SAPHIR atmospheric simulation chamber, and most recently, in the first Corona spring of 2020, using a flying blimp to determine how much short-lived climate pollutants have decreased during the lockdown. The answer is somewhat discouraging: Only by seven percent.

The good news is that we now know exactly where man-made methane emissions come from. Mainly from natural gas, livestock and landfills. And therefore, what measures we need to stop them: "We need to consistently switch to renewable energy, insulate our buildings and fix leaking gas lines," Kiendler-Scharr demands." "Less beef and milk, better waste cycles and so on."

The situation is more complicated with particulate aerosols. They come from exhaust gases and biomass burning, but also from natural sources such as desert dust and sea spray, and also form from substances released into the atmosphere by plants. Accordingly, they differ in their physical and chemical composition and in their effect on the climate. In the Jülich atmospheric chamber, it has nevertheless been possible to determine the basic processes of how different aerosols form and grow. Kiendler-Scharr's team is currently investigating how exactly they affect the climate.

Known pollutants such as carbon monoxide can be significantly reduced, for example, through better particulate filters, fewer combustion engine cars, or speed limits. This has a doubly positive effect: The air becomes cleaner, which means fewer people get lung and cardiovascular diseases, with and without heat waves. Which brings us back to the topic of health.

Kiendler-Scharr emphasizes: "Our model calculations show that it is possible to slow down global warming by reducing emissions. Now it's up to policymakers to act."