Focus on climate crisis
All articles | More information
Reporting on climate change is one of the major journalistic challenges of our time. The climate crisis is also one of the most important issues of humanity for SPIEGEL. For this reason, we support an international initiative that seeks to take a look this week: "Covering Climate Now" has been initiated by the Columbia Journalism Review and the Canadian newspaper "The Nation", with more than 200 media companies worldwide including the Guardian, El País, La Repubblica, The Times of India, Bloomberg or Vanity Fair. SPIEGEL is dedicating the cover story of the current issue to the climate crisis this week and every day pays special attention to mirror.de
When the temperatures climbed to 41 degrees Celsius, the authorities in Tokyo did not know what to do anymore: In 2013, in a suburb of the Japanese capital, they shot silver iodide particles into the sky in the hope that rain clouds would form. In fact, it rained in the two hours after that. The water not only cooled, but also reduced the consequences of weeks of drought.
Whether the silver iodide really caused the rain is controversial. But the experiment shows how determinedly Tokyo is fighting the heat, with dozens of people dying every summer. Some experts even warn that it could soon become too hot in Japan's big cities.
The cooling capacity of trees
In general, cities heat up faster than surrounding regions, for example because concrete stores heat. This world map shows the global urban heat islands in 2013. The red and orange dots represent cities where it was up to two degrees Celsius warmer than a ten-kilometer radius around the metropolises. However, the individual points are not representative of the size of the cities. The researchers have enlarged them so that they can still be recognized on the world map.
An international research team has investigated how urban heat can best be combated. Result: There is not one solution, scientists around Gabriele Manoli from ETH Zurich reported in the journal "Nature" at the beginning of September. The study also involved researchers from Duke University and Princeton University.
Most promising are plants as green air conditioners. Behind it is a physical process: when trees get too hot, tiny crevices open in the leaves, from which water evaporates. That consumes energy, it gets cooler. At the same time, there is a suction that drives ever new water from the roots into the leaves. Trees can exude hundreds of liters of water per day. The cooling capacity is about 70 kilowatt hours per 100 liters - enough to run two average household air conditioners.
photo gallery
31 pictures
Satellite image of the week: snapshots from space"We know that plants make the climate in a city more pleasant, now we wanted to find out how many green spaces it really needs to achieve a significant cooling effect," says first author Gabriele Manoli of ETH Zurich. To do so, the researchers analyzed average temperatures, population and rainfall of about 30,000 major cities worldwide. Weather data was provided by the Moderate Resolution Imaging Spectroradiometer (Modis) aboard the Nasa Earth Observation Satellite Terra.
Plants do not help everywhere
The cities clustered between two climate extremes: dry to tropical. It turned out that trees can not provide the desired cooling effect in all cities. "It all depends on the environment and regional climate characteristics," says Manoli. For example, in a city like Phoenix in the US, plants could make it cooler in the city than in the desert environment.
In tropical Singapore, on the other hand, it would take an extremely large amount of additional green space to cool the city considerably compared to the surrounding area. In addition, the air would also be much wetter. The problem: In high humidity, the cooling mechanism of the body does not work anymore, sweating. Even from 37 degrees Celsius, the heat can no longer be released to the outside, the body heated. At high humidity even lower temperatures can be dangerous.
Heat islands in tropical cities can not be achieved with trees alone, the researchers conclude. Instead, city planners should think about new heat-resistant materials, create shaded areas, and ensure that wind can circulate in the streets.
What role does climate change play?
The problem is therefore pressing, because already now half of the people live in cities, by the year 2050 it should be two-thirds. (Read more about the challenges facing cities due to climate change here.)
Climate change could increase the temperatures in the cities. In general, it is problematic to attribute individual betting events directly to climate change. After all, heat waves have already existed before humans began to mass-release CO2 into the atmosphere.
However, climate change increases the risk of such extreme weather conditions and they are becoming more intense, warn climatologists. This also applies to Germany. In the past 138 years, the 40-degree frontier has been broken only ten times, but 25 times this July - in just three days.