When US President Trump announced on June 1 that the US would withdraw from the Paris Climate Agreement, this caused international concern and incomprehension. For at the UN climate summit in Paris in December 2015, it was the first time that almost all countries had been able to swear on climate protection and the goal of limiting global warming to two degrees. Under her then President Barack Obama, the US also abandoned its former blockade position and this contributed significantly to the success of the negotiations. But Obama's successor, Donald Trump, had already made it clear in the election campaign that he does not believe in climate change and sees climate protection as an obstacle to his "America First" policy. With his dismissal of the Paris Agreement, which will become effective in four years, he has now drawn the line. Experts agree, however, that this decision, while a setback for international climate protection means that Trump will hurt the most of the US: "This is not only extremely short-sighted industrial policy, but also in terms of employment: Already working in the solar sector more than twice as many people in the US as in the coal industry. In the end, the US could therefore be the biggest loser, "commented Wolfgang Obergassel from the Wuppertal Institute for Climate, Environment and Energy.
Ozone shrinkage threatens
A previously unrecognized climate that directly affects the US has now been exposed by James Anderson of Harvard University at Cambridge and his colleagues. Accordingly, there is an increasing risk of pronounced ozone depletion in the stratosphere over the US. The starting point of their study was the NEXRAD radar measurement system, which researchers used to track the transport of water vapor, sulfur aerosols, and chlorine-containing compounds into the US stratosphere. It was found that especially the strong summer storms over the Midwest and in the South of the USA often hurl these substances into the stratosphere than previously assumed. Between 2004 and 2013, 38, 158 storms were at least two kilometers beyond the tropopause, "Anderson and his colleagues report. The tropopause forms the boundary between the troposphere, where clouds, storms and other betting events usually take place and the stratosphere above, the layer in which the ozone layer is located. The data now shows that on average every summer about 4000 storms break through this border.
For the ozone layer over the US, this has several negative consequences, as the researchers explain: First, the storms bring into the stratosphere substances that promote ozone depletion, including chlorine-containing substances and sulfur aerosols. The latter promote the decomposition of chlorine compounds into aggressive chlorine radicals, which then react with the ozone and thus decompose it. On the other hand, the storms transport large amounts of water vapor into the stratosphere. "Our measurements confirm that the stratosphere is unusually wet in the US over the summer, " Anderson and his colleagues report. The high level of transported water vapor causes the temperatures there to fall. However, as in the polar regions, low temperatures promote ozone depletion. Together with the limited air exchange over the central USA in the summer, this considerably increases the risk of measurable ozone depletion, the researchers explain.
"We do not know when it tilts"
"Every year, heavy stratospheric ozone losses are recorded in the polar regions. Our work now shows that the same chemistry can also take place across the central US, "says co-author Steven Wofsy of Harvard University. The emergency has not yet occurred and the ozone layer over the USA intact. However, the researchers fear that even the slightest change - additional storms, a volcanic eruption or increasing levels of CO2 in the atmosphere - could tip the fragile balance. "We still do not know how close this threshold is, " warns Wofsy. Climate change in particular could, according to the scientists, contribute to the tilting of the situation. Because global warming not only promotes strong storms, it also causes the stratosphere to become colder and favors air currents that trap chlorine-containing pollutants and sulfur aerosols over the USA for a particularly long time. "Overall, this leads to an increased risk of ozone loss over the Midwest, " warn Anderson and his colleagues. display
Should the ozone depletion occur, this would have consequences for the inhabitants of the USA. Because when the protective ozone layer expands, more damaging UV rays penetrate to the earth's surface. Similar to the polar regions and adjacent regions, this increases the risk of skin cancer, among other things. Studies show that even one percent ozone loss in the stratosphere leads to an increase in skin cancer rates of three percent. To make matters worse, ozone depletion over the USA would be the strongest in the summer and thus at the very time of the year when sunshine is most intense and people are the most exposed stop them. Researchers estimate that summer ozone depletion would increase one percent to 100, 000 additional skin cancers per year in the United States. "We need to closely monitor the situation and create the basis for weekly predictions of ozone loss, " says Anderson.
- James Anderson (Harvard University, Cambridge) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.1619318114