How global warming aggravates hurricanes

Climate change Longer and wetter: storms become more destructive

Storms slow down but bring more rain

James Kossin of the US Climate and Ocean Agency NOAA evaluated data on tropical cyclones between 1979 and 2016. He found: On average, the speed of the storms fell by 10 percent. In the western North Pacific, storms move 20 percent more slowly today, in the waters off Australia it is 15 percent. "These trends are very likely to increase local rainfall and freshwater flooding, with a very high risk of death," Kossin writes.

Kossin suspects global warming to be the cause. They weaken the tropical air circulation. There are less strong air currents today that could carry the storms further. In addition, warmer air can absorb more moisture, which increases the amount of rain that such a tropical storm brings.

These problems are likely to increase in the future. "The ten percent slowdown we observed happened over a period of time in which the planet warmed by 0.5 degrees Celsius," says Kossin. Climate policy is currently pursuing the goal of limiting total warming to 2 degrees by 2100. Whether this limit will be adhered to, however, appears to be questionable.

The trend is also visible in Europe

Tobias Geiger, scientist at the Potsdam Institute for Climate Impact Research, is not surprised by the new research results. "It has been observed several times in the last few decades that weather conditions move more slowly, that they are more persistent and the consequences can therefore become more extreme," he says in an interview with MDR-Wissen.

He and his colleagues have identified the changed temperatures in the Atlantic as a central cause. "A big factor is that the Arctic warms up more than the equator. This means that there is less temperature difference in seawater. These temperature differences are the engine for the weather in Europe," he explains. If the water temperatures now equalize, there are less strong compensatory winds that pull around the earth due to the Coriolis force.

This has now been confirmed several times for Europe that, for example, a high pressure situation is maintained for weeks in summer and that there are strong heat waves and droughts. Conversely, a low pressure area can also remain in the same place for weeks. Then extreme amounts of precipitation fall and there is flooding.

Dr. Tobias Geiger, Potsdam Institute for Climate Impact Research

Weaker hurricane season expected

This spring, Germany has only just seen the effects of such weather conditions. The situation was also similar with "Harvey". "A high pressure area was stuck in the middle areas and blocked the hurricanes from moving on," says Geiger. This shows how much global air currents affect the weather. They can even trap a storm several hundred kilometers in diameter in one place.

Geiger also expects that air circulation will decrease further and that storms will pass more slowly as a result. This increases the risk of damage from wind or from the concentration of rain, which in turn can lead to mudslides.

NOAA expects four severe cyclones in the Caribbean this year, as well as five to nine other, weaker hurricanes. This would make the season weaker than last year. Experts had expected below-average storms in 2017, but it turned out to be the worst hurricane year since 2005.