FLOODSGlobal Warming Doubled the Risk for Copenhagen’s historic 2011 Cloudburst

Researchers at the University of Copenhagen, in collaboration with the Danish Meteorological Institute (DMI), have used detailed weather models to clearly tie increased temperatures to the historic cloudburst over Copenhagen in July of 2011. New method involving counterfactual weather forecasts could link the weather event to global warming for the first time.

It is seven o’clock in the evening on July 2nd, 2011. A cloudburst of historic proportions has just struck north of Copenhagen. On the roof of his car, a taxi driver tries to save himself from the floodwaters as rain and hail plunge into the water and cars floating around him on Lyngbyvej.

On this day, the Danish capital experienced an extreme cloudburst that cost society billions of kroner. At Rigshospitalet, the situation was so dire that the floodwater was centimeters away from destroying the hospital’s generators and triggering an evacuation of 1400 patients.

Now, Niels Bohr Institute and DMI researchers have usedan unconventional tool to understand 2011’s extreme downpour. Counterfactual history is when you change something about an historical event to analyze the What if? Typically used by historians to understand our past, climate scientists have begun deploying the method in a similar way.

Their experiment demonstrates a clear correlation between the intensity of the cloudburst at the time and the heat in the atmosphere leading up to its occurrence.

“Yes, to put it simply you could say that on a planet one degree warmer, a similar weather situation would have likely prompted the evacuation of Rigshospitalet,” says Professor Jens Hesselbjerg Christensen of the Niels Bohr Institute.

Based on Historical Weather Data
By running different weather simulations for the day of the cloudburst based on DMI models, the researchers produced a number of counterfactual weather forecasts. These were divided into five different heat scenarios, each of which allowed the study to show the consequences of atmospheric temperature increases.

For the first time, the researchers were able to show that a century of human-caused increases in temperature doubled the risk of the historic cloudburst and increased its intensity.

The study also demonstrates that with increasing temperatures ahead of us, there will also be an increased risk of similar or even stronger cloudbursts whenever similar weather situations arise in the future.

The model calculations are based on historical weather data and are thereby supported by empirical evidence.