Global warming doubled the risk for Copenhagen’s historic 2011 cloudburst

Source(s): University of Copenhagen

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 centimetres away from destroying the hospital's generators and triggering an evacuation of 1400 patients.

Now, Niels Bohr Institute and DMI researchers have used an 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.

Cars stuck in flooding during 2011 cloudburst
Photo by Lisa Risager from Denmark, CC BY-SA 2.0, via Wikimedia Commons - (cropped)

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.

The cloudburst on 2 July 2011, Copenhagen

The most expensive natural catastrophe in Denmark since 1999. Insurance payments amounted to DKK 6.2 billion, divided into approx. 90,000 claims.

In some places, two months worth of precipitation fell in a few hours. In a single day, 135.4 mm fell at Copenhagen’s Botanical Garden. 31 mm fell within ten minutes in the suburb of Ishøj. More than 5,000 lightning strikes were recorded in 3 hours.

The heavy rain and hail caused traffic to come to a standstill in several places in the metropolitan area as roadways became rivers. Several highways were closed for 1-3 days.

Train traffic was disrupted for a week and in some places closed for days, due to everything from flooded stations to lightning strikes on equipment and landslides.

Approximately 10,000 households suffered power outages for up to 12 hours and approximately 50,000 homes lost heating and hot water for up to a week.

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