The July 2011, Copenhagen cloud burst what's climate change got to do with it?

In the evening on July 2, 2011 a severe cloud burst occurred over Copenhagen (the capital of Denmark). During late afternoon, deep moist convection (DMC) developed over southern Sweden in an airstream from east-northeast. In the early evening the DMC crossed the narrow strait between Sweden and Denmark intensifying strongly, creating severe flash floods over Copenhagen. Between 90 and 135 mm of precipitation in less than 2 hours was recorded across the city, which flooded cellars, streets, and key roads. The deluge caused more than one billion Euros in ensured damages. Although such extreme events are rare, the impact on society is important and clearly need to be understood in the context of global warming. But even with climate modelling capabilities reaching the convection permitting resolution, reproducing such an event is still challenging. Multiple studies confirm that extreme precipitation intensities increase in a warmer climate. Also, from climate modelling using transient simulations or a pseudo-warming approach, this has been demonstrated. Due to the fine scale at which this occurred, it is yet unclear how such an event would have been realized had the basic atmospheric state been similar to prevailing pre-industrial conditions, e.g. with global mean temperatures approximately 1 degree lower than at present. Or, in other words, how likely is it that global warming can be attributed as the cause of the severity of the event? Using a forecast-ensemble method with a convective permitting model, still keeping a climate perspective, this study assesses the risk for the occurrence of such an event under otherwise almost identical, but colder (and warmer) conditions. With this set-up, we compute a so-called upscaled risk of the event in the current climate as compared to pre-industrial but also for warmer scenarios. We find that not only is the risk of flooding increasing as climate warms but also the risk increases for unprecedented precipitation rate intensity. One implication of this is that the likelihood of realizing hourly intensities exceeding 60 mm is well above three times higher than would have been the case without anthropogenic forcing and possibly only reaching 90 mm or above with anthropogenic forcing present.