We looked into dozens of different climate models, as well as into observational data, and it turns out that the temperature distribution favouring planetary wave stalling increased in almost 70% of the simulations. Kai Kornhuber, co-author and scientist at the Potsdam Institute for Climate Impact Research (PIK) in Germany

Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity Michael Mann, study lead author and professor at Pennsylvania State University

Definition:

Planetary waves are a pattern of winds, of which the jet stream is a part, that encircle the northern hemisphere in lines that undulate from the tropics to the poles. Normally, the whole wave moves eastwards but, under certain temperature conditions, the wave can halt its movement. This leaves whole regions under the same weather for extended periods, which can turn hot spells into heatwaves and wet weather into floods.