Storylines of the 2018 Northern Hemisphere heatwave at pre-industrial and higher global warming levels

Extreme temperatures were experienced over a large part of the Northern Hemisphere during the 2018 boreal summer (hereafter referred to as NH2018 event), leading to major impacts on agriculture and society in the affected countries. Previous studies highlighted both the anomalous atmospheric circulation patterns during the event and the background warming due to human greenhouse gas emissions as main drivers of the event. In this study, we present Earth system model experiments investigating different storylines of the NH2018 event given the same atmospheric circulation and alternative background global warming for no human imprint, the 2018 conditions, and different mean global warming levels 1.5, 2, 3 and 4 degrees C. The results reveal that the human-induced background warming was a strong contributor to the intensity of the NH2018 event, and that resulting extremes under similar atmospheric circulation conditions at higher levels of global warming would reach dangerous levels. Compared to 9% during the NH2018 event, about 13% (34%) of the inhabited or agricultural area in the investigated region would reach daily maximum temperatures over 40 degrees C under 2 degrees C (4 degrees C) of global warming and similar atmospheric circulation conditions.