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

Abstract

<jats:p>Abstract. 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 to 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 for 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 °C, 2 °C, 3 °C, and 4 °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 very dangerous levels. About 32 % (61 %) of the inhabited or agricultural area in the investigated region would reach maximum temperatures over 40 °C under 2 °C (4 °C) of global warming and similar atmospheric circulation conditions. </jats:p>

A Methodology for Attributing the Role of Climate Change in Extreme Events: A Global Spectrally Nudged Storyline

“…The nudged global storyline method is an important step towards a holistic approach within the attribution of individual extreme events, which can quantify the role of both dynamical variability and known thermodynamic aspects of climate change, and the interplay between them, in great spatio-temporal detail. As shown by Wehrli et al (2020), the method can easily be expanded to a larger number of storylines for both past and future. The method could also be applied to other extreme events affected thermodynamically by climate change such as tropical cyclones (Feser and Barcikowska, 2012).…”

“…In previous applications of the storyline approach, individual extreme weather events have been dynamically constrained through boundary conditions applied to a regional model (Meredith et al, 2015) or by controlling the initial conditions in a weather forecast model (Patricola and Wehner, 2018). More recently, nudging the free atmosphere to reanalysis data (leaving the boundary layer free to respond) has been applied in a global medium-resolution atmospheric model to constrain the dynamical conditions leading to heat waves, first to determine the effect of soil moisture changes on selected recent heat waves (Wehrli et al, 2019), and subsequently to determine the effect of past and projected future warming on the 2018 Northern Hemisphere heatwave (Wehrli et al, 2020). The concept of nudging the atmospheric circulation in order to impose the dynamical conditions has a long history.…”

Global warming makes weather in boreal summer more persistent

“…Recent record-breaking heatwaves in the mid-latitudes caused massive socio-economic impacts (Coumou and Rahmstorf, 2012) and such record-heat has already increased 5-fold since pre-industrial times Rahmstorf and Coumou, 2011). Summer extremes often become most devastating when they last for many days and the recent decade has seen several such persistent extremes (Coumou and Rahmstorf, 2012;Fischer et al, 2007;Wehrli et al, 2020). In 2016, many municipalities in western Europe were flooded when a slow-moving continental-https://doi.org/10.5194/wcd-2020-40 Preprint.…”

Atmospheric moisture transport and river runoff in the mid-latitudes

“…All simulations were nudged towards the dynamical situation of 2018. They found maximum temperatures over 40°C in the future scenarios (Wehrli et al, 2020). Such a study provides insightful information that helps understanding risk and consequences of similar events in a future climate.…”

“…Another example of a storyline approach was given by Wehrli et al (2020) who studied the extremely dry summer of 2018 over western Europe under different warming scenarios and a scenario with no human imprint. All simulations were nudged towards the dynamical situation of 2018.…”