Compound and concurrent extremes: Attribution, projections and storylines under higher global warming levels

Abstract:

Compound events are receiving increasing attention in the literature because of the associated risks and potential impacts for ecosystems and society. However, there is still a limited number of studies assessing their attribution to human forcing or their projected changes under a warmer climate. In this presentation, two types of compound events are assessed: concurrent hot extremes across regions similar to those that happened in the 2018 Northern Hemisphere summer (Vogel et al. 2018; Wehrli et al., in review), and compound colocated hot/dry, hot/wet, and dry/wet events (Vogel et al., in press).

Storylines of the 2018 North-Hemisphere summer (NH2018) are presented at different global warming levels, including pre-industrial conditions and changes at higher global warming (+1.5°C, +2°C, +3°C, +4°C), applying the same circulation patterns as those experienced in 2018. 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°C under +2°C (+4°C) of global warming and similar atmospheric circulation conditions.

Analyses of colocated multi-variate (hot/dry, hot/wet, dry/wet) extremes in the CMIP6 projections reveal substantial increases in the probability of their occurrence at higher global warming levels, and a range of hot spots for these changes. Hot spot regions for hot and dry clusters are found in Brazil, Southern Europe, Turkey, Morocco, Algeria and Southern Africa. Hot spot regions for wet and hot clusters are found in Central Africa, Indonesia and in mountainous regions such as Peru, Chile, and the Himalayan region.

These analyses reveal substantial risk from compound events under higher global warming, even at +1.5°C or +2°C of global warming, and a substantial contribution of human climate forcing for their occurrence under present climate already. This highlights the need for a better assessment of compound events and correlated extremes to inform climate change adaptation and mitigation in the coming decades.