Summer weather becomes more persistent in a 2 °C world

Abstract

Heat and rainfall extremes have intensified over the past few decades and this trend is projected to continue with future global warming 1-3. A long persistence of extreme events often leads to societal impacts with warm-and-dry conditions severely affecting agriculture and consecutive days of heavy rainfall leading to flooding. Here we report systematic increases in the persistence of boreal summer weather in a multi-model analysis of a world 2 °C above pre-industrial compared to present-day climate. Averaged over the Northern Hemisphere mid-latitude land area, the probability of warm periods lasting longer than two weeks is projected to increase by 4% (2-6% full uncertainty range) after removing seasonal-mean warming. Compound dry-warm persistence increases at a similar magnitude on average but regionally up to 20% (11-42%) in eastern North America. The probability of at least seven consecutive days of strong precipitation increases by 26% (15-37%) for the mid-latitudes. We present evidence that weakening storm track activity contributes to the projected increase in warm and dry persistence. These changes in persistence are largely avoided when warming is limited to 1.5 °C. In conjunction with the projected intensification of heat and rainfall extremes, an increase in persistence can substantially worsen the effects of future weather extremes. Extreme weather events are commonly analysed in terms of intensity or frequency but often it is their persistence that leads to the most severe effects. Extended periods of warm and dry weather can strongly affect human health and agriculture and increase risks of wildfires 4. In 2018, dry and warm conditions in western Europe extended from April to September with only a few short interruptions of cooler and rainy weather 5-7 (see Fig. 1). This persistent drywarm compound extreme had a devastating effect on agriculture in Germany, with wheat harvests down by 15% (ref. 6). Similarly, most damaging flooding events occur following several consecutive days of heavy rainfall 8. In 2016, a slow-moving low-pressure system remained over Europe for two weeks, resulting in several days of heavy precipitation leading to floods in many municipalities in western Europe 9 (Fig. 1). Global warming is already increasing the frequency and intensity of heat and rainfall extremes 3 , as well as the duration of heat waves 10 , and these trends are projected to continue with future warming 11. However, if and how changes in persistence of local weather conditions might contribute to more severe weather extremes is poorly understood. Some observational studies suggest that summer persistence has increased over recent decades 12-16 but this is so far not supported by modelling studies.

Past and Projected Weather Pattern Persistence with Associated Multi-Hazards in the British Isles

“…As found in our analysis, greater A-type persistence and frequency during summer likely implies more blocking episodes with increased risk of poor air quality, drought and heatwaves [1,5,90,91]. A growing number of studies propose physical mechanisms that link Arctic Amplification (AA) [92] to more persistent weather patterns, which in turn enhance the likelihood of extreme weather events in the northern hemisphere mid-latitudes.…”

“…The AA may affect the polar jet stream by making Rossby waves more meridional (or wavier) and by weakening its flow. A wavier and weaker jet stream in summer favours more persistent extreme weather and it is also thought to extend ridges northward, enhancing such effects [1][2][3]90,91,[93][94][95]. In contrast, another study suggests that increasing trends in meridional extent of the jet stream, along with blocking events, may be an artefact of the methodologies used [85].…”

Increased likelihood of heat-induced large wildfires in the Mediterranean Basin

“…Compound dry-warm periods are projected to become more frequent and persistent in a warmer climate 28 and future scenarios of fire danger consistently point to an increase in the frequency and intensity of heat-induced FWTs across the Mediterranean. This will very likely increase the frequency and extent of wildfires, provided fuel remains abundant.…”

Observed Changes in Temperature and Precipitation Extremes Over the Yarlung Tsangpo River Basin during 1970–2017

“…Global mean surface temperature rose by approximately 0.85 • C during 1880-2012, and warming is projected to continue because of the future emission of greenhouse gases [1,2]. In recent decades, observed climate data and climate models have shown that warming of the climate system leads to increases in extreme weather and climate events, such as heatwaves, cold spells, floods, droughts, and rainstorms [3][4][5][6][7]. There is general agreement that changes in the frequency or intensity of extreme weather and climate events would have a profound socioeconomic and environmental impact [5,8,9].…”