Abstract
Abstract. Two long-lasting high-pressure systems in summer 2018 led to persisting heatwaves over Scandinavia and other parts of Europe and an extended summer period with devastating impacts on agriculture, infrastructure, and human life. We use five climate model ensembles and the unique 263-year-long Stockholm temperature time series along with a composite 150-year-long time series for the whole of Sweden to set the latest heatwave in the summer of 2018 into historical perspective. With 263 years of data, we are able to grasp the pre-industrial period well and see a clear upward trend in temperature as well as upward trends in five heatwave indicators. With five climate model ensembles providing 20 580 simulated summers representing the latest 70 years, we analyse the likelihood of such a heat event and how unusual the 2018 Swedish summer actually was. We find that conditions such as those observed in summer 2018 are present in all climate model ensembles. An exception is the monthly mean temperature for May for which 2018 was warmer than any member in one of the five climate model ensembles. However, even if the ensembles generally contain individual years like 2018, the comparison shows that such conditions are rare. For the indices assessed here, anomalies such as those observed in 2018 occur in a maximum of 5 % of the ensemble members, sometimes even in less than 1 %. For all of the indices evaluated, we find that the probability of a summer such as that in 2018 has increased from relatively low values in the pre-industrial era (1861–1890, one ensemble) and the recent past (1951–1980, all five ensembles) to higher values in the most recent decades (1989–2018). An implication of this is that anthropogenic climate change has strongly increased the probability of a warm summer, such as the one observed 2018, occurring in Sweden. Despite this, we still find such summers in the pre-industrial climate in our simulations, albeit with a lower probability.
Highlights
Long-lasting high-pressure-dominated weather resulted in remarkably warm and dry conditions in large parts of northern Europe during the summer of 2018 (Sinclair et al, 2019)
Similar pressure patterns to those observed in summer 2018 have previously been shown to be associated with warm temperature anomalies over different parts of Europe (Sousa et al, 2018; Zschenderlein et al, 2019); for example, Pfahl and Wernli (2012) found that most summer heatwaves (80 %) in northern Europe and Russia can be associated with atmospheric blocking situations
We investigate the likelihood of such a summer having occurred in the past century using five large global climate model ensembles, some of which cover the period since 1860 and others that start in the second half of the 20th century and extend to 2018
Summary
Long-lasting high-pressure-dominated weather resulted in remarkably warm and dry conditions in large parts of northern Europe during the summer of 2018 (Sinclair et al, 2019). Sweden experienced a very long warm period with an unusually high number of warm days. The high-pressure situation was already established in May, continued over summer until mid-August, and was only interrupted for short periods – mainly in June. The hot and dry conditions in Sweden in summer 2018 were associated with severe consequences for people, such as health problems and an excessive mortality rate (Åström et al, 2019), and for the environment, such as water shortages with adverse implications for arable land and pastures (Buras et al, 2020), including lack of forage, and unusually large areas being affected by forest fires (Krikken et al, 2019). Heatwaves are reported to have impacted Sweden prior to 2018 in terms of an increased mortality rate among people (e.g. Oudin Åström et al, 2013), ecological consequences (Rasmont and Iserbyt, 2012), and in connection with air pollution episodes (Struzewska and Kaminski, 2008)
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