The extremely warm summer of 2018 in Sweden – set in a historical context

Abstract

Abstract. Two long-lasting high pressure systems in summer 2018 lead to long lasting heat waves 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 whole Sweden to set the latest heat-wave in summer 2018 in historical perspective. With 263 years we are able to grasp the pre-industrial time well and see a clear upward trend in temperature itself as well as heat wave 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 as those observed in summer 2018 show up 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 hold individual years like 2018, the comparison shows that such conditions are rare. For the indices assessed here, anomalies such as observed in 2018 occur maximally in 5 % of the ensemble members, sometimes even in less than 1 %. For all indices evaluated we find that probabilities of a summer like in 2018 have increased from relatively low values for the one ensemble extending back to 1861–90 and for all five ensembles including 1951–80 to the most recent decades (1989–2018). An implication is that anthropogenic climate change has strongly increased the probability of a warm summer such as the one observed 2018 to occur in Sweden. Despite this, we still find such summers also in the pre-industrial climate, however, with a lower probability.