Abstract
Analysis of observations indicates that there was a rapid increase in summer (June–August) mean surface air temperature (SAT) since the mid-1990s over Western Europe. Accompanying this rapid warming are significant increases in summer mean daily maximum temperature, daily minimum temperature, annual hottest day temperature and warmest night temperature, and an increase in frequency of summer days and tropical nights, while the change in the diurnal temperature range (DTR) is small. This study focuses on understanding causes of the rapid summer warming and associated temperature extreme changes. A set of experiments using the atmospheric component of the state-of-the-art HadGEM3 global climate model have been carried out to quantify relative roles of changes in sea surface temperature (SST)/sea ice extent (SIE), anthropogenic greenhouse gases (GHGs), and anthropogenic aerosols (AAer). Results indicate that the model forced by changes in all forcings reproduces many of the observed changes since the mid-1990s over Western Europe. Changes in SST/SIE explain 62.2 ± 13.0 % of the area averaged seasonal mean warming signal over Western Europe, with the remaining 37.8 ± 13.6 % of the warming explained by the direct impact of changes in GHGs and AAer. Results further indicate that the direct impact of the reduction of AAer precursor emissions over Europe, mainly through aerosol-radiation interaction with additional contributions from aerosol-cloud interaction and coupled atmosphere-land surface feedbacks, is a key factor for increases in annual hottest day temperature and in frequency of summer days. It explains 45.5 ± 17.6 % and 40.9 ± 18.4 % of area averaged signals for these temperature extremes. The direct impact of the reduction of AAer precursor emissions over Europe acts to increase DTR locally, but the change in DTR is countered by the direct impact of GHGs forcing. In the next few decades, greenhouse gas concentrations will continue to rise and AAer precursor emissions over Europe and North America will continue to decline. Our results suggest that the changes in summer seasonal mean SAT and temperature extremes over Western Europe since the mid-1990s are most likely to be sustained or amplified in the near term, unless other factors intervene.
Highlights
European summer climate exhibits variability on a wide range of timescales
The direct impact of anthropogenic aerosols (AAer) changes act to increase diurnal temperature range (DTR), but change in DTR is countered by direct impact of greenhouse gases (GHGs) forcing
We have investigated the rapid summer mean warming over Western Europe and associated temperature extreme changes since the mid-1990s
Summary
European summer climate exhibits variability on a wide range of timescales. Understanding the nature and drivers of this variability is an essential step in developing robust climate predictions and risk assessments. Europe experienced record-breaking heat waves and extreme temperatures that imposed disastrous impacts on individuals, and society (Stott et al 2004; Fischer and Schär 2010; Barriopedro et al 2011; Christidis et al 2011, 2012; Hegerl et al 2011; Rahmstorf and Coumou 2011; Hoerling et al 2012; Schubert et al.2014; Sillmann et al 2014; Vautard et al 2007, 2013) Such climate events are often accompanied by prominent anomalies in atmospheric circulation and precipitation, as well as in the conditions of the nearby land and ocean surfaces. The main aims of this work are to determine the relative roles of changes in: (i) SST/SIE, (ii) GHGs, and (iii) AAer forcings in shaping the changes in the summer mean SAT and temperature extremes since the mid-1990s over Western Europe This will be achieved by performing a set of experiments using the atmospheric component of the stateof-the-art HadGEM3 global climate model.
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