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Biosphere Atmosphere Change Index
Abstract:
Summer 2015 in Europe. The summer 2015 in Europe was highly unusual, as persistent heat and dryness
prevailed in large parts of the continent. In central
and eastern Europe, a combination of record-low
seasonal rainfall (Orth et al. 2016) and record-high
monthly July/August temperatures were observed
over an area stretching from France to western Russia
(Supplemental Fig. S11.1). The anomalous temperatures
were caused by a sequence of four intense heat
waves that struck the region from the end of June
to early September (e.g., Fig. 11.1a). It is precisely
the few-day heat that causes problems with human
health, especially when combined with high humidity
(McGregor et al. 2010). We analyze seasonal maxima
of 3-day mean temperature (Tair3d, max) and seasonal
maxima of 3-day daily maximum wet bulb temperature
(WBTX3d, max), a measure of human thermal discomfort
that combines temperature and humidity and
is a proxy for heat stress on the human body (Fischer
and Knutti 2013; Sherwood and Huber 2010).
The series of heat waves began with a strongly
meandering jet stream, that is summertime “omegablocking”
(Dole et al. 2011), and the advection of very
warm subtropical air into central and western Europe
(Supplemental Fig. S11.1). Later in the season, the
jet stream was displaced to the north, so that stable
high-pressure systems could prevail over central and
eastern Europe bringing heat there. The first heat
wave in early July was hence most pronounced in
western parts of the continent, while south-central
and east-central Europe experienced the highest
temperatures in the subsequent heat waves later in
the season (Fig. 11.1b).
Anomalies in the hottest 3-day mean temperature
reached up to +6°C relative to climatology (Figs.
11.1c,d), and temperature records were broken, including
nationwide records (Kitzingen, Germany:
40.3°C; https://weather.com/news/climate/news/europe-
heat-wave-poland-germany-czech-august-2015),
various station records stretching from France to
the Balkan countries and southern Sweden (www
.meteofrance.fr/actualites/26913226-episode
-de-tres-fortes-chaleurs-en-france), nighttime temperatures
(Vienna, Austria: 26.9°C), record 3-day
mean temperatures across central Europe (Fig. 11.1e),
and inland water temperatures (e.g., Lake Constance).
Europe experienced the hottest August ever recorded
(NOAA 2016), and the entire summer season ranked
third after the unusual summers of persistent heat in
2003 and 2010 with hotspots in France and western
Russia, respectively (Barriopedro et al. 2011; Stott et
al. 2004). This extraordinary sequence of events raises
the question to what extent human-induced climate
change played a role in short-term heat waves beyond
natural climate variability.
A potential anthropogenic contribution to the
summer 2015 heat events had already been investigated
in near–real time (www.climatecentral.org
/europe-2015-heatwave-climate-change), and in the
present paper we build upon and substantiate the
previous analysis. We investigate two diagnostics
(Tair3d, max and WBTX3d, max) at four locations in longterm
station-based observational records and in a
large ensemble of consistently bias-corrected regional
climate model simulations
