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century? The indication is that for 2000-2100 the summers in this part of Europe
will gradually warm by 6.5 K on average. In addition, extremely hot summers will
continue, at least once or twice a decade, above this average warming. However,
because the average summer temperature is set to increase during the 21st century, an
exceptionally warm summer up to 2020 will become a normal summer by the 2040s.
From the 2040s exceptionally warm summers will all be far warmer than summer
2003, so that by the end of the century exceptional heatwaves can be expected to
be around 6.5 K warmer that those at the century's beginning. The researchers'
conclusion was that it seems likely that past human influence (greenhouse emissions)
has more than doubled the risk of European mean summer temperatures as hot as
those in 2003 for the early 21st century, and further that the likelihood of such warmth
is projected to increase 100-fold over the next four decades. They said, 'It is difficult
to avoid the conclusion that potentially dangerous anthropogenic interference in the
climatic system is underway' (Stott et al., 2004).
In 2011 two Swiss researchers (Fischer and Schar, 2010) attempted to model the
likely nature of European heatwaves for the 21st century. Their results suggested that
whereas extreme heatwaves are expected to increase across all Europe they will be
both more frequent and more intense in southern Europe and particularly Spain and
Portugal. For the Iberian peninsula and the Mediterranean region the frequency of
heatwave days is projected to increase from an average of about 2 days per summer
for the period 1961-90 to around 13 days for 2021-50 and 40 days for 2071-2100.
In a warmer world one might also expect fewer very cold winters. Although this
may be true there are likely to be exceptions, for a number of reasons. First, the
shifting of weather systems in a warmer world may result in the blocking of the jet
stream, such that cold Arctic air is drawn south (as happened in western Europe in the
winter of 2010-11). In winter, north-eastern North America and north-eastern Asia
are both colder than other regions at similar latitudes. In 2011 Yohai Kaspi and Tapio
Schneider, from the California Institute of Technology, showed that this anomalous
winter cold can result in part from westward radiation of large-scale atmospheric
waves - nearly stationary Rossby waves (large-scale meanders in high-altitude winds
caused by Coriolis force sheer) - generated by heating of the atmosphere over warm
ocean waters. They showed that warm ocean waters contribute to the contrast in mid-
latitude winter temperatures between eastern and western continental boundaries, not
only by warming western boundaries but also by cooling eastern boundaries. Such
reasons mean that cold weather extremes are likely to persist well into the 21st century
even as - in fact, because - the Earth warms.
Other work looking at the likelihood of winter extreme cold events as the 21st
century progresses still has them taking place by the end of the 21st century, albeit
with decreasing frequency, and by then on average 1 winter in 10 will be cooler than
the winter mean of the 20th century (Kodra et al., 2011).
In 2011 the IPCC published an advance copy of its Summary for Policymakers of
its 2012 Special Report on Managing the Risks of Extreme Events and Disasters to
Advance Climate Change Adaptation ( SREX ) 5 (IPCC, 2011, 2012). It concluded that
5
The SREX (2012) was due to be published after the submission of the manuscript for this topic's second
edition.
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