Climate change impacts and vulnerability in Europe 2016

Changes in the climate system
3.2.5 Heavy precipitation
Key messages
• The intensity of heavy precipitation events in summer and winter have increased in northern and north-eastern Europe
since the 1960s. Different indices show diverging trends for south-western and southern Europe.
• Heavy precipitation events are likely to become more frequent in most parts of Europe. The projected changes are
strongest in Scandinavia and eastern Europe in winter.
Relevance
Changes in the frequency and magnitude of heavy
precipitation events can have considerable impacts
on society, including agriculture, industry and
ecosystem services. An assessment of past trends and
future projections of heavy precipitation is therefore
essential for advising policy decisions on mitigation
and adaptation to climate change. The risks posed by
heavy precipitation hazards, such as flooding events
(including cloud burst and flash floods) are also
influenced by non-climatic factors, such as population
density, floodplain development and land-use changes.
Hence, estimates of future changes in such risks need
to consider changes in both climatic and non-climatic
factors.
To accurately assess trends in heavy precipitation at
local scales, high-resolution datasets are required.
These climatological datasets are compiled from the
observation networks from countries and additional
data from regional observations networks. As some
countries do not share all of their datasets, the spatial
and temporal coverage of the European dataset, and
consequently the accuracy of past trends, varies across
Europe (see Section 3.1).
Past trends
The majority of observation-based studies that
investigate trends in extreme rainfall intensity are
based on data recorded at the daily time scale. An
index for the maximum annual precipitation over five
consecutive days (Rx5d) shows significant increases up
to 5 mm per decade over northern and north‐western
Europe in winters and up to 4 mm in summers
(Map 3.9, left) (Donat, Alexander, Yang, Durre, Vose,
Dunn et al., 2013). The same index shows decreases
of more than 5 mm per decade in south-western
Europe in winter and between 2 and 3 mm in summer
(Map 3.9, right). The smaller trends in central and
south-eastern Europe are not statistically significant.
The increase in northern and north-eastern Europe
is a consequence of the observed shift polewards
of the North Atlantic storm track and weakening of
Mediterranean storms (Hov et al., 2013).
A wider literature review suggests that heavy
precipitation events have become more intense and
more frequent in Europe on average, but there are
important differences across regions, seasons, time
periods, heavy precipitation indices and underlying
datasets (Zolina et al., 2010; van den Besselaar et al.,
2013; Gallant et al., 2013; Donat, Alexander, Yang,
Durre, Vose, Dunn et al., 2013; Fischer et al., 2014;
Casanueva et al., 2014). Studies generally agree that
heavy precipitation has become more intense in
northern and north-eastern Europe since the 1950s,
even though not all changes are statistically significant.
Different studies and indices show diverging trends for
south-western and southern Europe.
Records of daily mean precipitation are often
insufficient to study trends and changes in heavy
precipitation. The damage associated with heavy
precipitation often originates from sub-daily localised
heavy precipitation events, which can lead to costly
flash floods. Owing to limited data availability, only
a limited number of studies have focused on large
regional scale assessments of sub-daily precipitation
(Hartmann et al., 2013). A recent review study
concludes that extreme sub-daily precipitation events
have generally increased in Europe, even in regions
with decreases in mean rainfall, but there is large
variability across regions, seasons and event durations
(Westra et al., 2014).
Projections
Global warming is projected to lead to a higher intensity
of precipitation and longer dry periods in Europe (IPCC,
2012; Hov et al., 2013). Projections show an increase
in heavy daily precipitation in most parts of Europe
in winter, by up to 35 % during the 21st century.
Heavy precipitation in winter is projected to increase
over most of Europe, with increases of up to 30 % in
north-eastern Europe (Map 3.10, left). In summer, an
increase is also projected in most parts of Europe, but
82 Climate change, impacts and vulnerability in Europe 2016 | An indicator-based report