Climate Change and the European Water Dimension - Agri ...

The temperature record over the last 1000 years (Figure I.1) has been constructed
using a combination of temperature measurements and proxies. It shows that the
temperature increase in the last 140 years is unprecedented within this time span.
I.B. Precipitation
Intuitively we expect that evaporation would increase with increasing temperatures,
and, in fact, all atmospheric general circulation models predict enhanced evaporation
of water. Also an increase in atmospheric moisture has been predicted by models
and confirmed by many observations.
Increased evaporation must obviously be balanced by increased precipitation. Also
the observation that atmospheric moisture is increasing leads to the expectation that
precipitation will increase. Increased evaporation means also that more latent heat
will be released in the atmosphere, and this may lead to more intense storms. The
overall result would be an intensification of the hydrological cycle with increased
precipitation at a global scale, but concentrated in more intense events. This
possible trend towards more extreme weather events will be discussed later.
Precipitation is measured at a large number at stations within Europe and globally.
Analysis of observations shows an increase of about 2% in global land precipitation
since the beginning of the 20 th century. This increase, however, is neither temporally
nor spatially uniform (see Figure I.2). The observed increase in the annual
precipitation for the zones between 30 0 N to 85 0 N is between 7 and 12% while the
increase in the Southern Hemisphere between 0 0 S and 55 0 S is about 2%. The
increase observed in the Northern Hemisphere is likely to be biased because the
data have not been corrected for the fact that an increasing fraction of the
precipitation is in liquid rather than frozen form, again due to increasing temperature.
However, this bias is expected to be small, and the increasing trend is significant,
although it is unsteady, e.g. interrupted by drought years in some areas. The trend
towards increasing precipitation has been confirmed by analyses of data from the
United States that show increasing streamflows.
A clearly increasing precipitation trend over the 20 th century has been reported for
Northern Europe, while the tendency for Southern Europe and the Mediterranean
Area in general has been towards less precipitation. Significant increases in
precipitation over the 20 th century have been found also for USA and Canada and
parts of the former USSR. China has experienced a slightly negative trend in
precipitation over the last 50 years, but with large geographical variations. The TAR
concludes that the present increase in precipitation over the middle and high northern
latitudes will continue at a rate between 0.5 and 1% per decade, except for over
Eastern Asia.
In contrast to the significant increases in precipitation in the mid- and high northern
latitudes, the trends observed in the tropics and sub-tropics show a more complex
picture; e.g. a decrease in precipitation in the northern sub-tropics. The effects of
aerosols on the hydrological cycle will be discussed later.
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