Projected precipitation changes in Italy
The changes in precipitation are shown in Figure 12 which visualizes the expected precipitation
changes in mm/day under the scenario A2 for a reference period (2060–2070) in the 21st century
with respect to a similar period in the 20th century (1980–1990). The differences between the
scenario and the control are expressed in the same display format as in the previous picture. The
differences are measured in mm/day of rain. There is a confused situation in summer when
precipitation decreases everywhere except in a large area over the south of Italy and Albania.
However, summer precipitation in the Mediterranean region is small and so we are looking at small
differences in small numbers that are easily affected by chance or random variation. It is, instead,
most interesting to inspect the winter precipitation in Figure 12, right. In this case we can see a
definite and well-formed pattern. The precipitation decreases over the entire Mediterranean belt
and we observe a corresponding increase in the precipitation in northern Europe. The magnitude is
about 0.5 mm/day that corresponds to an accumulated amount over the 90 days of the season of
45 mm. The observed precipitation for winter in northern Italy is around 200 mm for the 90 days
of winter. This means that we are facing a 25% possible decrease in precipitation by the late 21st
century. B2 is a more optimistic scenario regarding the increase in the amount of CO2 and it
accordingly generates a weaker greenhouse forcing. The changes follow the same general pattern
as in the A2 case, but in a weaker and more disorganized form. Still, it is possible to recognize the
distinctive pattern of warming and precipitation reduction evidences under A2. The expected
precipitation changes in mm/day under the scenario B2 for a reference period (2060–2070) in the
21st century with respect to a similar period in the 20th century (1980–1990) is shown in Figure
13.
The results obtained to date are certainly interesting, but the degree of reliability of the results of
the simulations has to be evaluated. A reduction of 25% in winter precipitation is a major reason
for concern. How can we believe this kind of result? It is, of course, a difficult and important
question and there are no sure answers. In general, we can get a feel for the reliability of the
model by checking the consistency of the simulation either internally or with other known results.
The choice of the global model to perform the simulation offers an illuminating opportunity
because we can investigate if the patterns that we have unveiled, and that we have inspected
from a purely regional viewpoint, are part of something larger and more consistent.
Figure 14 shows the expected global precipitation changes in mm/day under the scenario A2 for a
reference period (2060–2070) in the 21st century with respect to a similar period in the 20th
century (1980–1990). The differences are as discussed previously, but in this case we are showing
them globally. Focusing on the winter precipitation (bottom panel) we can see several interesting
patterns. The first observation is that the Mediterranean region is very small with respect to the
Earth; very large changes are visible elsewhere and changes in the Mediterranean region do not
particularly stand out. The second comment is that we can see that the changes we have
discussed in the previous sections are indeed part of a larger system that extends from the Atlantic
Ocean to Europe. It is composed of two parts: the northern branch is positive, indicating an
increase in precipitation and the southern part is negative, indicating a decrease. It is the southern
branch that ends up covering the entire Mediterranean region.