Geography Reference
In-Depth Information
5
Summary and Conclusions
The selected 27 indices are based on daily temperature and
precipitation data at some European climate stations (ranging
from 3 to 86 depending on period and variable), which have
among the most extensive climate records in the world. The
stations used here are required to have daily records start-
ing before 1901. Seasonal linear trends of the indices dur-
ing three periods (1801-2000, 1851-2000, and 1901-2000)
were estimated by simple regression. The signiicance of the
trends was determined by a t-test (see earlier) of the esti-
mated trend. For the most data-rich period 1901-2000, the
stations are grouped into three regions (northern, central and
southern Europe) and regional means are calculated as an
arithmetic mean of all the stations in each region. The long
term trends of the temperatures and precipitation at these sta-
tions are shown in igures and tables which provide valuable
information for past extreme climatic condition based on
reliable instrumental records over Europe.
In summary, the estimated trends for the temperature in-
dices indicate a shift in the frequency distribution of tem-
perature. Higher frequency and greater amplitude of warm
and hot extremes were detected for all the three periods. At
the same time, cold extremes have become rarer. A large
number of these trends are found to be statistically signifi-
cant at the 5 % level. On the other hand, the pattern of the
trends is much more heterogeneous and less significant
for the precipitation indices than for the temperature ones.
Nevertheless, a tendency towards increased precipitation
intensity, not necessarily combined with increased precipi-
tation totals, was established. There is strong evidence that
climate in Europe has changed during the three periods
analyzed, such that the occurrence and intensity of warm
temperature extremes have increased. Precipitation extremes
have also changed with a likely shift of the rainfall moving
towards higher precipitation rate.
Based on the summary statistics of the estimated trends,
the following conclusions can be highlighted:
• The majority of the trends estimated for temperature
indices over all the three periods is positive and a large
part of these positive trends are statistically significant. In
terms of regional difference, SEU stands out as a region
which experiences higher and more significant warming
trends, particularly in summer.
• The increased/decreased frequency and intensity of
high/low temperature extremes are associated with
increased mean temperatures. Extremely cold days and
nights have become fewer whereas extremely warm and
hot days and nights occurred more often.
• The majority of the trends for precipitation indices suggest
increased rainfall amount, increased extreme level and
frequency, although there are large regional differences.
There are also some differences in the trends of the
indices among the three time periods. Over the recent 100
years, NEU has most significant increases, especially in
autumn, while there is practically no significant change
in SEU. In terms of seasonal distribution, cold seasons
(SON and DJF) show more significant changes than those
of warm seasons (MAM and JJA).
• Generally, similar patterns of trends with regard to
season for all indices over different periods of time are
established. This is particularly true for the temperature
indices. The trends for the last 100 years are often higher
and more significant than the two longer time periods,
indicating higher speed of change over the most recent
100 years.
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