Summary of the Estimated Trends - European Trend Atlas of Extreme Temperature and Precipitation Records - page 167

4

Summary of the Estimated Trends

While the previous chapter can be used to ind trends for
all individual indices at all stations/regions, this chapter
attempts to provide an overview of the trends for the three
periods. Since there are many maps and igures in the previ-
ous chapter, summary statistics of the results are necessary to
obtain an overview. This chapter gives statistics of the trends
in terms of the trend estimates, fraction of positive and nega-
tive trends and their signiicance (see chapter 3 for deini-
tions) in the form of tables.

riod. The trends and the indication of significance are sum-
marized in Table 4.2 . Regarding the temperature indices
we find similar seasonal patterns for this period as for the
longer period described above. However, the fraction of
stations with negative trends is smaller, and the number of
significant positive trends is increased. This indicates that
the overall warming rate over this period is larger than that
in the previous period.
Nine precipitation stations are available and the summary
statistics for the stations are shown in Table 4.2 . In general
there is a clear tendency towards higher precipitation totals,
increased extreme levels, more frequent and more intensive
rainfall events. However, the fraction of significant trends
(both positive and negative) for the precipitation indices is
much lower than these for temperature indices.
There are more stations with increasing trend of total pre-
cipitation in cold seasons (SON and DJF) than those in warm
seasons (MAM and JJA), whereas the fraction of increasing
trend of heavy precipitation (PREC98P) is the largest in sum-
mer and lowest in spring. Almost half of the stations have
negative trends of PREC98P in spring, although the negative
trends are not as significant as the positive trends. Since the
heavy precipitation events in summer are often associated
with convection, this indicates that convective precipitation
has most likely increased at most of the stations. In terms of
number of days with precipitation rate larger than the 98th
percentiles over the reference period, most stations experi-
ence an increasing trend in autumn and winter, while spring
and summer both have a larger fraction of the stations with
negative trends. In terms of the measures for daily and 5day
extremely heavy precipitation (SDII98P, SDII, R5d, R1d),
the number of stations having positive trends are larger than
that with negative trends for all seasons. Only a small frac-
tion of stations show significant negative trends in summer
and winter. These results show that climate in Europe has
become wetter in general and there is an overall increase of
heavy precipitation both in terms of frequency and intensity
across the seasons. This tendency is confirmed by the overall
negative trends in the drought index CDD which shows the
max number of consecutive dry days. This feature is most
obvious in winter.

4.1 Temporal Trends of Indices at the Stations

4.1.1 1801 -2000

For indices based on Tmin/Tmax there are only three sta-
tions, while for indices based on Tmean we have seven
stations. There is no precipitation record for this period.
Table 4.1 summarizes the portions of the negative and posi-
tive trends. Statistically significant trends at two significance
levels are indicated separately.
In summary, an overall tendency towards higher tempera-
ture means and extreme levels (percentiles), an increase in
the frequency and duration of warm extremes, and a decrease
in the frequency of cold ones, which is consistent with glob-
al warming can be seen. This is especially true for winter,
but summer is always an exception. In fact, one third of the
stations show a weakly decreasing Tmin and Tmax and the
negative trends for Tmean appeared at more stations and be-
come significant. As a result, the heat wave index (HWDI) in
summer shows a slight negative trend (not significant) at one
third of the stations. However, all the stations experience an
increasing trend of warm spell duration (WSDI90). Thus, ex-
tremely high temperatures in summer for most stations still
show an upward trend.

4.1.2 1851 -2000

Nine stations with Tmin/Tmax observations and 13 sta-
tions with Tmean observations were analyzed for this pe-