Excerpts from IPCC:
Similarly, for the contiguous USA, Kunkel et al. (2003) and Groisman et al. (2004) confirmed
earlier results and found statistically significant increases in heavy (upper 5%) and very heavy
(upper 1%) precipitation of 14 and 20%, respectively. Much of this increase occurred during the last
three decades of the 20th century and is most apparent over the eastern parts of the country. In
addition, there is new evidence from Europe and the USA that the relative increase in precipitation
extremes is larger than the increase in mean precipitation, and this is manifested as an increasing
contribution of heavy events to total precipitation (Klein Tank and Können, 2003; Groisman et al., 2004).
(IPCC Ch. 3 pg. 302/ 68)
In South Africa, Siberia, central Mexico, Japan and the northeastern part of the USA, an increase
in heavy precipitation was also observed, while total precipitation and/or the frequency of days with an
appreciable amount of precipitation (wet days) was either unchanged or decreasing (Easterling et al., 2000;
Fauchereau et al., 2003; Sun and Groisman, 2004; Groisman et al., 2005).
(IPCC Ch. 3 pg. 302/ 68)
Observed changes in intense precipitation (with geographically varying thresholds between the
90th and 99.9th percentile of daily precipitation events) for more than one half of the global land area
indicate an increasing probability of intense precipitation events beyond that expected from changes in the
mean for many extratropical regions (Groisman et al., 2005; Figure 3.39, bottom panel). This finding
supports the disproportionate changes in the precipitation extremes described in the majority of regional
studies above, in particular for the mid-latitudes since about 1950.
(IPCC Ch. 3 pg. 302/ 68)
As well as confirming previous findings, the new analyses provide seasonal detail and insight into
longer-term variations for the mid-latitudes. While the increase in the USA is found primarily in the
warm season (Groisman et al., 2004), central and northern Europe exhibited changes primarily in winter
(DJF) and changes were insignificant in summer (JJA), but the studies did not include the extreme
European summers of 2002 (very wet) and 2003 (very dry) (Osborn and Hulme, 2002; Haylock and
Goodess, 2004; Schmidli and Frei, 2005). Although data are not as good, the frequencies of precipitation
extremes in the USA were at comparable levels from 1895 into the early 1900s and during the 1980s to
1990s (Kunkel et al., 2003).
(IPCC Ch. 3 pg. 303/ 69)
For the contiguous USA, Kunkel et al. (2003) and Groisman et al. (2004) analysed return periods
of 1 to 20 years, and interannual to inter-decadal variations during the 20th century exhibit a high
correlation between all return periods. In summary, from the available analyses there is evidence that the
changes at the extreme tail of the distribution (several-decade return periods) are consistent with changes
inferred for more robust statistics based on percentiles between the 75th and 95th levels, but practically no
regions have sufficient data to assess such trends reliably.
(IPCC Ch. 3 pg. 303/ 69)
Figure 3.39. (Top) Observed trends (% per decade) for 1951 to 2003 in the contribution to total
annual precipitation from very wet days (95th percentile). Trends were only calculated for grid boxes
where both the total and the 95th percentile had at least 40 years of data during this period and had data
until at least 1999. (Middle) Anomalies (%) of the global annual time series (with respect to 1961 to 1990)
defined as the percentage change of contributions of very wet days from the base period average (22.5%).
The smooth red curve shows decadal variations (see Appendix 3.A). From Alexander et al. (2006).
(Bottom) Regions where disproportionate changes in heavy and very heavy precipitation during the past
decades were documented as either an increase (+) or decrease (–) compared to the change in the annual
and/or seasonal precipitation (updated from Groisman et al., 2005). Thresholds used to define “heavy”
and “very heavy” precipitation vary by season and region. However, changes in heavy precipitation
frequencies are always greater than changes in precipitation totals and, in some regions, an increase in
heavy and/or very heavy precipitation occurred while no change or even a decrease in precipitation totals
was observed.
(IPCC Ch. 3 pg. 303/ 69)
In a study of precipitation extremes over California, Bell et al. (2004) find that changes in
precipitation exceeding the 95th percentile followed changes in mean precipitation, with decreases in
heavy precipitation in most areas. Leung et al. (2004) find that extremes in precipitation during the
cold season increase in the northern Rockies, the Cascades, the Sierra Nevada and British Columbia
by up to 10% for 2040 to 2060, although mean precipitation was mostly reduced, in accord with
earlier studies (Giorgi et al., 2001a). In a large river basin in the Pacific Northwest, increases in rainfall
over snowfall and rain-on-snow events increased extreme runoff by 11%, which would contribute to more
severe flooding. In their 25-km RCM simulations covering the entire USA, Diffenbaugh et al. (2005)
find widespread increases in extreme precipitation events under SRES A2, which they determine to
be significant.
(IPCC Ch. 11 pg. 891/ 45)
A long-standing result from global coupled models noted in the TAR is a projected increase in the
chance of summer drying in the mid-latitudes in a future warmer climate with associated increased risk of
drought. This is shown in Figure 10.12, and has been documented in the more recent generation of models
(Burke et al., 2006; Meehl et al., 2006b; Rowell and Jones, 2006). For example, Wang (2005) analyse 15
recent AOGCMs and show that in a future warmer climate, the models simulate summer dryness in most
parts of the northern subtropics and mid-latitudes, but with a large range in the amplitude of summer
dryness across models.
(IPCC Ch. 10 pg. 782/ 36)
Another aspect of these changes has been related to the mean changes in precipitation, with wet
extremes becoming more severe in many areas where mean precipitation increases, and dry extremes where
the mean precipitation decreases (Kharin and Zwiers, 2005; Meehl et al., 2005a; Räisänen, 2005a; Barnett
et al., 2006).
(IPCC Ch. 10 pg. 784/ 38)
Climate models continue to confirm the earlier results that in a future climate warmed by
increasing greenhouse gases, precipitation intensity (e.g., proportionately more precipitation per
precipitation event) is projected to increase over most regions (Wilby and Wigley, 2002; Kharin and
Zwiers, 2005; Meehl et al., 2005a; Barnett et al., 2006), and the increase in precipitation extremes is greater
than changes in mean precipitation (Kharin and Zwiers, 2005).
(IPCC Ch. 10 pg. 784/ 38)
The increase in mean and extreme precipitation in various regions has been attributed to
contributions from both dynamic and thermodynamic processes associated with global warming (Emori
and Brown, 2005). The greater increase in extreme precipitation compared to the mean is attributed to the
greater thermodynamic effect on the extremes due to increases in water vapour, mainly over subtropical
areas. The thermodynamic effect is important nearly everywhere, but changes in circulation also contribute
to the pattern of precipitation intensity changes at middle and high latitudes (Meehl et al., 2005a). Kharin
and Zwiers (2005) show that changes in both the location and scale of the extreme value distribution
produce increases in precipitation extremes substantially greater than increases in annual mean
precipitation. An increase in the scale parameter from the gamma distribution represents an increase in
precipitation intensity, and various regions such as the NH land areas in winter showed particularly high
values of increased scale parameter (Semenov and Bengtsson, 2002; Watterson and Dix, 2003). Time-slice
simulations with a higher-resolution model (~1°) show similar results using changes in the gamma
distribution, namely increased extremes in the hydrological cycle (Voss et al., 2002).
(IPCC Ch. 10 pg. 784/ 38)
Part of the reason for these results is shown in the geographic maps for these quantities, where
precipitation intensity increases almost everywhere, but particularly at middle and high latitudes where
mean precipitation also increases (Meehl et al., 2005a; compare Figure 10.18b to Figure 10.9). However, in
Figure 10.18d, there are regions of increased runs of dry days between precipitation events in the subtropics
and lower mid-latitudes, but decreased runs of dry days at higher mid-latitudes and high latitudes where
mean precipitation increases (compare Figure 10.9 with Figure 10.18d). Since there are areas of both
increases and decreases in consecutive dry days between precipitation events in the multi-model average
(Figure 10.9), the global mean trends are smaller and less consistent across models as shown in Figure
10.18.
(IPCC Ch. 10 pg. 784/ 38)