3.8.2.2 Precipitation

The conceptual basis for changes in precipitation has been given by Allen and Ingram (2002) and Trenberth et al. (2003; see Section 3.3 and FAQ 3.2). Issues relate to changes in type, amount, frequency, intensity and duration of precipitation. Observed increases in atmospheric water vapour (see Section 3.4.2) imply increases in intensity, but this will lead to reduced frequency or duration if the total evaporation rate from the Earth’s surface (land and ocean) is unchanged. The TAR states that it is likely that there has been a statistically significant 2 to 4% increase in the frequency of heavy and extreme precipitation events when averaged across the middle and high latitudes. Since then a more refined understanding of the observed changes in precipitation extremes has been achieved.

Many analyses indicate that the evolution of rainfall statistics through the second half of the 20th century is dominated by variations on the interannual to inter-decadal time scale and that trend estimates are spatially incoherent (Manton et al., 2001; Peterson et al., 2002; Griffiths et al., 2003; Herath and Ratnayake, 2004). In Europe, there is a clear majority of stations with increasing trends in the number of moderately and very wet days (defined as wet days (≥1 mm of rain) that exceed the 75th and 95th percentiles, respectively) during the second half of the 20th century (Klein Tank and Können, 2003; Haylock and Goodess, 2004). 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).

Despite a decrease in mean annual rainfall, an increase in the fraction from heavy events was inferred for large parts of the Mediterranean (Alpert et al., 2002; Brunetti et al., 2004; Maheras et al., 2004). Further, Kostopoulou and Jones (2005) noted contrasting trends of heavy rainfall events between an increase in the central Mediterranean (Italy) and a decrease over the Balkans. 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).

A number of recent regional studies have been completed for southern South America (Haylock et al., 2006), Central America and northern South America (Aguilar et al., 2005), southern and western Africa (New et al., 2006), the Middle East (Zhang et al., 2005) and central and southern Asia (Klein Tank et al., 2006). For southern South America, the pattern of trends for extremes between 1960 and 2000 was generally the same as that for total annual rainfall (Haylock et al., 2006). A majority of stations showed a change to wetter conditions, related to the generally lower value of the SOI since 1976/1977, with the exception of southern Peru and southern Chile, where a decrease was observed in many precipitation indices. In the latter region, the change in ENSO has led to a weakening of the continental trough resulting in a southward shift in storm tracks and an important effect on the observed rainfall trends. No significant increases in total precipitation amounts were found over Central America and northern South America (see also Figure 3.14), but rainfall intensities have increased related to changes in SST of tropical Atlantic waters. Over southern and western Africa, and the Middle East, there are no spatially coherent patterns of statistically significant trends in precipitation indices. Averaged over central and southern Asia, a slight indication of disproportionate changes in the precipitation extremes compared with the total amounts is seen. In the Indian sub-continent Sen Roy and Balling (2004) found that about two- thirds of all considered time series exhibit increasing trends in indices of precipitation extremes and that there are coherent regions with increases and decreases.

Alexander et al. (2006) also gridded the extreme indices for precipitation (as for temperature in Section 3.8.2.1). Changes in precipitation extremes are much less coherent than for temperature, but globally averaged over the land area with sufficient data, the percentage contribution to total annual precipitation from very wet days (upper 5%) is greater in recent decades than earlier decades (Figure 3.39, top panel, and Table 3.6, last line). 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. It is still difficult to draw a consistent picture of changes in the tropics and the subtropics, where many areas are not analysed and data are not readily available.

Table 3.6. Global trends in extremes of temperature and precipitation as measured by the 10th and 90th percentiles (for 1961–1990). Trends with 5 and 95% confidence intervals and levels of significance (bold: <1%) were estimated by REML (see Appendix 3.A), which allows for serial correlation in the residuals of the data about the linear trend. All trends are based on annual averages. Values are % per decade. Based on Alexander et al. (2006).

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). For Canada (excluding the high-latitude Arctic), Zhang et al. (2001a) and Vincent and Mekis (2006) found that the frequency of precipitation days significantly increased during the 20th century, but averaged for the country as a whole, there is no identifiable trend in precipitation extremes. Nevertheless, Groisman et al. (2005) found significant increases in the frequency of heavy and very heavy (between the 95th and 99.7th percentile of daily precipitation events) precipitation in British Columbia south of 55°N for 1910 to 2001, and in other areas (Figure 3.39, bottom panel).

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.

Since the TAR, several regional analyses have been undertaken for statistics with return periods much longer than in the previous studies. For the UK, Fowler and Kilsby (2003a,b), using extreme value statistics, estimated that the recurrence of 10-day precipitation totals with a 50-year return period (based on data for 1961 to 1990) had increased by a factor of two to five by the 1990s in northern England and Scotland. Their results for long return periods are qualitatively similar to changes obtained for traditional (moderate) statistics (Osborn et al., 2000; Osborn and Hulme, 2002), but there are differences in the relative magnitude of the change between seasons (Fowler and Kilsby, 2003b). 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. Similar results were obtained for several extratropical regions (Groisman et al., 2005), including the central USA, the northwestern coast of North America, southern Brazil, Fennoscandia, the East European Plain, South Africa, southeastern Australia and Siberia. 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.