3.8.2 Evidence for Changes in Variability or Extremes

3.8.2.1 Temperature

For temperature extremes in the 20th century, the TAR highlighted the lengthening of the growing or frost-free season in most mid- and high-latitude regions, a reduction in the frequency of extreme low monthly and seasonal average temperatures and smaller increases in the frequency of extreme high average temperatures. In addition, there was evidence to suggest a decrease in the intra-annual temperature variability with consistent reductions in frost days and increases in warm nighttime temperatures across much of the globe.

Evidence for changes in observed interannual variability (such as standard deviations of seasonal averages) is still sparse. Scherrer et al. (2005) investigated standardised distribution changes for seasonal mean temperature in central Europe and found that temperature variability showed a weak increase (decrease) in summer (winter) for 1961 to 2004, but these changes are not statistically significant at the 10% level. On the daily time scale, regional studies have been completed for southern South America (Vincent et al., 2005), Central America and northern South America (Aguilar et al., 2005), the Caribbean (Peterson et al., 2002), North America (Kunkel et al., 2004; Vincent and Mekis, 2006), the Arctic (Groisman et al., 2003), central and northern Africa (Easterling et al., 2003), southern and western Africa (New et al., 2006), the Middle East (Zhang et al., 2005), Western Europe and east Asia (Kiktev et al., 2003), Australasia and southeast Asia (Griffiths et al., 2005), China (Zhai and Pan, 2003) and central and southern Asia (Klein Tank et al., 2006). They all show patterns of changes in extremes consistent with a general warming, although the observed changes of the tails of the temperature distributions are often more complicated than a simple shift of the entire distribution would suggest (see Figure 3.38). In addition, uneven trends are observed for nighttime and daytime temperature extremes. In southern South America, significant increasing trends were found in the occurrence of warm nights and decreasing trends in the occurrence of cold nights, but no consistent changes in the indices based on daily maximum temperature. In Central America and northern South America, high extremes of both minimum and maximum temperature have increased. Warming of both the nighttime and daytime extremes was also found for the other regions where data have been analysed. For Australasia and Southeast Asia, the dominant distribution change at rural stations for both maximum and minimum temperature involved a change in the mean, affecting either one or both distribution tails, with no significant change in standard deviation (Griffiths et al., 2005). For urbanised stations, however, the dominant change also involved a change in the standard deviation. This result was particularly evident for minimum temperature.

Figure 3.38. Annual probability distribution functions for temperature indices for 202 global stations with at least 80% complete data between 1901 and 2003 for three time periods: 1901 to 1950 (black), 1951 to 1978 (blue) and 1979 to 2003 (red). The x-axis represents the percentage of time during the year when the indicators were below the 10th percentile for cold nights (left) or above the 90th percentile for warm nights (right). From Alexander et al. (2006).

Few other studies have considered mutual changes in both the high and low tail of the same daily (minimum, maximum or mean) temperature distribution. Klein Tank and Können (2003) analysed such changes over Europe using standard indices, and found that the annual number of warm extremes (above the 90th percentile for 1961 to 1990) of the daily minimum and maximum temperature distributions increased twice as fast during the last 25 years than expected from the corresponding decrease in the number of cold extremes (lowest 10%). Moberg and Jones (2005) found that both the high and the low tail (defined by the 90th and 10th percentile) of the daily minimum and maximum temperature distribution over Europe in winter increased over the 20th century as a whole, with the low tail of minimum temperature warming significantly in summer. For an even longer period, Yan et al. (2002) found decreasing warm extremes in Europe and China up to the late 19th century, decreasing cold extremes since then, and increasing warm extremes only since 1961, especially in summer (JJA). Brunet et al. (2006) analysed 22 Spanish records for the period 1894 to 2003 and found greater reductions in the number of cold days than increases in hot days. However, since 1973 warm days have been rising dramatically, particularly near the Mediterranean coast. Beniston and Stephenson (2004) showed that changes in extremes of daily temperature in Switzerland were due to changes in both the mean and the variance of the daily temperatures. Vincent and Mekis (2006) found progressively fewer extreme cold nights and cool days but more extreme warm nights and hot days for Canada from 1900 to 2003 and Robeson (2004) found intense warming of the lowest daily minimum temperatures over western and central North America. In Argentina, the strong positive changes in minimum temperature seen during 1959 to 1998 were associated with significant increases in the frequency of warm nights; there were also decreases in cold days (Rusticucci and Barrucand, 2004).

Alexander et al. (2006) and Caesar et al. (2006) have brought all these and other regional results together, gridding the common indices or data for the period since 1946. Over 74% of the global land area sampled showed a significant decrease in the annual occurrence of cold nights; a significant increase in the annual occurrence of warm nights took place over 73% of the area (Table 3.6, Figure 3.38 and FAQ 3.3). This implies a positive shift in the distribution of daily minimum temperature T_min throughout the globe. Changes in the occurrence of cold and warm days show warming as well, but generally less marked. This is consistent with T_min increasing more than maximum temperature T_max, leading to a reduction in DTR since 1951 (see Sections 3.2.2.1 and 3.2.2.7). The change in the four extremes indices (Table 3.6) also show that the distribution of T_min and T_max have not only shifted, but also changed in shape. The indices for the number of cold and warm events have changed almost equally, which for a near-Gaussian distributed quantity indicates that the cold tails of the distributions have warmed considerably more than the warm tails over the last 50 years. Considering the last 25 years only, such a change in shape is not seen (Table 3.6).