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.