A new global dataset of derived indicators has been compiled to clarify whether frequency and/or severity of climatic extremes changed during the second half of the 20th century. This period provides the best spatial coverage of homogenous daily series, which can be used for calculating the proportion of global land area exhibiting a significant change in extreme or severe weather. The authors chose 10 indicators of extreme climatic events, defined from a larger selection, that could be applied to a large variety of climates. It was assumed that data producers were more inclined to release derived data in the form of annual indicator time series than releasing their original daily observations. The indicators are based on daily maximum and minimum temperature series, as well as daily totals of precipitation, and represent changes in all seasons of the year. Only time series which had 40 yr or more of almost complete records were used. A total of about 3000 indicator time series were extracted from national climate archives and collated into the unique dataset described here. Global maps showing significant changes from one multi-decadal period to another during the interval from 1946 to 1999 were produced. Coherent spatial patterns of statistically significant changes emerge, particularly an increase in warm summer nights, a decrease in the number of frost days and a decrease in intra-annual extreme temperature range. All but one of the temperaturebased indicators show a significant change. Indicators based on daily precipitation data show more mixed patterns of change but significant increases have been seen in the extreme amount derived from wet spells and number of heavy rainfall events. We can conclude that a significant proportion of the global land area was increasingly affected by a significant change in climatic extremes during the second half of the 20th century. These clear signs of change are very robust; however, large areas are still not represented, especially Africa and South America.

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Trends in extreme daily temperature and rainfall have been analysed from 1961 to 1998 for Southeast Asia and the South Pacific. This 38-year period was chosen to optimize data availability across the region. Using high-quality data from 91 stations in 15 countries, significant increases were detected in the annual number of hot days and warm nights, with significant decreases in the annual number of cool days and cold nights. These trends in extreme temperatures showed considerable consistency across the region. Extreme rainfall trends were generally less spatially coherent than were those for extreme temperature. The number of rain days (with at least 2 mm of rain) has decreased significantly throughout Southeast Asia and the western and central South Pacific, but increased in the north of French Polynesia, in Fiji, and at some stations in Australia. The proportion of annual rainfall from extreme events has increased at a majority of stations. The frequency of extreme rainfall events has declined at most stations (but not significantly), although significant increases were detected in French Polynesia. Trends in the average intensity of the wettest rainfall events each year were generally weak and not significant.

[1] We analyzed a new data set of 54 high-quality homogenized daily maximum temperature series from western Europe (Austria, Belgium, Croatia, Czech Republic, Denmark, Finland, France, Germany, Ireland, Netherlands, Portugal, Spain, Sweden, Switzerland, United Kingdom) to define more accurately the change in extreme warm Daily Summer Maximum Temperature (DSMT). Results from the daily temperature homogeneity analysis suggest that many instrumental measurements in the late 19th and early 20th centuries were warm-biased. Correcting for these biases, over the period 1880 to 2005 the length of summer heat waves over western Europe has doubled and the frequency of hot days has almost tripled. The DSMT Probability Density Function (PDF) shows significant changes in the mean (+1.6 ± 0.4°C) and variance (+6 ± 2%). These conclusions help further the evidence that western Europe's climate has become more extreme than previously thought and that the hypothesized increase in variance of future summer temperature has indeed been a reality over the last 126 years.Citation: Della-Marta, P. M., M. R. Haylock, J. Luterbacher, and H. Wanner (2007), Doubled length of western European summer heat waves since 1880,