2002 Annual

Tuesday, 15 January 2002: 10:45 AM
Observed coherent changes in climatic extremes during 2nd half of the 20th century
Lisa Alexander, Met Office, Bracknell, Berks., United Kingdom; and P. Frich, P. Della-Marta, B. Gleason, M. Haylock, A. Klein Tank, and T. Peterson
A new global dataset of derived indicators has been compiled through international collaboration to clarify whether the 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. Ten indicators of extreme climatic events were chosen based on daily maximum and minimum temperature and precipitation totals. Only time series which had 40 years or more of almost complete records were used. Global maps showing significant changes from one multi-decadal period to another during the interval from 1946 to 1999 are shown for a selection of the indicators. 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 temperature-based 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 in the number of heavy rainfall events. We can conclude that a significantly increased proportion of the global land area was affected by a significant change in climatic extremes during the second half of the 20th century. However large areas are still not represented, especially Africa and South America.

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