J. H. Christensen, O. B. Christensen (2003): Climate modelling: Severe summertime flooding in Europe, in: Nature, S. 805-806, V. 421, siehe online
"Even as summers become drier, the incidence of severe precipitation could increase.
Using a high-resolution climate model, we are able to quantify the influence of greenhouse-gas-induced global warming upon heavy or extended precipitation episodes that inflict catastrophic flooding. We find that an increase in the amount of precipitation that exceeds the 95th percentile is very likely in many areas of Europe, despite a possible reduction in average summer precipitation over a substantial part of the continent. Our results indicate that episodes of severe flooding may become more frequent, despite a general trend towards drier summer conditions."
T. J. Osborn, M. Hulme (2002): Evidence for trends in heavy rainfall events over the UK, in Philosophical Transactions of the Royal Society series A, S. 1313 - 1325, Vol. 360, siehe online (Abstract)
"Daily precipitation in the UK has changed over the period 1961-2000, becoming on average more intense in winter and less intense in summer. Recent increases in total winter precipitation are shown to be mainly due to an increase in the amount of precipitation on wet days, with a smaller contribution in the western UK from a trend towards more wet days. If the wet-day amounts are modelled using a gamma distribution, then positive trends in its scale parameter are found across almost all of the UK, consistent with an increased frequency of heavy winter precipitation. Non-parametric analyses confirm an increase in the contribution of heavy events to winter precipitation totals. Analysis of multi-day sequences of heavy rainfall indicate a corresponding increase in their frequency. Results for summer show almost opposite trends: decreased precipitation totals (driven more equally by fewer wet days and reduced wet-day amounts), decreases in gamma scale parameter (although accompanied by a trend towards a less positively skewed distribution) and decreases in the occurrence of heavy precipitation (whether defined parametrically or non-parametrically). A more sparse network of weather stations with data back to 1901 suggests that the recent winter changes are unusual, while the recent summer changes are not, though the poorer coverage reduces the confidence in these longer-period results."
T. N. Palmer, J. Räisänen (2002): Quantifying the risk of extreme seasonal precipitation events in a changing climate, in: Nature, S.512-514, V.415, siehe online
"Increasing concentrations of atmospheric carbon dioxide will almost certainly lead to changes in global mean climate. But because—by definition—extreme events are rare, it is significantly more difficult to quantify the risk of extremes. Ensemble-based probabilistic predictions, as used in short- and medium-term forecasts of weather and climate, are more useful than deterministic forecasts using a 'best guess' scenario to address this sort of problem. Here we present a probabilistic analysis of 19 global climate model simulations with a generic binary decision model. We estimate that the probability of total boreal winter precipitation exceeding two standard deviations above normal will increase by a factor of five over parts of the UK over the next 100 years. We find similar increases in probability for the Asian monsoon region in boreal summer, with implications for flooding in Bangladesh. Further practical applications of our techniques would be helped by the use of larger ensembles (for a more complete sampling of model uncertainty) and a wider range of scenarios at a resolution adequate to analyse average-size river basins."
Milly et al.[Bearbeiten]
P. C. D. Milly, R. T. Wetherald, K. A. Dunne, T. L. Delworth (2002): Increasing risk of great floods in a changing climate, in: Nature, S. 514 - 517, V. 415, doi:10.1038/415514a, siehe online
"Radiative effects of anthropogenic changes in atmospheric composition are expected to cause climate changes, in particular an intensification of the global water cycle with a consequent increase in flood risk. But the detection of anthropogenically forced changes in flooding is difficult because of the substantial natural variability; the dependence of streamflow trends on flow regime further complicates the issue. Here we investigate the changes in risk of great floods—that is, floods with discharges exceeding 100-year levels from basins larger than 200,000 km2—using both streamflow measurements and numerical simulations of the anthropogenic climate change associated with greenhouse gases and direct radiative effects of sulphate aerosols. We find that the frequency of great floods increased substantially during the twentieth century. The recent emergence of a statistically significant positive trend in risk of great floods is consistent with results from the climate model, and the model suggests that the trend will continue."
Frich et al.[Bearbeiten]
P. Frich, L. V. Alexander, P. Della-Marta, B. Gleason, M. Haylock, A. M. G. Klein Tank, T. Peterson (2002): Observed coherent changes in climatic extremes during the second half of the twentieth century, in: Climate Research, S.193-212, V.19 (PDF)
"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."
L. V. Alexander, P. D. Jones (2001): Updated precipitation series for the U.K. and discussion of recent extremes, in Atmospheric Science Letters, S. 142 - 150, V. 1, doi:10.1006/asle.2000.0016, siehe online (Abstract)
"We present an automated method for updating existing long-running precipitation series in near-real time. Our analyses confirm the trend towards significantly drier summers in the south-east of England and significantly wetter winters in the west of Scotland. In 2000 England and Wales saw the wettest April since records began in 1766 and record-breaking daily precipitation in several regions in October led to the wettest autumn on record."
C. Frei, C. Schär (2001): Detection Probability of Trends in Rare Events: Theory and Application to Heavy Precipitation in the Alpine Region, in: Journal of Climate, S.1568-1584, V.14, siehe online (Abstract)
"A statistical framework is presented for the assessment of climatological trends in the frequency of rare and extreme weather events. The methodology applies to long-term records of event counts and is based on the stochastic concept of binomial distributed counts. It embraces logistic regression for trend estimation and testing, and includes a quantification of the potential/limitation to discriminate a trend from the stochastic fluctuations in a record. This potential is expressed in terms of a detection probability, which is calculated from Monte Carlo–simulated surrogate records, and determined as a function of the record length, the magnitude of the trend and the average return period (i.e., the rarity) of events.
Calculations of the detection probability for daily events reveal a strong sensitivity upon the rarity of events:in a 100-yr record of seasonal counts, a frequency change by a factor of 1.5 can be detected with a probability of 0.6 for events with an average return period of 30 days; however, this value drops to 0.2 for events with a return period of 100 days. For moderately rare events the detection probability decreases rapidly with shorter record length, but it does not significantly increase with longer record length when very rare events are considered. The results demonstrate the difficulty to determine trends of very rare events, underpin the need for long period data for trend analyses, and point toward a careful interpretation of statistically nonsignificant trend results.
The statistical method is applied to examine seasonal trends of heavy daily precipitation at 113 rain gauge stations in the Alpine region of Switzerland (1901–94). For intense events (return period: 30 days) a statistically significant frequency increase was found in winter and autumn for a high number of stations. For strong precipitation events (return period larger than 100 days), trends are mostly statistically nonsignificant, which does not necessarily imply the absence of a trend."
Osborn et al.[Bearbeiten]
T. J. Osborn, M. Hulme, P. D. Jones, T. A. Basnett (2000): Observed trends in the daily intensity of United Kingdom precipitation, in: International Journal of Climatology, S. 347-364, Volume 20, siehe online (Abstract)
"The intensity distribution of daily precipitation amounts in the UK has changed over the period 1961-1995, becoming on average more intense in winter and less intense in summer. This result is based on an analysis of 110 UK station records. In winter, and in terms of their relative contributions to total winter precipitation, there has been a decline in light and medium events and an increase in the heaviest events. This change is fairly uniform across the whole country and is apparent even when longer records (with reduced spatial coverage/detail) are analysed back to 1931 or 1908. The reverse is found in summer: over 1961-1995 there has been a decline in the proportion of the seasonal total being provided by the heaviest events. In the longer term context, however, the summer changes appear to be a return to earlier levels after a period in the 1960s when heavy summer rainfall made a greater than normal contribution. More complex changes have occurred in the intensity distribution of spring and autumn precipitation, with opposite changes in different regions of the UK."
Frei et al.[Bearbeiten]
C. Frei, C. Schär, D. Lüthi, H. C. Davies (1998): Heavy Precipitation Processes in a Warmer Climate, in: Geophysical Research Letters, S. 1431-1434, V. 25, siehe online (PDF)
"Climate simulations have suggested that a greenhouse-gas induced global warming would also lead to a moistening of the atmosphere and an intensification of the mean hydrological cycle. Here we study possible attendant effects upon the frequency of heavy precipitation events. For this purpose simulations with a regional climate model are conducted, driven by observed and modified lateral boundary conditions and sea-surface temperature distributions. The modifications correspond to a uniform 2K temperature increase and an attendant 15% increase of the specific humidity (unchanged relative humidity). This strategy allows to isolate the effects of an increased atmospheric moisture content from changes in the atmospheric circulation. The numerical experiments, carried out over Europe and for the fall season, indicate a substantial shift towards more frequent events of strong precipitation. The magnitude of the response increases with the intensity of the event and reaches several 10s of percent for events exceeding 30 mm per day. These results appear to apply to all precipitation events dominated by sea-to-land moisture transport."
T. R. Karl, R. W. Knight (1998): Secular trends of precipitation amount, frequency, and intensity in the United States, in: Bulletin of the American Meteorological Society, S. 231-241, Volume 79, siehe online (PDF)
"Twentieth century trends of precipitation are examined by a variety of methods to more fully describe how precipitation has changed or varied. Since 1910, precipitation has increased by about 10% across the contiguous United States. The increase in precipitation is reflected primarily in the heavy and extreme daily precipitation events. For example, over half (53%) of the total increase of precipitation is due to positive trends in the upper 10 percentiles of the precipitation distribution. These trends are highly significant, both practically and statistically. The increase has arisen for two reasons. First, an increase in the frequency of days with precipitation ]6 days (100 yr)−1[ has occurred for all categories of precipitation amount. Second, for the extremely heavy precipitation events, an increase in the intensity of the events is also significantly contributing (about half) to the precipitation increase. As a result, there is a significant trend in much of the United States of the highest daily year–month precipitation amount, but with no systematic national trend of the median precipitation amount.
These data suggest that the precipitation regimes in the United States are changing disproportionately across the precipitation distribution. The proportion of total precipitation derived from extreme and heavy events is increasing relative to more moderate events. These changes have an impact on the area of the United States affected by a much above-normal (upper 10 percentile) proportion of precipitation derived from very heavy precipitation events, for example, daily precipitation events exceeding 50.8 mm (2 in.)."