Climate alarmists contend that global warming is responsible for creating more frequent and greater extremes of various types of weather. This summary investigates this claim as it pertains to precipitation variability in Europe.
Alexandrov et al. (2004) analyzed various characteristics of climate in Bulgaria during the 20th century by applying Meteo-France homogenization procedures to many raw data sets of the country, which procedures included, in their words, "control of monthly data of precipitation and average air temperature from selected weather stations in Bulgaria; detection of breaks and outliers within the collected and controlled time series; correction of the climate long-term series according to the defined breaks and outliers in order to obtain homogenized climate series." Results indicated that "there was no significant warming trend during the last century in Bulgaria in spite of the slight increase of average air temperature during the last two decades." They also note that "a decreasing trend in annual and especially summer precipitation from the end of the 1970s was found," and that "variations of annual precipitation in Bulgaria showed an overall decrease." Thus, as Bulgaria experienced a slight increase in average air temperature over the past two decades, the variability of annual precipitation there decreased, which observation runs counter to extreme weather claims.
In introducing their study, Ntegeka and Willems (2008) write that "long-term temporal analysis of trends and cycles is crucial in understanding the natural variability within the climate system." Against this backdrop, the pair of authors conducted just such an analysis with respect to extremes of rainfall in Belgium over the past hundred-plus years. More specifically, Ntegeka and Willems performed "an empirical statistical analysis of trends in rainfall extremes ... based on the long-term high-frequency homogeneous rainfall series at the climatological station of the Royal Meteorological Institute of Belgium at Uccle." Amazingly, this series was recorded by "the same measuring instrument at the same location since 1898 and processed with identical quality since that time," and it was done at a measuring frequency of ten minutes, which has yielded more than 107 years of continuous data.
In describing their findings the Belgian researchers report that "significant deviations in rainfall quantiles were found, which persisted for periods of 10 to 15 years," such that "in the winter and summer seasons, high extremes were clustered in the 1910s-1920s, the 1960s and recently in the 1990s." "This temporal clustering," in the words of the authors, "highlights the difficulty of attributing 'change' in climate series to anthropogenically induced global warming," and they say that "no strong conclusions can be drawn on the evidence of the climate change effect in the historical rainfall series."
In one final study examining the variability of precipitation in Europe, Bohm (2012) first notes that "South Central Europe is among the spatially densest regions in terms of early instrumental climate data," citing Auer et al. (2007); and he goes on to state that this fact allows for successfully testing for homogeneity and developing "a larger number of very long instrumental climate time series at monthly resolution than elsewhere," - which he thus proceeds to do - noting that the resulting long time series subset of the greater alpine region provides a great potential for analyzing high frequency variability from the preindustrial (and mostly-naturally-forced) period to the "anthropogenic climate" of the past three decades. More specifically, he reports that "the unique length of the series in the region allowed for analyzing not less than 8 (for precipitations 7) discrete 30-year 'normal periods' from 1771-1800 to 1981-2010."
In describing the results of his analysis, Bohm writes that "the overwhelming majority of seasonal and annual sub-regional variability trends is not significant." In the case of precipitation, for example, he writes that "there is a balance between small but insignificant decreases and increases of climate variability during the more than 200 years of the instrumental period," while in the case of temperature he reports that "most of the variability trends are insignificantly decreasing." And in a "special analysis" of the recent 1981-2010 period that may be considered the first "normal period" under dominant greenhouse-gas-forcing, he finds all extremes "remaining well within the range of the preceding ones under mainly natural forcing." And he notes that "in terms of insignificant deviations from the long-term mean, the recent three decades tend to be less rather than more variable [italics added]."
The study's main result - in the concluding words of the Austrian researcher at the Central Institute for Meteorology and Geodynamics in Vienna - is "the clear evidence that climate variability did rather decrease than increase over the more than two centuries of the instrumental period in the Greater Alpine Region [GAR], and that the recent 30 years of more or less pure greenhouse-gas-forced anthropogenic climate were rather less than more variable than the series of the preceding 30-year normal period." And given the results of the two other research studies listed above, it does not appear that late 20th century warming -- be it CO2-induced or natural -- has led to more frequent and/or greater extremes in European precipitation, at least for the areas under study. Clearly, such findings do not support model-based projections to the contrary.
Alexandrov, V., Schneider, M., Koleva, E. and Moisselin, J.-M. 2004. Climate variability and change in Bulgaria during the 20th century. Theoretical and Applied Climatology 79: 133-149.
Auer, I., Boehm, R., Jurkovic, A., Lipa, W., Orlik, A., Potzmann, R., Schoener, W., Ungersboeck, M., Matulla, C., Briffa, K., Jones, P., Efthymiadis, D., Brunetti, M., Nanni, T., Maugeri, M., Mercalli, L., Mestre, O., Moisselin, J.-M., Begert, M., Mueller-Westermeier, G., Kveton, V., Bochnicek, O., Stastny, P., Lapin, M., Szalai, S., Szentimrey, T., Cegnar, T., Dolinar, M., Gajic-Capka, M., Zaninovic, K. and Majstorovic, Z. 2007. HISTALP - Historical Instrumental climatological Surface Time series of the greater ALPine Region. International Journal of Climatology 27: 17-46.
Bohm, R. 2012. Changes of regional climate variability in central Europe during the past 250 years. The European Physical Journal Plus 127: 10.1140/epjp/i2012-12054-6.
Ntegeka, V. and Willems, P. 2008. Trends and multidecadal oscillations in rainfall extremes, based on a more than 100-year time series of 10 min rainfall intensities at Uccle, Belgium. Water Resources Research 44: 10.1029/2007WR006471.Last updated 20 February 2013