Earth's climate, although typically described in terms of time-averaged statistics, is always in flux, varying simultaneously on a number of different timescales. In this Summary, we discus the results of several peer-reviewed scientific journal articles that describe the decadal-scale variability of a number of different climatic parameters in various parts of Europe.
Surface air temperature was investigated by Hasanean (2001) using data obtained from meteorological stations located in eight Eastern Mediterranean cities: Malta, Athens, Tripoli, Alexandria, Amman, Beirut, Jerusalem and Latakia. The period of analysis varied from station to station according to available data, with Malta having the longest record (1853-1991) and Latakia the shortest (1952-1991); and, as would be expected, all stations exhibited significant decadal variability.
Surface air pressure was studied by Slonosky et al. (2000), who analyzed data from 51 stations located throughout Europe and the eastern North Atlantic over the past 200-plus years. This work revealed, in Slonosky et al.'s words, that atmospheric circulation over Europe was "considerably more variable, with more extreme values in the late 18th and early 19th centuries than in the 20th century," in contradiction of the climate alarmist claim that weather becomes more variable as the planet warms.
North Atlantic Oscillation behavior was explored by Rodrigo et al. (2001), who studied its variation over the period 1851-1997. They found that "the recent positive temperature anomalies over western Europe and recent dry winter conditions over southern Europe and the Mediterranean are strongly related to the persistent and exceptionally strong positive phase of the NAO index since the early 1980s." However, the first decades of the past century were even more "exceptionally positive" than were its last decades; and were a linear regression to be run on the 20th century data, Rodrigo et al.'s results clearly indicate that the 100-year trend would be decidedly downward, indicative of a decreasing trend in NAO strength with the increasing temperatures of the past century.
Spring precipitation in southwestern Turkey over the period 1339-1998 was reconstructed by Touchan et al. (2003) from a large collection of tree-ring width measurements; and these reconstructions, in their words, "show clear evidence of multi-year to decadal variations." The most extreme of these precipitation events preceded the Industrial Revolution. They report, for example, that "all of the wettest 5-year periods occurred prior to 1756." Likewise, the longest period of reconstructed spring drought was the four-year period 1476-79, while the single driest spring was 1746, all of which extremes of wetness and dryness occurred prior to the Modern Warm Period.
Storminess in northwest Europe was investigated by Bijl et al. (1999), who analyzed well-correlated long-term sea level data from several coastal stations for trends and variations related to storminess there over the past century. They report there was considerable natural variability on decadal time scales, but they say they could find "no sign of a significant increase in storminess ... over the complete time period of the data sets." In the southern portion of the North Sea, on the other hand, natural variability was more moderate and they found "a tendency towards a weakening of the storm activity over the past 100 years."
River runoff (1920-1990), salinity (1886-1996) and sea level (1891-1993) were studied by Winsor et al. (2001) in the Baltic Sea region. They found that freshwater supply to the Baltic Sea exhibited large variations on time scales as long as several decades. As a result, the salinity of the Baltic Sea also showed large-scale variations on a time scale of several decades; but there was no long-term trend in salinity over the century. On the other hand, water exchange through the Danish Straits (calculated from sea level and river runoff data) showed relatively small variations; yet it too exhibited no trend during the century.
Salinity, temperature and ice cover were the focus of the investigation of Voronina et al. (2001) into sea-surface conditions in the southeastern Barents Sea throughout the mid- to late-Holocene, which they reconstructed from dinoflagellate cyst assemblages found in two sediment cores. Their work revealed that during the initial warm interval, there were no signs of significant climatic variability. The following cool periods, however, were much less stable. Also, they say their findings are similar to those of palaeoclimatic reconstructions from northwestern Eurasia and suggest "that sea-surface variations in the Barents Sea reflect large-scale changes in atmospheric and oceanic interactions between the North Atlantic and the Arctic," implying similar warming-induced reductions in climatic variability in those regions too.
Thermohaline circulation took center stage in the study of Clark et al. (2002), who reviewed the status of our knowledge of abrupt climate change, which is believed to be driven by changes in the rate of deep water formation in the Nordic Seas and the Southern Ocean along the Antarctic continental shelf in the Weddell and Ross Seas. In conducting this exercise, they found that abrupt climate change has never been a characteristic of the Holocene or current interglacial. Instead, rapid warmings, which can occur on decadal timescales, are phenomena of the past, and, in their words, "were characteristic of the last glaciation."
δ13C variability in the foraminifera Globigerinoides rubber over the past 1400 years was determined by Castagnoli et al. (2002) via study of a sediment core extracted from the Gallipoli terrace of the Gulf of Taranto. Using singular spectrum analysis, they were able to identify three important cyclical components of the record, having periods of approximately 11.3, 100 and 200 years. Comparison of their results with historical aurorae and sunspot time series revealed they were "associable in phase" and "disclose a statistically significant imprint of the solar activity in a climate record."
Last of all, we report the results of Hulme et al. (1999), who employed global climate simulations and two environmental response models to analyze the effects of natural climate variability and potential human-induced climate change on river runoff and agricultural wheat yield potential in Europe over the next 50 years. Interestingly, they found that the impacts of natural climate variability on runoff and wheat yields were "as great as, or greater than, the estimated impacts of human-induced climate change," providing a revealing new perspective on the issue. They note, for example, that "conventional impact analyses implicitly assume that ... multi-decadal natural climate variability can be ignored." However, as they showed in their study, "these assumptions are not correct." Indeed, Hulme et al. convincingly demonstrate that "human-induced climate change may not have as great an impact on natural resources as might multi-decadal natural climate variability."
What do we learn from this diverse assemblage of observations? We learn that decadal-scale variability is evident in essentially all climate-related variables, that it is likely driven by solar variability, that it tends to decrease as temperatures rise, and that its impact on certain natural resources (water availability) and agricultural productivity is likely to be greater than that of human-induced climate change for many decades to come, if not forever.
References
Bijl, W., Flather, R., de Ronde, J.G. and Schmith, T. 1999. Changing storminess? An analysis of long-term sea level data sets. Climate Research 11: 161-172.
Castagnoli, G.C., Bonino, G., Taricco, C. and Bernasconi, S.M. 2002. Solar radiation variability in the last 1400 years recorded in the carbon isotope ratio of a Mediterranean sea core. Advances in Space Research 29: 1989-1994.
Clark, P.U., Pisias, N.G., Stocker, T.F. and Weaver, A.J. 2002. The role of the thermohaline circulation in abrupt climate change. Nature 415: 863-869.
Hasanean, H.M. 2001. Fluctuations of surface air temperature in the Eastern Mediterranean. Theoretical and Applied Climatology 68: 75-87.
Hulme, M., Barrow, E.M., Arnell, N.W., Harrison, P.A., Johns, T.C. and Downing, T.E. 1999. Relative impacts of human-induced climate change and natural climate variability. Nature 397: 688-691.
Rodrigo, F.S., Pozo-Vazquez, D., Esteban-Parra, M.J. and Castro-Diez, Y. 2001. A reconstruction of the winter North Atlantic Oscillation index back to A.D. 1501 using documentary data in southern Spain. Journal of Geophysical Research 106: 14,805-14,818.
Slonosky, V.C., Jones, P.D. and Davies, T.D. 2000. Variability of the surface atmospheric circulation over Europe, 1774-1995. International Journal of Climatology 20: 1875-1897.
Touchan, R., Garfin, G.M., Meko, D.M., Funkhouser, G., Erkan, N., Hughes, M.K. and Wallin, B.S. 2003. Preliminary reconstructions of spring precipitation in southwestern Turkey from tree-ring width. International Journal of Climatology 23: 157-171.
Voronina, E., Polyak, L., De Vernal, A. and Peyron, O. 2001. Holocene variations of sea-surface conditions in the southeastern Barents Sea, reconstructed from dinoflagellate cyst assemblages. Journal of Quaternary Science 16: 717-726.
Winsor, P., Rodhe, J. and Omstedt, A. 2001. Baltic Sea ocean climate: an analysis of 100 yr of hydrographic data with focus on the freshwater budget. Climate Research 18: 5-15.