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Debris Flows -- Summary
Debris flows are a type of mass movement that frequently cause major destruction in alpine areas. Because debris flows are generally the product of heavy precipitation events, and because climate models project a future increase in the frequency and/or intensity of extreme precipitation events in consequence of CO2-induced global warming, many have become alarmed that such disastrous earthen flows will become more commonplace. In the present review, the likelihood of this claim is evaluated from three data-driven studies reported in the peer-reviewed scientific literature.

In setting the stage for their study, Stoffel et al. (2005) write that, since 1987, there has been an apparent above-average occurrence of debris flow events in the Valais region of the Swiss Alps, which they say has prompted some researchers to suggest that the apparent increase was the result of global warming (Rebetez et al., 1997). In an attempt to place the recent debris flow events in a broader context, Stoffel et al. used dendrochronological methods to reconstruct the history of such events in this region over the past 400 years, which exercise allowed them to determine if the apparent recent increase in debris-flow events was indeed real and if it was caused by CO2-induced global warming.

In extending the history of recent debris-flow events (1922-2002) back to the year 1605, the authors found that "phases with accentuated activity and shorter recurrence intervals than today existed in the past, namely after 1827 and until the late nineteenth century." What is more, the nineteenth century period of high-frequency debris flow was shown to coincide with a period of higher flood activity in major Swiss rivers, while less frequent debris flow activity after 1922 corresponded with lower flooding frequencies. In addition, debris flows from extremely large mass movement events, similar to one that occurred in 1993, were found to have "repeatedly occurred" in the historical past, and to have been of such substantial magnitude that the "importance of the 1993 debris-flow surges has to be thoroughly revised."

With respect to the apparent above-average occurrence of debris flow events in the Valais region since 1987, Stoffel et al.'s work revealed it to be just that - apparent, revealing that debris flows occurred "ever more frequently in the nineteenth century than they do today." And with respect to a cause, they conclude that "correlations between global warming and modifications in the number or the size of debris-flow events, as hypothesized by, e.g., Haeberli and Beniston (1998), cannot, so far, be confirmed in the study area."

Also working in the Valais region of the Swiss Alps, Bollschweiler and Stoffel (2010) developed a history of debris-flow frequencies for eight different areas in the Zermatt Valley -- a dry inner-alpine valley with central coordinates of 46°10'N, 47°7'E -- based on data obtained from "tree-ring series of affected conifers and complemented, where available, with data from local archives," which work entailed the sampling of 2467 individual trees that had been impacted by debris-flow activity in order to obtain 4491 pertinent increment cores.

Results indicated that there were peaks in debris-flow activity "toward the end of the Little Ice Age and in the early twentieth century when warm-wet conditions prevailed during summers in the Swiss Alps," but they say they also observed "a considerable decrease in frequency over the past decades which results from a decrease in the frequency of triggering precipitation events." Most importantly, two Swiss scientists report that when longer-term changes were sought, they could not identify "any significant trends in the debris-flow series between 1850 and 2009."

In discussing their real-world debris-flow results, Bollschweiler and Stoffel say they "contradict the widely accepted assumption that climatic changes will univocally lead to an increase in event frequency." But they add that their findings "are in concert with data from Jomelli et al. (2007), indicating that the most recent past (2000-2009) represents the period with the lowest frequency of debris-flow events since AD 1900," in spite of the fact that this latter decade is frequently touted by climate alarmists as having been the warmest such period of the past millennium or more.

Lastly, Matthews et al. (2009) analyzed "the frequency and timing of debris flows since c. 8500 cal. BP which, in turn, are related to climatic variability, extreme climatic events and site conditions" at three alpine slope-foot mires located above the treeline among some of the highest mountains in southern Norway. In doing so, the seven researchers report that they could find "no obvious correlation between debris-flow frequency and a relative warm climate." In fact, they say that "debris-flow frequency was lowest post-8000 cal. BP during the Holocene Thermal Maximum," and that most of the "century- to millennial-scale phases of enhanced debris-flow activity appear to correlate with Neoglacial events," one of which was the "Little Ice Age." In addition, they write that their record "agrees quite closely with a compilation of other debris-flow records from widely distributed sites in east and west Norway." What is more -- citing the work of Berrisford and Matthews (1997), Stoffel and Beniston (2006), Pelfini and Santilli (2008) and Stoffel et al. (2008) -- they report that "there appears to be no consistent upward trend in debris-flow frequencies over recent decades," when one might have expected them to be growing in both number and magnitude if climate-alarmist claims were correct. As a result, the seven Norwegian and UK researchers conclude that there is little real-world evidence "for the association of higher debris-flow frequencies with an increasingly warm climate." In fact, they say that "the evidence suggests the opposite."

Such findings, coupled with those presented in the prior two papers, clearly demonstrate the importance of evaluating the uniqueness of Earth's contemporary climatic state, or the uniqueness of recent trends in various climate-related phenomena, over a much longer time period than just the past century or, even worse, merely a portion of it. Only when a multi-centennial or millennial view of the subject is at hand can the uniqueness of a climate-related phenomenon's recent behavior be adequately evaluated. And when such an evaluation is performed, it is found there is nothing unusual, unprecedented or unnatural about current trends in debris flows, leaving one absolutely no reason to believe in the climate-alarmist claim that they will increase in the future as a result of CO2-induced global warming.

Berrisford, M.S. and Matthews, J.A. 1997. Phases of enhanced rapid mass movement and climate variation during the Holocene: a synthesis. In: Matthews, J.A., Brunsden, D., Frenzel, B., Glaser, B. and Weiss, M.M. (Eds.) Rapid mass movement as a source of climatic evidence for the Holocene. Palaoklimaforschung 19: 409-440.

Bollschweiler, M. and Stoffel, M. 2010. Changes and trends in debris-flow frequency since AD 1850: Results from the Swiss Alps. The Holocene 20: 907-916.

Haeberli, W. and Beniston, M. 1998. Climate change and its impacts on glaciers and permafrost in the Alps. Ambio 27: 258-265.

Jomelli, V., Brunstein, D., Grancher, D. and Pech, P. 2007. Is the response of hill slope debris flows to recent climate change univocal? A case study in the Massif des Ecrins (French Alps). Climatic Change 85: 119-137.

Matthews, J.A., Dahl, S.O., Dresser, P.Q., Berrisford, M.S., Lie, O., Nesje, A. and Owen, G. 2009. Radiocarbon chronology of Holocene colluvial (debris-flow) events at Sletthamn, Jotunheimen, southern Norway: a window on the changing frequency of extreme climatic events and their landscape impact. The Holocene 19: 1107-1129.

Pelfini, M. and Santilli, M. 2008. Frequency of debris flows and their relation with precipitation: a case study in the Central Alps, Italy. Geomorphology 101: 721-730.

Rebetez, M., Lugon, R. and Baeriswyl, P.-A. 1997. Climatic change and debris flows in high mountain regions: the case study of the Ritigraben torrent (Swiss Alps). Climatic Change 36: 371-389.

Stoffel, M. and Beniston, M. 2006. On the incidence of debris flows from the early Little Ice Age to a future greenhouse climate: a case study from the Swiss Alps. Geophysical Research Letters 33: 10.1029/2006GL026805.

Stoffel, M., Conus, D., Grichting, M.A., Lievre, I. and Maitre, G. 2008. Unraveling the patterns of late Holocene debris-flow activity on a cone in the Swiss Alps: chronology, environment and implications for the future. Global and Planetary Change 60: 222-234.

Stoffel, M., Lièvre, I., Conus, D., Grichting, M.A., Raetzo, H., Gärtner, H.W. and Monbaron, M. 2005. 400 years of debris-flow activity and triggering weather conditions: Ritigraben, Valais, Switzerland. Arctic, Antarctic, and Alpine Research 37: 387-395.

Last updated 22 February 2012