What was done
Debris flows are a type of mass movement that frequently causes major destruction in alpine areas. Since 1987, there has been an apparent above-average occurrence of such events in the Valais region of the Swiss Alps, which has prompted some researchers to suggest that the apparent increase was the result of global warming (Rebetez et al., 1997). In the present study, Stoffel et al. attempt to place recent debris flow events in a broader context by reconstructing the history such events in this region over the past 400 years. Using dendrochronological methods, they sought to determine if the apparent recent increase in debris-flow events was indeed real, and if it was, to determine if it was caused by CO2-induced global warming.

What was learned
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."

What it means
The results of this study indicate that the apparent above-average number of debris flow events since 1987 was just that - apparent. In fact, Stoffel et al. report that debris flows occurred "ever more frequently in the nineteenth century than they do today." Thus, 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." These findings 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 we adequately evaluate the uniqueness of a climate-related phenomenon's recent behavior, let alone link that behavior to late 20th-century or early 21st-century global warming.

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

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.