Extremely warm temperatures as a potential cause of recent high mountain rockfall

A gradual reduction in the stability of steep bedrock slopes is recognized as one potential impact of warming in high mountain regions. Recently, consideration has turned to the potential direct role of extremely warm temperatures in triggering rockfalls. We provide here a first systematic assessment of the timing of 53 recent rockfalls relative to defined seasonal extremes of daily maximum air temperature. Rockfall observations from the Swiss Alps, Mont Blanc Massif, and Southern Alps of New Zealand, are combined with climate analyses based on the nearest available long-term records. At four high elevation climate stations in Switzerland, there has been significant warming across all quantiles of daily maximum temperature during the spring and summer months, with corresponding increases in both the frequency and magnitude of extremely warm days, and generally less warming (even cooling) during winter and autumn. In the same region, an unusually high occurrence of extremely warm days occurring in the week leading up to rockfalls has been observed over recent decades, with 14 out of 24 rockfalls preceded by one or more extremely warm days. At the neighboring Mont Blanc Massif, based on only two years of observations, few rockfalls can be linked to extremely warm temperatures, although an early and extremely warm onset of seasonal thawing in spring 2007 may have contributed to the large number of rockfalls observed that same summer. In the Southern Alps of New Zealand, the available evidence provides no basis to suggest extremely warm temperatures have triggered unusual rockfall activity. (C) 2013 Elsevier B.V. All rights reserved.