Fig. 4: Rockfall rates at Täschgufer closely follow summer air temperatures, especially if warmer conditions persist over several summers.

Evolution of raw (black dots) and bias-corrected (blue and yellow) annual rockfall activity and nine-year spline-smoothed rockfall trends (black curve) at Täschgufer between 1920 and 2020. Both temperatures and rockfall rates are plotted as deviations (z-scores) from their 1920–2020 means. The thin (annual) and bold (spline smoothed) purple curves show z-score data for 13 July–26 August (Fig. 3); the red dotted curve shows smoothed June–August (JJA) air temperatures at the site. The latter correlation remains significant at decadal timescales with r = 0.69 (p < 0.01). Rockfall activity first increased in the late 1940s and early 1950s to peak massively after the mid-1980s. a, Rockfall correlates with annual JJA (r = 0.41 (p < 0.01)) and 13 July–26 August (r = 0.48 (p < 0.01)) air temperatures. b, The combination of DBH with the mean impact circles (defined by the mean diameter of rockfall fragments) yields an estimate of the percentage of the slope for which rockfall will be intercepted by trees in any given year or decade and therefore allows definition of periods for which activity is either over- (>100%) or underestimated (<100%).