In September, British climber Andrew Foster threw himself over his wife, Lucy, as a rockfall descended on them from the world-famous El Capitan at California's Yosemite National Park. Andrew was killed, and appears to have saved his wife's life, using his body to shield her. It was the first fatal rockfall at the park since 1999, and one of a series of rockfalls that occurred that week — seven in all.
Of course, rockfalls are part of the natural erosion process that affects all mountains. The Appalachians, for example, used to look more like the Rockies until they were worn down over millennia. And, of course, the underlying type of rock makes a difference in how and how often rockfalls occur. So it would seem that human influence — however significant it is over some aspects of our environment — wouldn't have much impact on these natural disasters.
Perhaps surprisingly, however, new science connects rockfalls to climate change.
Large rockfall off the East Buttress of El Cap in Yosemite. I heard and saw it from Glacier Point at 1:51-1:52pm pic.twitter.com/youAujh3pA— Jon Kameen (@JonKameen) September 28, 2017
Rocks naturally develop weak spots over time, which can eventually weaken enough for big chunks to break off. Scientists have long attributed rockfalls to specific weakening factors, namely “freeze-thaw action, wetting and drying, temperature changes, and (of course) human disturbances (even rock climbing),” Roy Sidle, a geography professor at Australia's University of the Sunshine Coast, tells the Atlantic.
And until 2016, many geologists thought it was the freeze-thaw action that had the biggest influence on the likelihood of a rockfall at Yosemite. Like potholes, rockfalls are mostly caused by this frost-wedging, as water trickles into weak spots during winter storms and then expands when it later freezes. Sounds right, doesn't it?
But a paper in Nature Geosciences, published by Yosemite's park geologist, found something else: According to data collected over three years, it was actually hot, sunny days that triggered rockfalls — not the freeze-at-night, thaw-in-the-day effect. "Our data indicate that the warmest times of the day and year are particularly conducive to triggering rockfalls, and that cyclic thermal forcing may enhance the efficacy of other, more typical rockfall triggers," the study's authors wrote.
That's because as rock heats up in sunlight, it expands slightly, then contracts again at night. The hotter the days, the more expansion and contraction. The more hot days, the more the first-tiny, then-larger cracks grow, until they eventually trigger a rockfall. Climate change has led to a spate of temperatures hotter than modern historical norms in Yosemite, and in many other places as well.
In other parts of the world, like the Alps, the melting of glaciers and permafrost due to rising temperatures has been connected to landslides.