Putnam said that, until recently, most geologists thought freeze-thaw cycles, also called frost wedging, caused the majority of rockfalls. The idea was that water seeped into cracks during winter rainstorms, then expanded when it froze—the same process that forms potholes. Then, in 2016, Yosemite’s park geologist published a paper in Nature Geoscience that found most rockslides in Yosemite happen on hot, sunny days, not in freezing temperatures.
Like sticky wood doors in the summer, rock expands ever so slightly when it’s heated. The more the temperature rises and falls between day and night, or summer and winter, the more rock will expand and then contract, causing cracks to propagate through it. As temperatures rise worldwide, those hot, sunny days will become more frequent, which could mean more rockslides.
Rising global temperatures are also melting the glaciers and permafrost that support rock formations. According to Allen Glazner, a professor of geology at the University of North Carolina at Chapel Hill, this kind of melting “has really no effect on what’s happening in Yosemite.” But there are still glaciers in the Sierra Nevada, and they’ve ben retreating for a century. In colder places like the Alps, melting glaciers and permafrost have been linked to an expected uptick in rockslides.
Climate change could also impact the wetting and drying cycles that may trigger rockslides. According to Glazner, the sheer weight of water can sometimes be too much for a rock formation, and cause sections to break off. As hurricanes and other storms become less frequent and more intense, the water weight from large storms could also trigger more rockfalls.
It’s hard to tell what the cumulative effect of a changing climate will be on the frequency of rockslides. On the one hand, fewer freezing nights could reduce the number of rockslides caused by freeze-thaw cycles; on the other hand, a warmer and wetter climate could increase the number of slides caused by heat-based rock expansion and water weight. “It could be that the effects cancel out,” said Glazner. “We’ll have to wait and see.” All three geologists were careful to point out that there’s no conclusive data yet to prove rockslides are getting more frequent in Yosemite.
Sidle was cautious about predicting a rockfall-driven apocalypse in a warming Yosemite, but didn’t rule out more rockfall activity. “I would say the impact of climate change on rockslides in Yosemite is currently rather small, but could increase slightly with time if freeze-thaw and wetting-drying cycles become more common and diurnal temperature changes become greater,” he said. “But this embodies several assumptions.”
As for preparing for rockslides to make them less dangerous, said Sidle, “in real time, prediction is not really feasible.” Efforts like a 3-D map of the rock types that make up El Capitan by Putnam and Glazner could help identify areas that are likely to erode, but according to Glazner, that study didn’t point out the areas that fell last week as particularly vulnerable. Putnam said that, because smaller rockfalls often trigger larger slides, the NPS frequently issues warnings to park visitors when they observe smaller falls. But it’s still very difficult to predict: “Everything could have stopped on the 27th,” he said, before the much larger rockslide on the 28th.
“Every time there’s a tiny rockfall in Yosemite, you can’t shut down every road,” Putnam said, “because then you wouldn’t have a national park.” Glazner similarly highlights the relationship between rockfalls and Yosemite’s landscape. “These kinds of catastrophic events are why those beautiful cliffs are there,” he said. “When you see big boulders on the valley floor, almost all of them fell off the cliff nearby. Nature always wins on this stuff.”