Global Warming and Rockfalls

Over at The Island of Doubt, James Hrynyshyn has a brief post up about this Guardian article. The article in the Guardian discusses a large rockfall that is expected to happen in the next few days - about two million cubic meters of Mt. Eiger in the Swiss Alps are expected to take a short, quick trip down. The Guardian reports that a scientist monitoring the situation calls it an indirect result of global warming. James correctly notes that the article provides no explanation of why this might be the case, so I thought I would.

Increased erosion rates probably aren't the first thing that pops to mind as a result of climate change, but it's actually a very reasonable expectation. A major cause of erosion in areas where there are cold winters is the freeze-thaw cycle. Water is pretty good at getting into cracks in rock, and other such confined places. Water also, as you are probably aware, expands when it freezes. When water freezes in cracks in the rock, it acts like a wedge, forcing the cracks wider. When the ice melts, the crack can hold more water, and the next time it freezes, the water will expand again and force the crack wider still. Given enough time, and enough freeze-thaw cycles, rock will break as a result. That's pretty basic and non-controversial science, and it was probably covered when you took science in high school.

Now for the global warming tie-in. As temperatures rise, there are going to be places that see more freeze-thaw cycles per year than they did in the past. In colder times, they might have gone through only a few cycles per year, because the temperatures remained below freezing for the vast majority of the winter - remember, it's the repeated action of freezing and thawing that causes the erosion. Once the water has frozen, the damage has been done for that cycle, and you need to thaw and re-freeze to work the cracks wider. If the climate is such that the temperature only climbs above freezing two or three times a year, then the rocks will only go through two or three freeze-thaw cycles per year. In places like that, a little bit of warming can have a huge effect.

In terms of the overall climate, an increase in warming that results in temperatures climbing over the freezing point ten or twelve times in a year, instead of the three or four that was common in the past, really isn't much of a change. In terms of the number of freeze-thaw cycles, it's an enormous increase. I expect that this is what's happening in the Swiss Alps, and that this is why lots of rock is soon going to be coming down that mountain.