Climate Change and Slope Instability

Abstract

Pronounced step-wise atmospheric warming during the 20th century reduced ice cover in most of Earth’s mountains by 25–50 percent. Net changes in temperatures responsible for this remarkable deglacierization are less than 2°C, a small fraction of the warming that occurred at the end of the Pleistocene. Yet the effects of warming of the past century have been profound. Alpine permafrost, which expanded during the Little Ice Age, is now thinning. Large areas at high elevations may become completely free of permafrost by the end of the 21st century. Loss of alpine permafrost and glacier downwasting appear to be partly responsible for accelerated mass wasting and catastrophic slope failures in high mountains in recent decades. New lakes appeared during the Little Ice Age when glaciers advanced across streams and rivers and blocked drainage. Most of these lakes drained one or more times during the past century, producing catastrophic floods orders of magnitude larger than normal nival or rainfall floods. In some instances, lakes have appeared upvalley of former, drained ones as glaciers have continued to retreat under a warming climate. Lakes also formed behind Little Ice Age end moraines when glaciers retreated in the early 20th century. Moraine dams are vulnerable to failure because they are steep-sided and consist of loose sediment. Outburst floods from lakes dammed by glaciers and moraines erode, transport, and deposit huge amounts of sediment over distances of tens of kilometers. They broaden floodplains, destroy pre-flood channels, and create a new braided planform. Outburst floods from glacier- and moraine-dammed lakes have claimed thousands of lives in the Andes and Himalayas and continue to be a hazard in these and other mountain ranges.