Climate Change Effects on Debris Flows

In mountain areas with human infrastructure, debris flows are a curse. They are abundant and dreaded as they arrive, usually without warning, and damage or destroy much of what they encounter along their path. It has been widely shown that climate is changing and that the past few decades of global warming are almost entirely attributable to human greenhouse gas emissions. There are, unfortunately, very few signs that emissions will be reduced as much as necessary to avoid further warming. In fact, the current trajectory projects between 3.3 and 5.7 °C warming by the end of the century, with amplified warming rates at high latitudes and high altitudes. Recent geopolitical events have led to a weaponization of energy, which in turn has led to another surge in fossil fuel production. Surely, changes in debris-flow activity may pale in comparison to the myriad of other catastrophic changes that would be unleashed commensurate with such unprecedented warming in human history. Nonetheless, debris flows (including their volcanic expression as lahars), especially as part of complex hazard chains in high mountains, have the potential for vast destruction of human habitat and above-ground and buried infrastructure. In this chapter, we provide a review of recent advances in climate change effects on debris flows. We demonstrate that, for some watersheds, an increase in frequency may result in a decrease in magnitude and hence runout distance. This, in turn, could result in a net risk reduction. In other cases, however, higher rainfall intensities and magnitudes will unleash more vigorous debris-flow activity. We also highlight the secondary effects of warming through tree mortality by insect infestations and drought stress, as well as the consequences of wildfires on debris-flow occurrence, frequency, and magnitude. Similarly, we emphasize the importance of glacial retreat and permafrost degradation that destabilize steep, high mountainous terrain worldwide. Importantly, we stress that quantifying debris-flow changes in a warming world is highly complex and subject to much uncertainty.