Timescales of Glacier Responses to Environmental Perturbations: What They Mean for the 21st Century

Much debate concerns the timescales of glacier responses to environmental change. Glaciers respond with multiple glacier dynamical mechanisms which may have distinct response times. In an idealized case of a cold-based, debris-free glacier, the response time, tau, given by J&oacutehanneson et al. (1989) is tau = H/-b, where H is a thickness parameter (e.g., thickness near the equilibrium line altitude), and b is the ablation rate near the terminus. Raper and Braithwaite (2009) made a modification to account for a crude measure of glacier hypsometry. A logical consequence is that for those glaciers indicating a lingering response to the Little Ice Age and post-Little Ice Age warming (sometimes thought to be a cause of ongoing wastage), the more we can expect 21st Century glacier responses to be an inexorable acceleration of wastage as they begin to lose memory of the Little Ice Age and increasingly respond to industrially modified climate of the 20th and 21st Centuries. Where basal melting and basal sliding occurs (in most temperate glaciers) the response time to an anomalous weather event can be just hours or days. For the shorter response timescales, consistent with basal sliding and deforming till, and kinematic wave propagation, we can expect glacier changes to be more tightly coupled to climate variations, which means that there will be many reversals of general temporal trends due to natural decadal climate oscillations. However, surges can lead to overextended and subsequently stagnating ice, and the recovery to equilibrium can be very slow along the lines of J&oacutehanneson et al. (1989). The behavior can be very complex: stagnant debris-covered glacier tongues may respond with different timescales within the same glacier due to heterogeneous debris loads. These considerations derived from mainly temperate glaciers point to a complex evolution of response times in polar glaciers as climate warms, as basal sliding initiates, and as supraglacial debris production increases. The behavior of polar ice will eventually depart from historical behavior and slow responses and take on increasingly complex behavior of surge-and-waste cycles, stagnating debris-covered lobes, supraglacial lake formation, and fast responses that are today more associated with Alaskan and Himalayan glaciers. Response times in Greenland and the Canadian Arctic may be lengthy until the transition to temperate behavior occurs, when responses will become extremely rapid.