The tidewater glacier cycle is the typically centuries-long behavior of tidewater glaciers that consists of recurring periods of advance alternating with rapid retreat and punctuated by periods of stability. During portions of its cycle, a tidewater glacier is relatively insensitive to climate change.
Calving rate of tidewater glaciers
While climate is the main factor affecting the behavior of all glaciers, additional factors affect calving (iceberg-producing) tidewater glaciers. These glaciers terminate abruptly at the ocean interface, with large pieces of the glacier fracturing and separating, or calving, from the ice front as icebergs. Climate change causes a shift in the equilibrium line altitude (ELA) of a glacier. This is the imaginary line on a glacier, above which snow accumulates faster than it ablates, and below which, the reverse is the case. This altitude shift, in turn, prompts a retreat or advance of the terminus toward a new steady-state position. However, this change in terminus behavior for calving glaciers is also a function of resulting changes in fjord geometry, and calving rate at the glacier terminus as it changes position. Calving glaciers are different from land terminating glaciers in the variation in velocity along their length. Land terminating glacier velocities decline as the terminus is approached. Calving glaciers accelerate at the terminus. A declining velocity near the terminus slows the glacier response to climate. An accelerating velocity at the front enhances the speed of the glaciers response to climate or glacier dynamic changes. This is observed in Svalbard, Patagonia and Alaska. A calving glacier requires more accumulation area than a land terminating glacier to offset this higher loss from calving. The calving rate is largely controlled by the depth of the water and the glacier velocity at the calving front. The process of calving provides an imbalance in forces at the front of the glaciers, that raises velocity. The depth of the water at the glacier front is a simple measure that allows estimation of calving rate, but is the amount of flotation of the glacier at the front that is the specific physical characteristic that is important. Water depth at the glacier terminus is the key variable in predicting calving of a tidewater glacier. Debris flux and sediment recycling at the glacier grounding-line, particularly rapid in the temperate glaciers of Alaska, can alter this depth, acting as a second-order control on terminus fluctuations. This effect contributes to the insensitivity of a glacier to climate when its terminus is either retreating or advancing in deep water. Austin Post was one of the first to propose that water depth at the calving margin strongly affects the rate of iceberg calving. Glaciers that terminate on a morainal shoal are generally stable, but once a glacier retreats into water that deepens as the ice front recedes, calving rate increases rapidly and results in drastic retreat of the terminus. Using data collected from 13 Alaskan tidewater calving glaciers, Brown et al. (1982) derived the following relationship between calving speed and water depth: V C = C H w + D {\displaystyle V_{C}=CH_{w}+D} , where V C {\displaystyle V_{C}} is the mean calving speed (m⋅a−1), C {\displaystyle C} is a calving coefficient (27.1±2 a−1), H w {\displaystyle H_{w}} is the mean water depth at glacier front (m) and D {\displaystyle D} is a constant (0 m⋅a−1). Pelto and Warren (1991) found a similar calving relationship with tidewater glaciers observed over longer time periods, with slightly reduced calving rate to the mainly summer rates noted by Brown et al. (1982). Calving is an important form of ablation for glaciers that terminate in freshwater, also. Funk and Röthlisberger determined a relationship between calving speed and water depth based on analysis of six glaciers that calve into lakes. They found that the same basic calving relationship developed for tidewater calving glaciers was true for freshwater calving glaciers, only the calving coefficients led to calving rates 10% of that for tidewater glaciers.
Tidewater glacier phases Observations of Alaskan tidewater calving glaciers prompted Austin Post to describe the tidewater calving glacier advance/retreat cycle: (1) advancing, (2) stable-extended, (3) drastically retreating, or (4) stable-retracted. The following is a detailed review of the tidewater glacier cycle derived by Post, with numerous cited examples, the cycle is based on observations of temperate tidewater glaciers in Alaska, not outlet glaciers from large ice sheets or polar glaciers. The accumulation area ratio of a glacier, AAR, is the percentage of a glacier that is a snow-covered accumulation zone at the end of the summer melt season. This percentage for large Alaskan glaciers is between 60 and 70 for non-calving glaciers, 70–80 for moderately calving glaciers and up to 90 for very high calving rate glaciers. By using accumulation area ratio (AAR) data for Alaskan tidewater calving glaciers, Pelto (1987) and Viens (1995) produced models showing that climate acts as a first-order control on the advance/retreat cycle of calving glaciers during most of the advance retreat cycle, but there are climate insensitive periods as well. Pelto (1987) examined the terminus behavior of 90 Alaskan glaciers and found that the terminus behavior of all 90 were correctly predicted based on the AAR and calving rate.
Advancing
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