The Greenland Ice Sheet is the second largest ice sheet on Earth and is considered to be especially vulnerable to global warming. There are two reasons for this: first it spans a range of latitudes from 60° to 83°N; and second it is surrounded by ocean waters, which unlike Antarctica do not possess ocean currents that isolate the ice sheet from mid-latitude heat transfer. Studies show that the ice sheet has experienced record amounts of surface melting in recent years (Figure 2.5). Many of the outlet glaciers that drain the Greenland Ice Sheet have also accelerated and thinned
in the past decade. The net loss in volume of the Greenland Ice Sheet has more than doubled in recent years, from 90 to 220 km3 per year. The main cause of this increase is the acceleration of several large outlet glaciers. Three outlet glaciers in particular, Jakobshavn Isbrae, Helheim Glacier and Kangerdlugssuaq Glacier, have seen an exceptional speed-up, surface lowering and recession. These changes are important because, between them, these three glaciers drain around 12% of the ice from the Greenland Ice Sheet. The glaciers could potentially draw-down large volumes of ice from the interior of the ice sheet and increased ice discharge to the oceans would contribute directly to sea-level rise.
Glaciologists are trying to understand the mechanisms behind these recent changes using both field studies and remotely sensed data. It is widely accepted that the observed thinning of the glaciers results from changes in the flow regime of the glaciers rather than changes in surface mass balance, but the reasons for the initiation of these dynamic changes remains to be fully explained. Two possible processes have been suggested.
1. The first mechanism is the Zwally effect, which relies on meltwater reaching the bed of the ice sheet and reducing friction through a higher basal water pressure. This follows from the observation that increased air temperatures in the region in the past few decades have led to longer or more widespread surface melting. The mechanism is simple: warmer air temperatures mean increased surface melting, so there is more water on the ice-sheet surface. As this surface meltwater drains through moulins, crevasses or other fractures to the glacier bed, it provides lubrication and reduces friction so that the glacier velocity due to sliding increases. This process, originally proposed by Jay Zwally, was observed to be the cause of a brief seasonal acceleration of up to 20% on the Jakobshavns Glacier in 1998 and 1999. The acceleration lasted two to three months. Zwally hypothesised that the coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming. More recent studies also show that rapid supraglacial lake drainage events are related to short-term velocity fluctuations. However, it is unclear if these events are on a sufficient scale to have a lasting or significant effect on the annual flow of large outlet glaciers.
2. The second mechanism is the Jakobshavn effect, a term coined by Terry Hughes to describe a situation where a small imbalance of forces in a calving glacier causes a substantial non-linear response in its behaviour. In this case the imbalance of forces is at the calving front and propagates up-glacier. For example, thinning at the calving front might cause the glacier to become more buoyant, or to float at the snout, making it more responsive to tidal changes. Reduced friction due to greater buoyancy causes the rate of iceberg calving to increase, leading to glacier acceleration and draw-down. This process has been invoked to explain the exceptional thinning of the three outlet glaciers. Other possible triggers for perturbations in tidewater glacier behaviour are changing marine influences, such as warmer waters and the decreasing influence of sea-ice in the fjords, or decreases in water depth due to sedimentation.
Understanding the relative importance of these two mechanisms and their link to climate change is critical if we are to make century-scale predictions about future changes in the Greenland Ice Sheet and its contribution to sea-level rise. This is because the second mechanism (the 'Jakobshavn effect') will cease to operate once the ice sheet retreats beyond the influence of the ocean, whereas the first mechanism (the 'Zwally effect') will not. Calving-induced dynamic change is therefore self-limiting, whereas meltwater-drainage-induced dynamic change is not.
One major problem with the meltwater-drainage hypothesis is the assumption that an increase in subglacial meltwater availability automatically leads to increased ice flow, partly because our observations of this effect on the Greenland Ice Sheet cover only a few years. Also, observations on smaller valley glaciers suggest that increased ice flow is associated only with the initial (e.g., 'spring event'; see Section 4.7) phase of meltwater drainage to the bed, and that later phases, although characterised by far greater amounts of meltwater, are not associated with rapid flow. This is because the subglacial drainage system changes its configuration to accommodate the increased water flux. If the Zwally effect is the key then, because meltwater is a seasonal input, velocity will have a seasonal signal. If the Jakobshavn effect is the key, the velocity will propagate up-glacier and there will be no seasonal cycle. This is clearly testable given good real-time observations on glacier velocity. It should also be remembered that just as ice discharge can increase suddenly, so it can decrease suddenly: Helheim and Kangerdlugssuaq Glaciers doubled their discharge within a year in 2004, but 2 years later this discharge had quickly dropped back to close to its former rate. It is therefore important to note that so far no one has actually shown that increased surface meltwater is responsible for the acceleration of the outlet glaciers.
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