Box 31 The Mass Balance Of The Polar Ice Sheets And Global Sealevel Rise

One of the most important advances in glaciology in the past two decades has been the ability to monitor how the Earth's two polar ice sheets are changing using satellite measurements. As global temperatures have risen, so have rates of snow accumulation, ice melting and glacier flow in the Polar Regions. Although

Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd the balance between these opposing processes has varied considerably on a regional scale, satellite data show that the large Antarctic and Greenland Ice Sheets are each losing mass overall. Rignot and Thomas (2002) used satellite data to determine the mass balance of the two ice sheets and showed that: (i) the Greenland Ice Sheet is losing mass by near-coastal thinning; and (ii) the West Antarctic Ice Sheet, with thickening in the west and thinning in the north, is thinning overall. Shepherd and Wingham (2007) later estimated that the combined imbalance of the two ice sheets is about 125 gigatons (1.25 x 1011 tons) per year of ice, enough to raise sea level by 0.35 mm per year. This is only about 10% of the observed present rate of sea-level rise of 3.0 mm per year. This means that the Earth's smaller mountain glaciers and ice caps, which react more quickly to changes in mass balance, must be contributing the largest amounts of water to the oceans (Meier et al., 2007). However, much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and outlet glaciers. As this has accelerated over the past decade, we might expect to see a rise in the sea-level contribution from the two polar ice sheets over the course of the twenty-first century (see Sections 2.1 and 2.2). In both polar continents there are suspected triggers for the accelerated ice discharge, namely surface and ocean warming.

Sources: Meier, M.F., Dyurgerov, M.B., Rick, U.K., et al. (2007) Glaciers dominate eustatic sea-level rise in the 21st Century. Science, 317, 1064-7. Rignot, E. and Thomas, R.H. (2002) Mass balance of polar ice sheets. Science, 297, 1502-6. Shepherd, A. and Wingham, D. (2007) Recent sea-level contributions of the Antarctic and Greenland Ice Sheets. Science, 315,1529-32.

Figure 3.1 shows how the total amount of accumulation and the total amount of ablation each year defines the mass balance of a glacier. Ablation will tend to dominate in the warm summer months and accumulation in the winter months. If the amount of ablation equals the amount of accumulation over a year the net balance of the glacier will be zero and its size will remain constant (Figure 3.1). On the other hand, if there is more accumulation than ablation then the net balance will be positive and the glacier will grow and expand. If it is has a negative mass balance then the glacier will gradually disappear.

The study of glacier mass balance is, therefore, the study of inputs and outputs to the glacial ice system. Inputs to a glacier's mass balance include: snow, hail, frost, avalanched snow and rainfall. If these inputs survive summer ablation they will begin a process of transformation into glacier ice. The term firn or neve is used for snow that has survived a summer melt season and has begun this transformation. The transformation involves: (i) compaction; (ii) the expulsion of air; and (iii) the growth of an interlocking system of ice crystals. Dry fresh snow is about 97% air by volume and has a density of 100 kg m-3 while glacier ice has almost no air within it and a density of 900 kg m~3. The rate at which this transformation takes place is dependent on climate. If snow fall is high and significant melting occurs then the

3.1 Annual Mass Balance

3.1 Annual Mass Balance

Figure 3.1 Accumulation and ablation curves define the mass balance year for a glacier. The winter balance is positive and the summer is negative. If the winter and summer balances are exactly equal, then the net mass balance will be zero and the glacier will neither advance or retreat. [Modified from: Sugden and John (1976) Glaciers and Landscape, Edward Arnold, figure

Figure 3.1 Accumulation and ablation curves define the mass balance year for a glacier. The winter balance is positive and the summer is negative. If the winter and summer balances are exactly equal, then the net mass balance will be zero and the glacier will neither advance or retreat. [Modified from: Sugden and John (1976) Glaciers and Landscape, Edward Arnold, figure

process can be rapid, because older snow is quickly buried by fresh accumulation, which compacts the firn, while alternate melting and refreezing encourages the growth of new ice crystals. In contrast, where accumulation rates are low and little melting occurs the transformation can be extremely slow. For example, in the interior of the East Antarctic Ice Sheet, where there is very little accumulation and even less melt, the transformation may take up to 3500 years. By contrast on the Seward glacier in Alaska the transformation is achieved in as little as 3-5 years, due to the high accumulation and melt rates.

Outputs from the mass balance system are collectively termed ablation. This ablation can occur in three ways, by: ice melt, iceberg calving and sublimation. Glacial meltwater is derived from direct melting of ice on the surface of, or within, a glacier. On the surface this is a function of solar radiation received, whereas within and at the base of the glacier heat is supplied by: (i) friction due to ice flow; and (ii) by heat derived from the Earth's crust beneath the glacier (geothermal heat). Melting therefore can occur both on the surface of the glacier and within the glacier itself. Surface melting is primarily a result of warm air temperatures and is therefore highly seasonal, whereas melting within the glacier is not. It is important to emphasise that melting is not simply confined to the ice margin but may occur across the whole of the glacier surface.

Where glaciers terminate in water, either in the sea or in a lake, blocks of ice will break from the front (snout or terminus) of the glacier as icebergs (Figure 3.2). This process is known as iceberg calving. It can be a particularly rapid way of losing mass from a glacier. In very cold and dry environments mass may also be lost through sublimation, which is the direct evaporation of water from its solid state as ice.

In summary, it is the relative balance between inputs and outputs to a glacier that determine its mass balance and therefore whether it will expand, contract or remain unchanged. Accumulation and ablation do not occur equally over the whole surface of a glacier. Accumulation dominates in the upper regions, where temperature

Figure 3.2 Photograph of the San Rafael Glacier in Chile, a calving glacier. [Photograph:

Figure 3.2 Photograph of the San Rafael Glacier in Chile, a calving glacier. [Photograph:

and precipitation are suitable for snowfall, whereas ablation dominates at the terminus of a glacier, where the climate is relatively warm and where mass can be lost by iceberg calving. It is this spatial imbalance between accumulation and ablation which creates the surface slope that drives ice flow, aided by gravity.

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