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Altitude-Mass Balance Feedback
To determine whether a particular ice sheet (e.g., Greenland or Antarctica)
will gain or lose mass under global warming, we need to know where its mean
annual temperature is located on the previous graph. It turns out that the
Greenland mean annual temperature is in region b and Antarctica's is in
region a. From this we will conclude that, for a small amount of warming,
Antarctica will gain ice mass (causing sea level to drop) and Greenland
will lose mass (leading to sea-level rise).
Figure 8 reveals an interesting factor relating to the change in a continental ice sheet. The top figure is a schematic representation of how an ice mass responds to changes in temperature. In certain temperature ranges, depending on the characteristics specific to each ice mass, irreversible changes can occur. An ice mass experiencing warming will at some temperature not be able to sustain ice continuously over the annual cycle of warming and cooling. Once this happens, the ice mass cannot become re-established by a modest cooling. As indicated by the line at zero ice volume, the temperature must lower substantially (the amount depending on local conditions) for the ice mass to survive the annual cycle to become re-established. This is due to the altitude - mass balance feedback as is shown in the bottom sketch.This is due to the altitude-mass balance feedback. An explanation of this feedback is given in Figure 9. In this figure, Tcw is the global mean temperture at which permanent ice last exists on the mountain during a global warming, and Tcc is the global mean temperature at which permanent ice first exists on the mountain during a global cooling.
The bottom diagram of Figure 6 gives symbolic representations of ice masses on Antarctica and Greenland. The closed curves represent cross-sections of the continents from sea level to the peaks of the continents. These symbolic continents are placed on a mass-balance diagram that has regions of mass accumulation (positive numbers) and regions of mass loss (negative numbers). Note that accumulation increases with altitude to a maximum and then decreases at higher altitude (lower temperatures) because of the lower water-vapor content of cold air. If the glacier did not slide down the continent toward sea level, the glacier would not exist below the zero line on the sketch. But because of glacier movement, the region of accumulation feeds ice to the region of net mass loss.
Global warming causes the pattern of horizontal lines to move upward on this diagram. As the zero line moves upward toward the highest elevation on the continent, the accumulation zone becomes very small (see Greenland). Eventually the glacier disappears. To re-establish the glacier, the zero line must move well below the top of the continent (i.e., the temperature must cool well below the value that led to demise of the glacier). This is the altitude - mass balance feedback that gives the lower critical point on the upper sketch.
Note that Antarctica is not near a critical point and, from the lower graph, this continent will experience increased mass accumulation with initial warming, but more intense warming (far greater than is likely due to present projections) also will send it to a critical point.
Because the mass accumulation region of Antarctica extends to down to sea level, there is very little loss of mass by melting and runoff, as shown in Figure 10. The primary mass-loss mechanism is calving, as illustrated for Antarctica in Figure 11. The West Antarctic ice sheet extends out over the water well beyond the point of attachment to the continent, known as the pinning point. The glacier slowly moves out over the ice and at some point creates enough stress to break off. The next image gives another illustration of this process.
There is some speculation that, because it extends so far out from its pinning point, the whole West Antarctic ice sheet may collapse. If that happens it is estimated that global sea level would rise approximately 7 meters. Such a rise in sea level would inundate the southern third of Florida. Recent estimates suggest collapse is not imminent although it is not inconceivable.
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