Another important cause of sea-level rise is ice melt. The term "ice melt" may well evoke an image of floating icebergs dissolving into the sea and raising the level of the ocean. In fact, however, icebergs are the one kind of ice that scientists do not worry about when they think of rising sea levels. That is because icebergs are already floating. Just as the water level in a glass rises when ice cubes are dropped into it, so the ocean level is already higher because it has icebergs floating in it.
Icebergs melting can have very important effects. In particular, sea ice has a much higher albedo than water. As discussed in the previous chapter, if too much sea ice melts, there will not be as much ice to reflect the sun's rays, and the overall temperature of the ocean will rise. But these effects do not have any direct effect on ocean level. In fact, if you melted all the sea ice on the earth, the ocean level would not rise at all, just as, if you melted the ice in a glass of water, the level of water in the glass would not change.
When scientists think about sea-level rise, therefore, they are contemplating glaciers, rather than icebergs. Glaciers are basically large masses of ice on land. They are created in areas where much more snow falls in the winter than can melt in the summer. Compressed under its own weight over many years, the snow eventually turns into ice.
Glaciers can be enormous: some are more than a hundred miles long. "Almost 10 percent of the world's landmass is currently covered with glaciers, mostly in places like Greenland and Antarctica."5 There are also glaciers at high altitudes on the tops of mountains. Altogether, glaciers contain about 75 percent of the world's freshwater supply.
If a glacier melted and turned into water, and that water flowed into the ocean, then the ocean level would in fact rise. But
The melting of Ice as global temperatures rise has effects not only on humans, but on other species as well. One animal that has been especially hard hit by the decrease of ice is the polar bear.
Polar bears typically hunt in water, often surfacing to cling to ice floes and then swim to land. Although bears can swim 100 miles without much trouble, "as the ice gets farther out from shore because of warming, it's a longer swim that costs more energy and makes them more vulnerable," according to Ian Stirling of the Canadian Wildlife Service as quoted by Bill Mouland in the Daily Mail on February 1, 2007.
In addition, polar bears' winter shelters in snowdrifts and on sea ice have been disappearing as the temperature warms. As their habitat diminishes, polar bear numbers have declined, dropping by a quarter in the past twenty years. In addition, individual polar bears have become smaller and thinner. Females especially have diminished in size. Partially as a result, polar bear litters have shrunk; triplets, which used to be fairly common, are now unheard of.
With current predictions of global temperature increases and continued northern ice melt, the outlook for polar bears seems grim. Some scientists hope that special conservation areas may help the bears survive. If not, extinction in the near future seems like a real possibility.
measuring the exact amount of glacial melt, and its contribution to sea-level rise, is not easy to do. As just one problem, "Neither the USA nor Canada has completed a national glacier inventory," making it impossible to collect a complete set of data, according to Roger Braithwaite, who worked on a study of glacial melt for the Alfred Wegener Institute for Polar and Marine Research.6 Despite this and other difficulties, however, scientists have estimated that glacial melting seems to be raising the sea level by about .4 inches (1mm) a year, according to University of Victoria climate scientist Andrew Weaver. Weaver noted that, "These numbers may not seem like much in any given year, but if you consider changes over longer timescales, they start to add up."7
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