Sea Ice and Climate
In conversation, frozen means "unmoving." But in fact, the frozen world is constantly on the move. Take sea ice, for instance. This floating ice layer thickens and becomes more extensive during the long, dark winter, then shrinks in the brief polar summer, as shown by the animation on this globe. The thinning doesn't raise sea level because the ice is already in the ocean.
But sea ice responds to more than the familiar seasonal rhythms. In the Arctic, the circulation patterns of ocean and atmosphere seesaw on a steady, natural, five- to seven-year beat. In one mode, sea ice tends to build up; in the other, it tends to dissipate. This pattern, the Arctic Oscillation, is a natural fluctuation in Northern Hemisphere climate.