tential of modifying the THC. Another class of catastrophic event associated with land glaciers is the formation of large lakes of meltwater, held back only by fragile ice dams. The breaking of an ice dam can lead to the sudden delivery of massive amounts of freshwater to the ocean. As noted above, it is believed that the draining of an ice-dammed lake (Agassiz) was at least partly involved in the initiation of the Younger Dryas event and was most likely responsible for the event about 8,200 years ago (Barber et al., 1999; Broecker et al., 1988; Teller, 1990; Teller, in press).

Sea ice, which forms by freezing of ocean water, is an important amplifier of climate forcing. When sea ice forms, it increases the planetary albedo, enhancing cooling. Sea ice also insulates the atmosphere from the relatively warm ocean, allowing winter air temperatures to decline precipitously (as much as tens of degrees C, compared to conditions over open water), and reducing the supply of moisture to the atmosphere, which in turn reduces precipitation downwind. The rectifying and amplifying effects of sea ice are important in connection with changes in the location of North Atlantic deep-water formation. The site of North Atlantic deep-water formation is roughly where a substantial fraction of the heat transported northward in the Atlantic Ocean is deposited. Changes in the location affect the sea-ice margin and can have a net effect on the planetary radiation budget.

Formation of sea ice also leads to rejection of very dense brine. This is particularly important around the Antarctic margin. Brine formation there is the major contributor to formation of the world’s deepest ocean water.

Sea ice must be considered dynamically. Its movements are rather like those of a viscous fluid in coexistence with ocean water when viewed over a large enough area, but with brittle behavior in smaller regions. The creation of leads, or cracks, in sea ice affects albedo and air-sea exchange, and the movement of sea ice from one place to another has important effects on the global distribution of ice cover. Similar transport issues arise with regard to transport of icebergs discharged from land glaciers; here, the main interest concerns the distribution of freshwater ultimately delivered by the icebergs.

Snow cover can also serve as an amplifier of climate change and a source of persistence. Snow-covered land maintains cold conditions because of its high reflectivity and because its surface temperature cannot rise above freezing until the snow melts. There are interesting interactions between snow cover and vegetation. A modest snow cover on a flat surface, such as tundra, suffices to cause high albedo. However, in terrain covered with ever-



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