green trees, snow falls to the surface without completely coating the dark canopy, allowing absorption of much solar radiation.

Atmosphere

The atmosphere is involved in virtually every physical process of potential importance to abrupt climate change. The atmosphere provides a means of rapidly propagating the influence of any climate forcing from one part of the globe to another. Atmospheric temperature, humidity, cloudiness, and wind fields determine the energy fluxes into the top of the ocean, and the wind fields dictate both the wind-driven ocean circulation and the upwelling pattern (of particular interest in the tropics and around Antarctica). Atmospheric moisture transport helps govern the freshwater balance, which plays a crucial role in the THC, and precipitation patterns provide the hydrological driving of glacial dynamics. The atmospheric response to tropical seasurface temperature patterns closes the feedback loop that makes El Niño operate. Atmospheric dust transport can affect the planet’s radiation balance and on longer time scales might affect ocean carbon dioxide uptake via iron fertilization (Mahowald et al., 1999).

How do oceans affect the air-temperature pattern? The primary—although by no means the only—effect of oceans on air temperature derives simply from the heat capacity of the oceans and has little to do with horizontal ocean heat transport. Being composed of a fluid that can mix heat vertically, oceans are slow to cool in winter and slow to warm in summer. Land temperature, in contrast, responds rapidly to adjust to changes in the seasonal cycle of solar radiation. The primary winter pattern of surface air temperature thus consists of warm oceans and cold land (see Plate 5). To some extent, that contrast depends indirectly on ocean dynamics, which affect the upper ocean stratification and so the mixed-layer depth and the amount of seasonal heat storage (Plate 5).

Complete shutdown of the THC would remove about 8 W/m2 from the Northern Hemisphere extratropical heat budget (Pierrehumbert, 2000). To restore balance, the northern atmosphere-ocean system must cool down until the infrared radiation lost to space is correspondingly reduced. On the basis of a conventional sensitivity factor incorporating water-vapor feedback, the perturbation in heat budget implies an extratropical cooling of about 4°C, and growth of sea ice could amplify this cooling. This is roughly the cooling found in simulations (Seager et al., 2001), in which ocean heat transport was suppressed. Because the Northern Hemisphere THC is pri-



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