North Atlantic Oscillation/Arctic Oscillation (NAO/AO). Among many possible causes, Shindell et al. (1999) suggested that these trends are a response to greenhouse-gas-induced warming. Large impacts could result from continuation and strengthening of such trends, perhaps leading toward a locking of the system in one of the preferred patterns (Palmer, 1999; Corti et al., 1999). Feedbacks on the THC have received the most attention (see below); other complex questions, including linkages with ENSO and other modes, merit greater attention in the future.

Most hypotheses with respect to possible abrupt changes in natural modes are based on but a few model experiments. Some of the models still have known biases (e.g., with respect to ENSO frequency). Nevertheless, such simulations indicate that changes in natural modes are a possibility when climatic conditions change.

Oceanic circulation modes may also experience major changes with greenhouse warming. Alteration of the THC thus can involve changes in structure in addition to a simple decrease or increase in amplitude. The present North Atlantic circulation involves two important modes of THC: the deep overflow branch originating in the Greenland-Iceland-Norwegian-Barents Seas, and the rapidly responding middle-depth THC driven by convection in the Labrador Sea (Plate 4). Wood et al. (1999), using a climate model with no flux corrections, suggested that greenhouse warming of the Nordic Seas between Greenland and Norway will cause reduction in density of the deep overflows. As they move south of Greenland they then will fail to circulate westward as boundary currents into the Labrador Sea. This, in turn, promotes a collapse of Labrador Sea deep convection, which is the primary driver of intermediate-depth THC (Häkkinen, 1999). The paleoclimatic record based on benthic and planktonic foraminifera (Hillaire-Marcel et al., 2001) suggests that Labrador Sea convection might indeed have been shut down for long periods during the previous interglacial warm period. This ironic scenario shows how the two modes of THC can interact, and in this case alter the balance in favor of the overflow mode.

POSSIBLE FUTURE ABRUPT CHANGES IN THE HYDROLOGICAL CYCLE

One of the most striking predictions of climate models and theory is that global warming will put more moisture into the atmosphere in the tropics and generally accelerate freshwater transport to higher latitudes. Melting of land-fast ice, sea ice, and permafrost and biological or geological



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