The THC is maintained by density contrasts in the ocean, which themselves are created by atmospheric forcing (air-sea heat and water fluxes) and modified by the surface circulation. A crucial question is which density contrast one should consider—the one between the equator and the poles, or the one between North and South Atlantic, or perhaps even between North Atlantic and North Pacific. The choice matters in assessing what order of magnitude of change in surface density it might take to change the THC drastically. “Pole-to-pole” density differences are about 1 order of magnitude smaller than “pole-to-equator” ones and hence much more easily influenced.

Surface-density contrasts can be influenced by a wide variety of processes, both internal to the ocean and coupled to the atmosphere. For example, the THC transports relatively warm and salty waters from the subtropical North Atlantic into the convection regions, with opposite effects on density. The import of warm water tends to reduce the density in the high latitudes, and the import of saline water increases density. An indirect effect is associated with evaporation (and later export of water vapor), which occurs preferentially over warm water. Analysis of atmospheric data suggests that the Atlantic drainage basin loses moisture to the Pacific (Warren, 1983; Zaucker and Broecker, 1992). The resulting accumulation of salinity in the Atlantic is compensated for by a net influx of freshwater from the Southern Ocean to the Atlantic. Changes in the water balance of the tropics (for example, changing El Niño patterns) might influence the THC if sustained long enough (Schmittner et al., 2000; Latif et al., 2000). Another important driver is sea ice (Aagard and Carmack, 1989), which is important for the freshwater budget of the North Atlantic convection regions. Thus, the central question in understanding and simulating the role of THC changes in abrupt climate change is: What is the combined effect of all these feedback mechanisms in a climate-change scenario?

An understanding of abrupt climate change thus requires a detailed quantitative knowledge about the various driving processes and their combined effect on the THC. In principle, a change in any of these processes can generate substantial climate change in areas influenced by the THC. However, those climate changes will not necessarily be limited to those areas. Effects could be widespread, and vary from region to region, because of changes in patterns of natural climate variability (such as NAO and ENSO) and their associated teleconnections. This is largely unexplored terrain that needs enhanced research.