changes in drainage basins might provide extra abruptness to the freshwater cycle. Increasing precipitation and streamflow has been documented in North America. During the twentieth century, the zone 30°N to 85°N has experienced a 7-12 percent increase (Intergovernmental Panel on Climate Change, 2001a); this is the region critical to the Arctic and Atlantic Oceans. In 1998, the region north of 55°N was the wettest on record; and in the middle northern latitudes, precipitation has exceeded the 1961-1990 mean every year since 1995.

In global-warming scenarios, the GFDL coupled-climate model suffers a major decrease in THC amplitude owing to the resulting load of buoyant freshwater on top of the far northern Atlantic. The increase in precipitation minus evaporation (plus runoff from land) north of 45°N is nearly 50 percent under doubled carbon dioxide (Manabe and Stouffer, 1994). In other climate models, the freshening of the surface ocean is less strong, but high-latitude warming has the same effect. Adequate observations to establish crucial rates of the freshwater cycle are not being carried out.

One of the most important issues is the possibility of increase in extreme events related to land-surface hydrology. As summarized in Intergovernmental Panel on Climate Change (2001b), model projections of global warming find increased global precipitation, increased variability in precipitation, and summertime drying in many continental interiors, including “grain belt” regions. Such changes might produce more floods and more droughts. On the basis of the inference from the paleoclimatic record, it is possible that the projected changes will occur not through gradual evolution proportional to greenhouse-gas concentrations, but through abrupt and persistent regime shifts affecting subcontinental or larger regions. The inability to conduct long simulations with coupled models validated against paleoclimatic records, owing to resource limitations, leaves many uncertainties.

Shrinkage or disappearance of all or part of a large ice sheet in response to natural or human-caused forcing remains a poorly quantified possibility. Changes in the balance between snowfall and melting at the ice-sheet surface might be important in the future (e.g., Intergovernmental Panel on Climate Change, 2001b), but attention is focused on changes in ice flow because they might affect sea level more rapidly and probably will be more difficult to predict.