cannot rule out the possibility of net cooling over the North Atlantic if the THC decrease is very fast. Such rapid cooling would exert a large strain on natural and societal systems. The probability of this occurring is unknown but presumably much smaller than that of any of the more gradual scenarios included in the Intergovernmental Panel on Climate Change report (Plate 7). The probability is not, however, zero. Obtaining rational estimates of the probability of such a low-probability/high-impact event is crucial. It is worth remembering that models such as those used in the Intergovernmental Panel on Climate Change report consistently underestimate the size and extent of anomalies associated with past changes of the THC; if the underestimate results from lack of model sensitivity possibly linked to overly coarse resolution or other shortcomings rather than from improper specification of forcing, future climate anomalies could be surprisingly large.

Even if no net cooling results from a substantial, abrupt change in the Atlantic THC, the changes in water properties and regional circulation are expected to be large, with possibly large effects on ecosystems, fisheries, and sea level. There are no credible scenarios of these consequences, largely because the models showing abrupt change in the THC have too crude spatial resolution to be used in regional analyses. To develop these scenarios would require the combination of physical and biological models to investigate the effects on ecosystems, and the “nesting” of large-scale and coastal models to investigate sea-level change.

If we are to develop the ability to predict changes in the THC, we must observe its strength and structure as a fundamental requirement, akin to the necessity to observe the equatorial Pacific if one wants to forecast El Niño. So far, however, no observational network exists to observe the THC on a continuous basis. We also need to learn more about which upstream processes and regions are the source of the observed changes in the THC. Present-day observations show substantial decadal changes in the temperature of the warm Atlantic currents flowing toward the Arctic Ocean and in the outflows of freshwater and ice from the Arctic Ocean (e.g., Dickson et al., 2000). Both have been observed to affect the characteristics of the cold deep overflows that cross the Greenland-Scotland Ridge southward to drive the THC (e.g., Hansen et al., 2001b). Systematic, long-term observations of the fluxes influencing the THC are needed. Moreover, remote influences on the THC must be monitored, in particular the low-latitude atmospheric water-vapor transport from the Atlantic to the Pacific and the influence of Southern Ocean changes.

Meltwater from the Greenland ice sheet and glaciers and permafrost

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