the grid-point variables, Existing parameterizations of cloud amounts in general circulation models are physically very crude. When empirical adjustments of parameters are made to achieve verisimilitude, the model may appear to be validated against the present climate. But such tuning by itself does not guarantee that the response of clouds to a change in the CO2 concentration is also tuned. It must thus be emphasized that the modeling of clouds is one of the weakest links in the general circulation modeling efforts.

The above uncertainties, and others such as those connected with the modeling of ground hydrology and snow and ice formation, create uncertainties in the model results that will be described in Chapter 4.

3.3
OCEANS

Existing numerical models of the atmosphere, which treat the ocean as having no meridional heat transports of its own, may give somewhat improper accounts of the CO2 impact. It is currently estimated that at some latitudes the ocean transports as much as 50 percent of the poleward heat flux in the existing climatic system. A proper accounting for oceanic dynamics has several possible consequences as levels of CO2 continue to rise.

The role of the ocean as an active transporter of heat meridionally leads one to consider several possible feedback mechanisms. Atmospheric models suggest that the warming at high latitudes will be larger than at low latitudes. If this reduced atmospheric baroclinicity reduces the wind stress at the ocean surface (and there are not good estimates of the anticipated size of such a reduction), it is possible that oceanic meridional heat flux might be reduced, Because of the required overall radiative heat balance of the total system, the atmosphere would then be required to compensate for reduced oceanic heat transport by steepening the equator-to-pole temperature gradient, thus ameliorating somewhat the predicted polar warming, However, the total atmospheric warming would not likely be greatly affected, merely its distribution in latitude.

The only part of the ocean that has been included in the general circulation modeling of the CO2 effects is the mixed layer. The rationale for this simplification is that only the mixed layer needs to be modeled in order to deal with the annual cycle, while the heat capacity of the deeper ocean does not matter once thermal equilibrium has been reached.

On time scales of decades, however, the coupling between the mixed layer and the upper thermocline must be considered. The connections between upper and lower ocean are generally presumed to have response times of the order of 1000 years, the essential coupling being local vertical diffusion and formation of bottom water at high latitudes. This ignores the mechanism of Ekman convergence of the surface mixed layers in the large subtropical gyres,



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement