ABSTRACT
In the preceding chapters, projections have been presented of the buildup of GHG concentrations in response to anthropogenic emissions, of the gradual warming of the climate in response to the radiative heating caused by the increase in GHG concentrations, and of the long-term rise in sea level. These projections have been made with a variety of mostly simple models which, by their very design, preclude the possibility of abrupt changes in the rate of warming or in climatic parameters. The most sophisticated models - the coupled AOGCMs - allow for and indeed sometimes simulate abrupt (multi-decadal time scale) changes in oceanic circulation, but the details and timing of such changes are highly dependent on uncertain model parameters, such as the subgrid-scale diffusion coefficients or the precipitation and evaporation fluxes obtained through coupling with the atmosphere. Furthermore, the way in which most such models are coupled probably distorts their ability to correctly respond to the kinds of things that could trigger abrupt changes, as will be explained later. Thus, the models used to project future climatic change either cannot simulate possible abrupt changes in the climate system, or where the possibility of simulating such changes exists, the results cannot be relied upon. All that can be said with high confidence is that the physics of the climate system permit several reorganizations of the atmosphere-ocean subcomponents, and were such reorganizations to occur, the effects would be profound.
