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Natural Climate Variability on Decade-to-Century Time Scales
A mature (albeit incomplete) understanding of the atmospheric component of the climate system is now available, thanks to the models of varying complexity and detail that have been developed over the last few decades. Because of the dominant role the atmosphere plays in weather variability on time scales of hours to weeks, as well as the practical importance of weather forecasting, observations of atmospheric phenomena began before observations of other climate subsystems, and are still the most plentiful. On the other hand, on the decade-to-century time scale, the atmosphere—with its characteristic time of months to years, at most—does not play as dominant a role in the complicated collective behavior of the above-mentioned family of subsystems. (This topic is discussed further in the introduction to Chapter 4.)
The purpose of reviewing atmospheric modeling in the decade-to-century context is therefore twofold: (1) as a useful in-depth illustration of the broader climate modeling enterprise, since it has the longest history and largest variety of models, as well as the best data sets for validation, and (2) as the source of indispensable building blocks for a hierarchy of coupled climate models. The papers in this section discuss useful aspects of the validating data sets and of the methodology for their analysis, as well as an important subset of the great variety of atmospheric models. This introduction provides a quick review of the breadth of atmospheric models, as well as of validation methodology. Attention is drawn to those aspects that are covered in greater depth and detail by the papers that follow.
THE MODELING HIERARCHY
Decade-to-century-scale variability, natural and anthropogenic, is but one of the problems of climate dynamics. Various problems, in terms of temporal and spatial scales, have been addressed with a variety of models. These models span the entire spectrum of detail and complexity, from zero-dimensional models that lump the entire atmosphere into one single variable—global temperature—to very highly resolved, three-dimensional general-circulation models (GCMs). Intermediate models can have one or two spatial dimensions.
Each type of model has its strengths and weaknesses; none is perfectly reliable, nor can it address all climate problems on decade-to-century or any other time scales. To obtain, therefore, a reliable estimate of climate sensitivity to external forcing—natural or anthropogenic—two approaches must be employed: (1) the systematic use of a complete hierarchy of climate models, with simpler models being tailored to the in-depth study of specific climate processes or phenomena, while more complex models are geared mainly to simulating a wide range of observed details (Schneider and Dickinson, 1974); and (2) the validation of the models in this hierarchy against each other and against existing data sets.
The simplest zero-dimensional (0-D) model is applied in this section by Wigley and Raper (1995) and by Lindzen (1995) to decade-to-century-scale problems. It equates the rate of change of the climate system's heat content to the balance of incoming-minus-outgoing radiation. The heat