I have been alerted to an important recent paper on modeling the climate system. It is
Bates, J., 2010: Climate stability and sensitivity in some simple conceptual models, Climate Dynamics (28 December 2010), pp. 1-19.
The abstract reads [highlight added]
A theoretical investigation of climate stability and sensitivity is carried out using three simple linearized models based on the top-of-the-atmosphere energy budget. The simplest is the zero-dimensional model (ZDM) commonly used as a conceptual basis for climate sensitivity and feedback studies. The others are two-zone models with tropics and extratropics of equal area. In the first of these (Model A), the dynamical heat transport (DHT) between the zones is implicit, in the second (Model B) it is explicitly parameterized.
It is found that the stability and sensitivity properties of the ZDM and Model A are very similar, both depending only on the global-mean radiative response coefficient and the global-mean forcing. The corresponding properties of Model B are more complex, depending asymmetrically on the separate tropical and extratropical values of these quantities, as well as on the DHT coefficient. Taking Model B as a benchmark, a criterion for assessing the sensitivities given by the ZDM and Model A is found. This criterion is not always satisfied for parameter ranges of physical interest.
The 2 × CO2 sensitivities of the simple models are studied and compared. Possible implications of the results for sensitivities derived from GCMs and palaeoclimate data are suggested. Sensitivities for more general scenarios that include negative aerosol forcings (inadvertent or geoengineered) in the tropics are also studied. Some unexpected outcomes are found in this case. These include the possibility of a negative global average temperature response to a positive global average forcing, and vice versa.”
The paper starts with the insightful text
“There is a long-established tradition in dynamic meteorology and oceanography of using simple models to study the basic nature of observed phenomena in the atmosphere and oceans. This tradition has proven fruitful, leading not only to increased scientific understanding but also providing essential practical guidance in the development of the complex models used for numerical prediction and data assimilation.
Simple models have also played a role, though a considerably less extensive one, in the study of climate. Here, the overwhelming emphasis has been on the development and use of GCMs. Nevertheless, conceptual discussions of climate sensitivity and feedback have been based very largely on the simplest of models, the global and annual-mean top-of-the-atmosphere (TOA) radiation budget model known as the zero-dimensional model (ZDM) (e.g., Charney et al. 1979; IPCC, 2001; NRC, 2005; Bony et al., 2006; Wallace and Hobbs, 2006; Forster and Gregory, 2006; IPCC, 2007; Roe, 2009). The ZDM has also been used as a basis for estimating climate sensitivity from palaeoclimate data (e.g., Hansen et al. 1984; Hoffert and Covey, 1992; Hansen et al., 1993). Basic issues of climate sensitivity and feedback have been addressed by some authors using simple TOA radiation budget models that are slightly more differentiated than the ZDM. Recent examples are the studies of Lindzen et. al. (2001; hereafter LCH01) and Lindzen and Choi (2009; hereafter LC09) in which a two-zone (tropical and extratropical) TOA model is used.”
The conclusions include the findings that
‘These results may also have implications for the reliability of 2 × CO2 sensitivity estimates based on the ZDM using paleoclimate data (e.g., Hansen et al., 1984; Hoffert and Covey, 1992; Hansen et al., 1993). Lindzen (1993) has critcized such estimates on the grounds that they do not include DHT [dynamical heat transport]. The present results indicate that the estimates may be deficient not only in this regard, but also in depending only on single global-mean values of the radiative forcing and the radiative response coefficient. For given global-mean values of both quantities, the manner in which they are distributed between the tropics and extratropics have been shown to have potentially large effects on the sensitivity.”
As often reported on my weblog (e.g. see), it is the regional variations in climate that matter much more than any global average surface temperature anomaly or trend. The excellent paper by Bates provides further support for this view.