Compositing techniques can be used to compare models and observations in a way that can be relevant for evaluating cloud-climate feedbacks (see Illingworth and Bony, this volume). These methodologies permit an assessment of how clouds change with dynamic and thermodynamic conditions in the present-day climate (e.g., Bony et al. 2004). In these studies, it is essential to eliminate, as much as possible, the dynamic contribution to observed relationships between cloud properties and temperature. Other studies have decomposed the global cloudiness into a small number of prominent cloud regimes and used this decomposition to understand and assess the response of clouds to long-term climate changes (e.g., Williams and Tselioudis 2007). Of course, a perturbed climate will be characterized by both perturbed thermodynamics and dynamics, both of which are a challenge to model.
Rapid changes in clouds as a response to instantaneous CO2 doubling have been identified by Gregory and Webb (2008) in a slab model ensemble. Changes in radiative cooling in the atmosphere and fast surface responses, which are associated with the CO2 change, operate on timescales of months; that is, before variations in longer-term sea surface temperature (SST) are established. Such changes constitute a substantial and, in some cases, dominant fraction of longer-term changes. They may be detectable in the satellite record of changes of cloud forcing. However, issues related to confounding changes in the atmosphere (e.g., attributable to aerosols) may complicate detection of a signal.
The fluctuation dissipation (FD) theorem states that the transient behavior (and sensitivity) of dynamic systems to perturbations can be determined from their natural variability. This theorem has been adapted from statistical physics, where it applies to a wide array of classical and quantum mechanical systems. It provides a potential mechanism for estimating or constraining climate sensitivity from short-term unforced variations in the climate system (Leith 1975; Schwartz 2007). To date, however, there is no rigorous determination of whether the FD theorem is applicable to the Earth's climate and, if so, which fields and f uctuations constrain climate sensitivity. The prospect that the theorem might apply to spectral features of the Earth's radiation field is under active investigation.
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