On the Diagnosis of Radiative Feedback in the Presence of Unknown Radiative Forcing (Invited)
Abstract
Phase space analysis applied to output from both a simple forcing-feedback model, and output from the IPCC AR4 coupled climate models, demonstrates that much of the radiative variability observed in satellite radiation budget data is likely due to internal radiative forcing -- non-feedback, internally-generated variations in clouds, and possibly water vapor. It is shown that, within the classic forcing-feedback paradigm of climate variability, time-varying radiative forcing causes an uncorrelated temperature response which almost totally obscures the signature of radiative feedback. From a practical point of view, this has corrupted the diagnosis of radiative feedback from satellite data, leading to a variety of feedback estimates, usually biased toward zero. The problem does not seem to be widely appreciated, despite several published studies. Part of the persistent confusion stems from the common (but unrealistic) example of an instantaneously imposed radiative imbalance on the climate system. This example just happens to be the only case in which radiative feedback can be accurately diagnosed in response to an unknown amount of radiative forcing. This is because all of the subsequent radiative change is due only to feedback - not forcing. In contrast, for any real-world situations radiative feedback can only be clearly observed when it is in response to non-radiative forcing of temperature. Non-radiative forcing, mostly due to changes in the rate of convective heat flux from surface to the atmosphere, results in a highly correlated radiative response simply because the radiative variations are then totally composed of feedback - not forcing. Thus, the accurate diagnosis of radiative feedback from satellite measurements of natural climate variability depends upon either (1) analysis of time periods when internal radiative forcing is small relative to non-radiative forcing, or (2) the development of new methods of feedback diagnosis which do not currently exist. In simple conceptual terms, the basic issue is ultimately one of cause-versus-effect -- that is, determining the direction of causation when observing variations in radiative flux and temperature.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.A32A..03S
- Keywords:
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- 1616 GLOBAL CHANGE / Climate variability;
- 3310 ATMOSPHERIC PROCESSES / Clouds and cloud feedbacks