Shortcomings of IPCCs Radiative Forcing Concept

The forcing summary graph put forth by the IPCC (2007) includes only the direct forcing and Twomey aerosol indirect effect. It does not reflect the full magnitude or uncertainty of cloud-mediated aerosol forcing. However, some effects (e.g., the aerosol "cloud lifetime effect,") for which confidence in understanding is limited do not necessarily have the smallest magnitude, and therefore they cannot be omitted. The inclusion of such uncertain effects emphasizes the imperative need to narrow the gap in our understanding.

Forcing, as used by IPCC, is well-defined for the cloud albedo effect when simulated in a simplified manner, but it cannot capture the microphysical responses to aerosol-related changes in cloud particle number. For well-mixed greenhouse gases, a consistent scaling between forcing and climate change is a reasonable expectation, but this is less so for agents such as aerosols, which are not well-mixed. Aerosol-cloud interactions break this consistency further (Haywood et al., this volume).

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