TABLE 4-1 Metrics for Climate Forcing

Climate Forcing Metric



IPCC TAR adjusted tropopause or TOA global average radiative forcing with adjustment of stratospheric temperatures


  • Changes in global mean surface temperature are nearly linearly related to global mean TOA radiative forcing for a wide range of forcing agents

  • Simple and computationally efficient

  • Enables comparison of different forcing agents

  • Enables comparison of different models with one another, with benchmarks, and with estimates in the literature

  • Can be used in simple climate models for policy analysis

  • Already introduced into the policy dialogue


  • Conveys insufficient information about hydrological response

  • Does not fully characterize the climate impact of light-absorbing aerosols

  • Does not characterize regional response

  • Does not accommodate nonlinear response from large perturbations

  • Does not fully characterize the climate impact of nonradiative forcing, the indirect aerosol effect (other than the first), and the semidirect aerosol effect

Radiative forcing calculated with fixed sea surface temperature (SST; Hansen et al., 2002) or fixed surface temperatures (Shine et al., 2003) and adjusted atmospheric temperatures (in both troposphere and stratosphere)


  • Allows calculation of indirect and semidirect effects of aerosols

  • Incorporates fast atmospheric feedbacks in the simulation of climate forcing and response

  • Insensitive to the altitude at which forcing is calculated


  • Subject to limitations of the standard TOA forcing except for ability to calculate indirect and semidirect effects of aerosols

  • Not as computationally expedient as the standard radiative forcing calculation

  • Not as readily comparable across models as the standard radiative forcing calculation

Global mean radiative forcing at the surface


  • Provides a characterization of the surface energy budget

  • If reported with TOA forcing, may provide information on how forcing affects the lapse rate, with implications for precipitation and mixing


  • Does not allow characterization of regional structure

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