On aspects of the concept of radiative forcing |
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Authors: | P M F Forster R S Freckleton K P Shine |
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Institution: | (1) Department of Meteorology, Reading University, Whiteknights, PO Box 239, Reading, RG6 6BB, UK, GB |
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Abstract: | The concept of radiative forcing has been extensively used as an indicator of the potential importance of climate change
mechanisms. It allows a first order estimate of the global-mean surface temperature change; and it is possible to compare
forcings from different mechanisms, on the assumption that similar global-mean forcings produce similar global-mean surface
temperature changes. This study illustrates two circumstances where simple models show that the conventional definition of
radiative forcing needs refining. These problems arise mainly with the calculation of forcing due to stratospheric ozone depletion.
The first part uses simple arguments to produce an alternative definition of radiative forcing, using a time-dependent stratospheric
adjustment method, which can give different forcings from those calculated using the standard definition. A seasonally varying
ozone depletion can produce a quite different seasonal evolution of forcing than fixed dynamical heating arguments would suggest.
This is especially true of an idealised and extreme “Antarctic ozone hole” type scenario where a sudden loss of ozone is followed
by a sudden recovery. However, for observed ozone changes the annually averaged forcing is usually within 5% of the forcing
calculated using the fixed dynamical heating approximation. Another problem with the accepted view of radiative forcing arises
from the definition of the tropopause considered in the second part of this study. For a correct radiative forcing estimate
the “tropopause” needs to separate the atmosphere into regions with a purely radiative response and those with a radiative-convective
response. From radiative-convective model results it is found that radiative equilibrium conditions persist for several kilometres
below the tropopause (the tropopause being defined as where the lapse rate reaches 2 K km-1). This region needs to be included in stratospheric adjustment calculations for an accurate calculation of forcing, as it
is only the region between the surface and the top of the convection that can be considered as a single, forced, system. Including
temperature changes in this region has a very large effect on stratospheric ozone forcing estimates, and can reduce the magnitude
of the forcing by more than a factor of two. Although these experiments are performed using simple climate models, the results
are of equal importance for the analysis of forcing-response relationships using general circulation models.
Received: 25 October 1996/Accepted: 14 April 1997 |
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