Greenhouse gas radiative forcing: Effects of averaging and inhomogeneities in trace gas distribution
Authors: Freckleton R. S. , Highwood E. J. , Shine K. P. , Wild O. , Law K. S. , Sanderson M. G. 
Published: February 2007

Radiative forcing is a useful diagnostic quantity for indicating the size of climate change mechanisms, and for interpreting the results of General Circulation Model (GCM) experiments. It is also ideal for exploring the potential sensitivity of climate change simulations to assumptions regarding, for example, the horizontal and vertical distribution of changes in greenhouse gases, as a guide to whether these effects need to be incorporated into GCMs. This paper examines this issue for a range of greenhouse gases, and also examines the dependence of the forcing on horizontal averaging and the definition of tropopause position.

By comparison with calculations at a high horizontal resolution, it is shown that the use of a single global mean profile results in global mean radiative forcing errors of several percent for CO2 and chlorofluorocarbon CFC‐12 (CCI2F2); the error is reduced by an order of magnitude or more if three profiles are used, one each representing the tropics and the southern and northern extratropics. Three alternative definitions of tropopause position lead to a spread in forcing of up to 9%.

The effect of the spatial variation of a number of greenhouse gases, which are often regarded as well‐mixed, is then explored. For methane, three‐dimensional distributions are used for present and pre‐industrial cases generated from chemical‐transport models. The inhomogeneities are shown to have little impact on the forcing, because they are concentrated at the surface. The assumption that methane is well‐mixed in both the vertical and horizontal results in errors in the global mean forcing of only 2% compared to the full calculation; at individual locations the error never exceeds 4%.

For CFC‐11 (CCI3F) and CFC‐12, recent satellite data from the Cryogenic Limb Array Etalon Spectrometer are used to define the vertical and latitudinal variation in the stratosphere. For the global mean forcing, the assumption that CFC‐11 and CFC‐12 are well‐mixed in the horizontal and vertical results in errors of less than 4%, which is significantly smaller than current spectroscopic uncertainties. For CFC‐11 this error is a factor of two smaller than that implied in recent work, where the forcing from a well‐mixed distribution was compared with that of an idealised vertical distribution. However, the zonal variation of the forcing is more seriously affected by the use of the satellite distributions, and errors can reach 30% at high latitudes on assuming a well‐mixed distribution.

Finally, the distribution of hydrohalocarbons generated from a 2–D chemical model is used, assuming an idealised, predominantly northern hemisphere source distribution. Five gases are included, with lifetimes ranging from 2 to 26 years. The error in global mean radiative forcing arising from the assumption that these gases are well‐mixed is 10% for gases with lifetimes above 10 years, but can reach 30% for gases with lifetimes of about 2 years. It is found that almost all of the error is due to the vertical rather than horizontal distribution; the fall‐off above the tropopause, determined by the stratospheric lifetime, is an important determinant of the size of the error. The errors in the zonal distribution of the forcing are much larger, and can reach a factor of two for the shorter‐lived gases, particularly at high latitudes.