Radiative forcing by well‐mixed greenhouse gases: Estimates from climate models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4)

Abstract

[1] The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO 2 , CH 4 , N 2 O, CFC-11, CFC-12, and the increased H 2 O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH 4 and N 2 O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.Citation: Collins, W. D., et al., (2006), Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the

Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

“…For the current offline validation, we use the Discrete-Ordinate 4 stream scattering model (Stamnes et al, 1998) coupled to the line by line Reference Forward Model (RFM, Dudhia, 1997). This sophisticated radiative transfer model has previously been used at the University of Reading as a reference calculation for GCM radiation schemes and found to be in excellent agreement with other line by line models (see Collins et al, 2006); this model is referred to here as LBL.…”

Radiative forcing at high concentrations of well‐mixed greenhouse gases

“…We compare (1) the level at which the lapse rate changes sign (temperature minimum tropopause); (2) the lowest level at which the temperature lapse rate between this and all higher levels within 2 km falls bellow 2 K km −1 (lapse rate tropopause) [WMO, 1986]; (3) the 200 hPa pressure level (200 hPa pseudotropopause) [Collins et al, 2006]; and (4) the lowest level at which the difference in net flux between this level and the next higher level is below an arbitrary threshold, taken as 3 W m −2 here (radiative tropopause) (Figure 1). …”

A very simple model for the water vapour feedback on climate change

“…Even a systematic error in simulated humidities of a factor of 2 would have negligible effect on wavelengths that are already almost saturated or still almost transparent: only at the fraction of wavelengths where the optical depth is of order one is there much scope for error in the contribution to t t . Also, the surface downward LW flux, which is often used for GCM verification (Ellingson et al, 1991;Morcrette, 2002;Collins et al, 2006) and so presumably also often tuned, can be approximated as (1 − t t ) times the surface Planck flux, because the exponential decrease in q with height in the troposphere means that photons coming down at the surface are unlikely to have come far (equivalently: that temperature drops off little in a q scale height)-in fact, even with other gases and clouds present the downward surface LW flux can be so approximated (e.g. Dines, 1921;Philipona et al, 2004).…”