How Well Do Multi-Decadal Global Climate Models Represent The Radiative Effect of Well-Mixed Greenhouse Gases? Part II

Gavin’s comment in the weblog Draft American Meterological Society Statement on Climate Change [comment #7] made the important statement that,

“As to how the forcings are calculated, it is essential that you do the analysis over the globe and over the seasonal cycleâ€?,

I agree.

The Collins et al 2006 paper that I weblogged on yesterday did not do this analysis. Thus where does the conclusion that the current radiative forcing is around “1.5W/m2″ [Gavin’s Comment #1] come from?

The most complete paper that does summarize how radiative forcings are defined (as communicated to me by Gavin Schmidt) is

Hansen, J., Mki. Sato, R. Ruedy, L. Nazarenko, A. Lacis, G.A. Schmidt, G. Russell, I. Aleinov, M. Bauer, S. Bauer, N. Bell, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, M. Kelley, N. Kiang, D. Koch, J. Lean, J. Lerner, K. Lo, S. Menon, R. Miller, P. Minnis, T. Novakov, V. Oinas, Ja. Perlwitz, Ju. Perlwitz, D. Rind, A. Romanou, D. Shindell, P. Stone, S. Sun, N. Tausnev, D. Thresher, B. Wielicki, T. Wong, M. Yao, and S. Zhang 2005. Efficacy of climate forcings. J. Geophys. Res. 110, D18104, doi:10.1029/2005JD005776.
This is the paper where they concluded that

“Attempts to slow global warming must focus primarily on restricting CO2 emissions.”

With respect to the climate forcing from CO2, or other forcings, they provide the following definition,

“The simplest forcing, and the only pure forcing, is the instantaneous forcing, Fi. Fi is the radiative flux change at the tropopause after the forcing agent is introduced with the climate held fixed.”

The Hansen et al paper, unfortunately, does not provide the vertical variation of this forcing (i.e. the radiative flux divergence), nor its spatial structure. These are also “pure forcingsâ€?. The need to report on the vertical and regional variation of the radiative forcings was one of the main recommendations in the 2005 NRC Report “Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties” (see page 5).

Moreover they state,

“The climate forcing by CO2 in the present GISS model III is at the high end of the range estimated by IPCC [Ramaswamy et al., 2001]. Specifically, doubled CO2 in our current model, from the 1880 value of 291 ppm to 582 ppm, yields Fi = 4.52 W/m2.”

What was the specific procedure used to obtain this value of Fi? Did they run the model’s radiation parameterization off-line for each grid point in the model over a long time period? They are also silent on what is their current estimate of Fi, since the climate presumably has equilibrated to some fraction of the CO2 added after 1880.

As admitted by Hansen et al, the other forcings that they consider in their paper include feedbacks.

The summation globally and over long averaging times of Fi and its vertical divergence in the models should be a priority and would be a valuable addition to the quantification of CO2 radiative forcing. Regional maps of this forcing would also be valuable to address the issue that was raised in

Matsui, T., and R.A. Pielke Sr., 2006: Measurement-based estimation of the spatial gradient of aerosol radiative forcing. Geophys. Res. Letts., 33, L11813, doi:10.1029/2006GL025974.

Click to access R-312.pdf

where we compared the climate forcings of CO2 and aerosols in terms of the horizontal gradients of diabatic heating that these forcings produce.

To better quantify radiative forcing, including its vertical and horizontal structure, I propose an experiment, following what we report on with respect to the analysis by Norm Woods in the second Edition of Cotton, W.R. and R.A. Pielke, 2007: Human impacts on weather and climate, Cambridge University Press, New York (to be available late January 2007) and building on the Hansen et al paper.

The radiation codes that are used in each AOGCM should be applied to vertical profiles extracted from the NCAR-NCEP and ECMWF global reanalyses (e.g. at 6 hour intervals for a year globally) in order to diagnose the instantaneous long and short radiative flux divergences associated with each well-mixed greenhouse gas. This includes their vertical and horizontal structure. Then rerun these diagnoses but increment the well-mixed greenhouse concentrations to selected different values (e.g. adding 100 ppm to the current concentration of CO2; adding 100 ppb to CH4, etc).

Once this experiment is completed, the same framework can be used with the non-well mixed climate forcings.

This framework would provide a clear signal of the relative and absolute radiative forcings of each well-mixed greenhouse gases (and when completed, the other climate forcings) without involving climate feedbacks in the assessment. These would be “pure forcings” using the terminology given in the Hansen et al paper. These “pure forcings” are what should be presented when comparing the relative contribution of each radiative forcing to climate system heat changes.

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