The IPCC has used a figure of estimated radiative forcing since preindustrial times to present its overview of the human impact on the climate system. This figure was reproduced in the 2005 National Research report as Figures ES-1 and Figure 2-1.
As stated in the 2005 NRC report starting on page 28,
“According to the IPCC definition, applied to the data in Figure 2-1, ‘The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent is the change in net irradiance at the tropopause after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with the surface and tropospheric temperatures and state held fixed at the unperturbed values.’ This definition of forcing is restricted to changes in the radiation balance of the Earth-troposphere system imposed by external factors, with no changes in stratospheric dynamics, without any surface and tropospheric feedbacks in operation, and with no dynamically induced changes in the amount and distribution of atmospheric water. A somewhat broader perspective is applied in this chapter to include, in particular, volcanic aerosols, the effects of land-use changes and aerosols on precipitation, and the radiative forcing due to changes in ocean color.”
The figure caption to the IPCC figure in the 2005 NRC Report states,
“FIGURE 2-1 Estimated radiative forcings since preindustrial times for the Earth and troposphere system (TOA radiative forcing with adjusted stratospheric temperatures). The height of the rectangular bar denotes a central or best estimate of the forcing, while each vertical line is an estimate of the uncertainty range associated with the forcing, guided by the spread in the published record and physical understanding, and with no statistical connotation. Each forcing agent is associated with a level of scientific understanding, which is based on an assessment of the nature of assumptions involved, the uncertainties prevailing about the processes that govern the forcing, and the resulting confidence in the numerical values of the estimate. On the vertical axis, the direction of expected surface temperature change due to each radiative forcing is indicated by the labels ‘warming’ and ‘cooling.’ SOURCE: IPCC (2001).”
The first comment on the IPCC figure is that the magnitude of the warming and cooling terms listed on the left-hand vertical axis are not the current radiative forcing. The magnitudes plotted are the difference of the estimated fluxes between the preindustrial times and 2000. For example, some of the radiative forcing of the well-mixed greenhouse gases presumably resulted in an adjustment in the Earth’s radiative budget before the 20th century. This is the reason for the last sentence in the NRC figure caption with respect to the IPCC figure where the Committee elected to clarify that the terms “warming” and “cooling” do not refer to the current magnitudes of radiative forcing. The current radiative forcing of the well-mixed greenhouse gases is not 2.4 Watts per meter squared.
Secondly, the IPCC figure recognizes the ” very low” level of scientific understanding of a number of the radiative forcings. This uncertainty should have immediately raised concerns about using the climate models as skillful projections. Recently, even the radiative forcings of the well-mixed greenhouse gases have seen a new uncertainty (with respect to methane), as discussed on this weblog (More Complications on Quantifying the Radiative Effects of Well-mixed Greenhouse Gases“).
On page 30, the 2005 NRC report summarized the perspective on this figure as follows,
“Figure 2-1 has been an effective way to portray the relative magnitudes of different radiative forcings, the associated scientific uncertainties, and an assessment of the current level of understanding. It has been used widely in the scientific and policy communities. However, it has some important limitations, including the following:
The figure does not provide information about the timescales over which each of the forcings is active. For example, the greenhouse gases in the first bar (CO2, CH4, N2O, and halocarbons) remain in the atmosphere for decades or longer, whereas the various aerosols persist for days to weeks.
The figure shows globally-averaged forcings and therefore does not provide information about regional variation in forcing or vertical partitioning of forcing.
The figure does not provide information about other climate effects of each forcing agent, such as impacts on the hydrological cycle.
The figure gives the impression that one can simply sum the bars to determine an overall or net radiative forcing; however, such a calculation does not give a reasonable description of the cumulative effect of all the forcings.
The uncertainty ranges are generally estimated from the range of published values and cannot be readily combined to determine a cumulative uncertainty.
The figure does not consistently indicate the forcing associated with specific sources (e.g., coal, gas, agricultural practices).
The figure omits nonradiative forcings as discussed in this report.
Although it would be unrealistic to expect a single figure to fully portray all of these aspects of radiative forcings, there are clearly opportunities to improve upon Figure 2-1 and to introduce new figures that address these limitations in the next IPCC report.”
Clearly, there is a National Research Council recommendation that the new IPCC Report address these issues. My reading of the IPCC Report so far, however, (see the post entitled “Overlooked Issues in Prior IPCC Reports and the Current IPCC Report Process: Is There a Change From the Past?“) indicates that the IPCC is ignoring the National Research Council’s report. This is very disappointing, if we want to advance our scientific understanding of climate, rather than use the IPCC process as political advocacy.