Climate Science is going to present observational data and other information in coming web postings that raises questions about the validity of claims in the 2007 IPCC report. One of the issues is whether climate feedbacks amplify or mute radiative forcings caused by human activities. The IPCC asserts that climate feedbacks in fact amplify the human effect. We can test this assertion using observational data.
If the magnitude of the IPCC estimates of radiative forcings from human causes are greater than or equal to the sum of the total observed radiative forcings and feedbacks (i.e. the total climate system radiative imbalance), then the feedbacks have actually reduced the effect of radiative forcings caused by human activities. By contrast, if the magnitude of radiative forcing caused by humans is less than the sum of the total observed radiative forcings and feedbacks than the feedbacks have amplified the human radiative forcings.
In this first reality check, the information that is used is
1. Total Radiative Forcing from Human Causes
The radiative forcings from human causes are provided by the 2007 IPCC Report [see page 4 of the Statement for Policymakers; Fig. SPM.2].
Their value is +1.6 [with a range of +0.6 to +2.4 Watts per meter squared]
This value, as reported in a footnote in the IPCC report, is supposed to be a difference with between current and pre-industrial values (but note that that this is not what is stated in the figure caption).
2. Total Observed Radiative Forcings and Feedbacks
Ocean heat content data can be used to diagnose the actual observed climate forcings and feedbacks [Pielke Sr., R.A., 2003: Heat storage within the Earth system]. Here I will use Jim Hansen’s value for the end of the 1990s of
+0.85 Watts per meter squared
(even though this is probably an overstatement (see)).
Thus, the total observed radiative forcing and feedback of 0.85 W/m^2 lies below the IPCC central estimate of 1.6 W/m^2 for just the human contribution to radiative forcing. This suggests that the climate feedbacks most likely act to diminish the effects of human contributions to radiative forcing, though it is important to recognize that a small part of the IPCC range (0.6 to 0.85) falls under the observed value from the work of Hansen.
This suggests that, at least up to the present, the effect of human climate forcings on global warming has been more muted than predicted by the global climate models.
This issue was inadequately discussed by the 2007 IPCC report. Climate Science has weblogged on this in the past (e.g. see), but so far this rather obvious issue has been ignored.