The IPCC Claim Regarding A Linear Relationship Between The Global Average Surface Temperature Trends And Global Average Radiative Forcing Is Quantitatively Inaccurate

In our paper

 Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr., J.R. Christy, and R.T. McNider, 2009: An alternative explanation for differential temperature trends at the surface and in the lower troposphere. J. Geophys. Res., in press [with edits still to be made in the final published version; see],

we show that the surface and lower tropospheric temperature trends are diverging in time. We offer an explantion for some of this related to the use of minimum temperatures over land as part of the construction of the global average surface temperature trend. Other sources of bias and uncertainty are reported in our 2007 JGR paper [Pielke et al 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends].

This also means that the diagnosis of the radiative forcing using the surface temperature trends introduces errors. The assumption of a linear relationship between the radiatve forcings and surface temperature is clearly stated in the 2007 IPCC WG1 report (e.g. see Chapter 2 page 133), where it is written

“Radiative forcing [RF] can be related through a linear relationship to the global mean equilibrium temperature change at the surface (delta Ts): delta Ts = lambda * RF, where lambda is the climate sensitivity parameter (e.g.,Ramaswamy et al., 2001).”

The lower troposphere is also expected to have a linear relationship to the radiative forcing although amplified relative to the surface; e.g. see Figure 5.6 for the tropics in CCSP 1.1. Chapter 5.

Our Klotzbach et al. 2009 paper shows that there is not a temporally invariant linear relationship between the global average surface and lower tropospheric temperature trends.  Based on this paper, and our other papers such as JGR 2007, there is also not a linear relationship between the global average surface temperature trends and the radiative forcing. This view is at significant variance to the IPCC view, which used the CCSP 1.1 report in its assessment. In our new Klotzbach et al paper, we present additional observational evidence that the paragraph above from the IPCC report is not correct, and we discuss one of the reasons for the discrepancy.

The 2005 NRC report supports our view where it is written

“The simplification of complex, mechanistically disparate processes to the same radiative forcing metric, with the implication that positive forcings may cancel negative forcings, provides a way of easily communicating climate forcing factors and their relative importance to general audiences. However, a net zero global mean radiative forcing may be associated with large regional or nonradiative (e.g., precipitation) changes. Further, when forcings are added, uncertainties in individual forcings must be propagated, resulting in large uncertainties in the total forcing. Adding forcings also belies the complexity of the underlying chemistry, physics, and biology. It suggests that all effects on climate can be quantified by a similar metric without knowing, or needing to know, the details of the climate response as captured in feedback effects. Yet there are many aspects of climate change—including rainfall, biodiversity, and sea level—that are currently not related quantitatively, much less linearly, to radiative forcings.”

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