A probabilistic quantification of the anthropogenic component of twentieth century global warming T. M. L. Wigley B. D. Santer http://link.springer.com/content/pdf/10.1007%2Fs00382-012-1585-8 Significant excerpts Abstract - This paper examines in detail the statement in the 2007 IPCC Fourth Assessment Report that Most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. We use a quantitative probabilistic analysis to evaluate this IPCC statement, and discuss the value of the statement in the policy context. For forcing by greenhouse gases (GHGs) only, we show that there is a greater than 90 % probability that the expected warming over 19502005 is larger than the total amount (not just most) of the observed warming In the following, we confirm the IPCC statement regarding the GHG component of anthropogenic warming, and show that this statement is probably too conservative. In addition to the GHG only component of warming, we also consider the total effect of all anthropogenic emissions, where GHG warming is partly offset by the cooling effect of sulfate aerosols. Our method for assessing the GHG only and total anthropogenic components of observed global-mean surface warming produces probabilistic estimates of these components We first consider results for the 56-year period from 1950 to 2005. The key result here is that for the GHG forcing only case (Fig. 4a), which relates directly to the 2007 IPCC statement. Over this interval the observed (NOAA/NCDC) robust trend, with ENSO removed, is 0.610C (see Table 3). The median model robust trend is 0.940C. The probability that the model-estimated GHG component of warming is less than the observed trend is approximately 7 %. Using IPCC terminology, therefore, it is very likely that GHG-induced warming is greater than the total warming observed over this 56-year period (i.e., the model GHG-only trend is not just greater than most of the observed warming, but very likely greater than the full amount of observed warming). As noted above, there is ambiguity in the meaning of the word most in the IPCC statement that we have focused on here. If most means anything more than 50 % (i.e., for NOAA/NCDC data, a warming greater than about half of 0.610C = 0.305C), then the IPCC statement is far too conservative. For GHG forcing alone, the lowest warming value over 19502005, in all of the 625 cases considered (25 sensitivities and 25 diffusivities), is 0.469C, much greater than 0.305C. If, however, most means almost all, then our result is entirely consistent with the IPCC statement and provides strong support for this statement. Results for the all anthropogenic forcing case (Fig. 4b) are of equal interest, and provide information on the overall consistency between the model-predicted changes and observations. When one accounts for both positive (GHG) and negative (net aerosol) forcings, the median model expectation for the net warming over 19502005 (0.557C) is very similar to the observed warming (0.610C) In the 19,375 simulations the model predicted warming is smaller than the observed warming in 61 % of the cases and larger than the observed warming in 39 % of the cases. This result is relatively insensitive to the inclusion of the effects of natural solar and volcanic forcing (see next section) For trends over 19502005, when one accounts for both greenhouse gas and aerosol forcings, the model expectation for the net warming is consistent with the observations. Including the effects of solar and volcanic forcings slightly reduces the median warming (see Fig. 6), but does not lead to a statistically significant difference between the median model expectation and any of the observed warming trends. Overall, our results provide strong confirmation of the IPCC findings, and are in close agreement with pattern based fingerprint analyses, such as those of Stott et al.(2006). Very impressive and rigorous analyses amounting to what most consider compelling support.