Int. J. Environ. Res. Public Health 2011, 8 999 Figure 4 is a schematic of how this feedback might take place to amplify the effects of increasing atmospheric CO2 levels. By assuming a strong feedback, the IPCC AR4 report [5] and the models it uses reach the conclusion that the climate sensitivity factor (CSF) relating the increase in global temperatures (in degrees centigrade) to a doubling in the level of atmospheric CO2 is between 2 and 4.5 °C with a best estimate of 3 °C. By using the IPCC or related model results, economists have implicitly used these estimates in determining the benefits of CO2 emissions mitigation. Now the major scientific issue is whether this hypothesis that there is strong feedback is supported by the scientific method on the basis of real world data. Figure 4. Schematic explanation for possible feedback effects. Source: Lindzen [21]. 2.3.1. Four Critical Comparisons with Real World Data There are at least four particularly telling physically-based basic comparisons which can be made to determine the scientific validity of the UN strong feedback hypothesis. According to the scientific method an inconsistency even in one of these comparisons means that the hypothesis should be rejected from a scientific viewpoint. In order to determine this it is necessary to compare the physical effects of the assumed strong feedback with actual real world data. The CAGW concerns are based on a hypothesis that there are significantly positive feedbacks from the effects of the CO2 increase on water vapor and clouds that would greatly amplify the greenhouse effect of the increase in CO2. This is the key hypothesis which can be compared with real world data to determine whether the IPCC findings are valid or not. Although there are other tests (see [22], Slide 44), hypothesis 2 requires that each of the following four observations discussed in this subsection are present: