Int. J. Environ. Res. Public Health 2011, 8 1011 they would be beneficial. If significant warming occurred, Northern areas of the Northern Hemisphere would presumably be opened to cultivation and use, as Southern Greenland was in the Medieval Warm Period. The scientific validity of physical science hypotheses such as the value of the CSF can only be determined by comparisons with real world data using the scientific method and not by the use of peer review or atmospheric models or statements by experts. Although Lindzen/Choi [21,39] and Spencer/Braswell [41] have derived their estimates of the CSF by using real world data, that does not prove that their estimates will ultimately prove accurate. They do, however, appear to be much more solidly grounded in real world observations than the IPCC AR4 primarily model-based estimates. And they are consistent with the four negative tests of the high CSFs used by the IPCC AR4 discussed in Section 2.3. 2.6.3. Combined Effect of the CSF and Residence Time Corrections The net result of combining both the hypotheses 1 and 2 correction adjustments suggest above would be a combined factor of roughly 0.04 times 0.25, or 0.01. This would mean that all benefit estimates that have not adjusted for the postulated invalidities of hypotheses 1 and 2 would need to be divided by about 100. The result would be that most benefit estimates would be so small as not to be worth further discussion. 3. The Economic Costs of Climate Change Mitigation I believe that there are also significant differences in the costs of reducing CAGW than usually assumed by many economists who have examined this question. First, the much smaller CSF and shorter residence times for CO2 in the atmosphere explained in Sections 2.2, 2.3, and 2.6 above greatly increase the cost of reducing CO2 emissions per degree of assumed global cooling achieved since it takes much more of a change in CO2 emissions to reduce temperatures by a given quantity. In other words, just as an increase in atmospheric GHG levels increases temperatures much less than assumed by the IPCC, reductions in CO2 emissions would be much less effective in decreasing temperatures. Note that this does not change the total cost of mitigation because there will presumably be proportionately less of a temperature increase to mitigate. The second difference is that most efforts to estimate costs of CO2 emissions reductions appear to have greatly underestimated the actual costs. The third difference is that with one exception only one of the analysts discussed in this paper looked at alternative less expensive methods for achieving such mitigation should it ever be needed and desired. These three differences will be discussed in the next three subsections. 3.1. Why the Effectiveness of Proposed Reductions in CO2 Emissions in Reducing Temperature Increases Will Be Much Lower than Assumed by Many Economists The much lower CSF found by Lindzen/Choi [21,39] and Spencer/Braswell [41] affects the costs as well as the benefits of global warming mitigation. As outlined in Section 2.6, the hypothesized increase in global temperatures would be more than four times less than shown by the IPCC AR4 models because the CSF appears to be at least four times less. If the economic damages are proportional to the temperature increase, then damages (benefits of mitigation) would be more than