Int. J. Environ. Res. Public Health 2011, 8 993 such as solar or cloud cover variability, which they view as being of minor or little importance in influencing climate. A much more logical and plausible set of CAGW hypotheses might be as follows: Hypothesis 1: Anthropogenic releases of CO2 are the primary cause of increases in atmospheric CO2. Hypothesis 2: Increases in atmospheric CO2 levels interact with the major greenhouse gas, water vapor, to create a large positive feedback capable of creating catastrophic global warming. If these two hypotheses were correct, then it might follow that: Hypothesis 3: Anthropogenic GHG emissions, particularly CO2, will result in CAGW. This appears to be the primary hypothesis offered by climate change mitigation promoters. Unfortunately, there is no easy way to determine the validity of this statement with current knowledge other than to examine hypotheses 1 and 2 separately. But a related but still important hypothesis that can be examined is the following: Hypothesis 3a: Changes in global temperatures are primarily influenced by rising levels of GHGs other than water vapor in the atmosphere. Most of the remainder of this section will discuss these hypotheses using a variety of sources. The emphasis will be on a few of the scientific issues, which have a major influence on the economic analysis of climate mitigation since economic analysis is the focus of this paper. There are many scientific issues that this article will not discuss because of the time and space that would be required. It is possible, of course, that some of these other issues, if analyzed in the same depth, would result in offsetting the effects of those discussed. Given the history of the UN IPCC, however, which has consistently emphasized the adverse effects of increasing CO2 in the atmosphere, this possibility is not very likely. 2.2. Hypothesis 1: Anthropogenic Releases of CO2 Are the Primary Cause of Increases in Atmospheric CO2 There have been slow but fairly steady increases in CO2 levels in the atmosphere since measurements began on Mauna Loa in Hawaii in the mid-1950s based on measurements there. Some have argued that these increases are primarily a result of emissions from burning fossil fuels; others believe it is due to natural causes. The IPCC claims that 21 percent of atmospheric CO2 is from burning fossil fuels [8], and that CO2 may remain in the atmosphere for a long period, with a ―rough indication‖ of 50 to 250 years, as discussed in the next paragraph. The atmospheric residence time (or RT) is less than clearly stated in the various IPCC reports, but a recent article [9] whose lead author was the Co-chair of the IPCC Working Group 1 (concerning physical sciences) claims that CO2 remains in the atmosphere almost indefinitely. The issue of how long CO2 emissions remain in the atmosphere has become even more confusing because global warming supporters have coined a variety of terms related to the residence time of CO2 in the atmosphere besides the simple residence time, equal to the average time that molecules remain in the atmosphere.