Int. J. Environ. Res. Public Health 2011, 8 997 than the ~7 GT C annually released from fossil fuel burning‖ [8]. Further, ―anthropogenic CO2 is less than ½ W/m2, less than 0.1 percent, judged from C isotopes. Clouds are a real thermostat, with far more temperature regulating power than CO2‖ [8]. He does not mention it, but it appears widely agreed that the atmospheric models used by the IPCC do not handle clouds very well. An important point here is that water can absorb less CO2 as temperatures rise. So an alternative explanation to the IPCC hypothesis that the observed rise in atmospheric CO2 is due to anthropogenic emissions is that ocean temperatures have at some time in the past warmed enough so that some of its CO2 is now being out gassed. The issue of CO2 residence times in the atmosphere is of great importance to economic analyses of the benefits of climate mitigation since it determines the period of time during which the economic benefits of emissions reductions would take place. Short atmospheric residence times would mean that the economic benefits of emissions reductions would occur over a much shorter period than if residence times were very long. The greenhouse effect of added CO2 in the natural world is only effective when the CO2 is in the atmosphere, not when it is in plants or the oceans, or in ocean sediments. Added CO2 (or the carbon in it) in plants or oceans may have important effects, of course, but not for the greenhouse effect on global temperatures. The economic implications of this will be spelled out in more detail in Section 2.6.1 below. The observed increasing atmospheric CO2 levels may be primarily due to increasing ocean temperatures hundreds of years ago since water cannot absorb as much CO2 at higher temperatures. In other words, the CAGW supporters may have reversed cause and effect. Instead of increases in CO2 causing rising temperatures, rising temperatures result in higher CO2 in the atmosphere, probably with a time lag. It is not known exactly what the lag time might be for changes in water temperatures to result in increases or decreases in out gassing from or absorption of CO2 by the oceans. Research shows, however, that at the bottom of an ice age 250 thousand years ago CO2 increased with about an 800 year lag after temperatures increased [19]. So the current increases in CO2 levels might be due to increases in temperatures 800 years ago or could be a result of anthropogenic emissions. Or since much of Earth‘s CO2 appears to be derived from and vary with changes in volcanic activity [8], this may be another possibility. In sum, there appears to be substantial evidence that CO2 residence times in the atmosphere are about 5 years, that only a small percentage of CO2 from fossil fuel sources can be found in the atmosphere, that there are alternative natural explanations for rising CO2 levels, and that there is little reason to think that CO2 emissions from fossil fuel burning are likely to be a significant cause of the gradually increasing atmospheric CO2 levels. 2.3. Hypothesis 2: Increases in Atmospheric CO2 Levels Interact with the Major Greenhouse Gas, Water Vapor, to Create a Large Positive Feedback Capable of Causing Catastrophic Global Warming Undoubtedly the most controversial of the hypotheses underlying the UN‘s conclusions is the question of feedback. The IPCC and all the computer models it uses assume that there is a strong positive feedback between an increase in trace greenhouse gases in the atmosphere (such as CO2) and the major greenhouse gas (water vapor). Without this strong feedback there is no real basis for CAGW since the atmosphere contains only trace amounts of other greenhouse gases besides water vapor, and