Int. J. Environ. Res. Public Health 2011, 8 994 Segalstad reports [10] as follows (with reference number added): ―[The] IPCC defines lifetime for CO2 as the time required for the atmosphere to adjust to a future equilibrium state if emissions change abruptly, and gives a lifetime of 50–200 years in parentheses (Houghton et al., 1990) [11]. Their footnote No. 4 to their Table 1.1 explains: For each gas in the table, except CO2, the "lifetime" is defined here as the ratio of the atmospheric content to the total rate of removal. This time scale also characterizes the rate of adjustment of the atmospheric concentrations if the emission rates are changed abruptly. CO2 is a special case since it has no real sinks, but is merely circulated between various reservoirs (atmosphere, ocean, biota). The "lifetime" of CO2 given in the table is a rough indication of the time it would take for the CO2 concentration to adjust to changes in the emissions…” Segalstad [12] argues ―the IPCC has constructed an artificial model where they claim that the natural CO2 input/output is in static balance, and that all CO2 additions from anthropogenic carbon combustion being added to the atmospheric pool will stay there almost indefinitely.‖ In this regard, the IPCC and its supporters have sometimes tried to support their case by arguing, sometimes with models, that CO2 once emitted will stay around for quite some time in one reservoir or another and will continue to pass between reservoirs until it is ultimately transported into ocean sediments. They have sometimes argued that it will take many years for the system to fully return to pre-industrial levels. Although this is probably true, the immediate question from an economic and environmental standpoint is the average residence time of CO2 in the atmosphere since that is the only CO2 reservoir where it may impact the greenhouse effect. It is possible that CO2 emissions might have adverse effects in other reservoirs, of course, but this is highly unlikely in that it represents food for plants and in Segalstad‘s view there are extremely large, multiple buffering systems found in the oceanic reservoir [8]. But it is the atmospheric reservoir that is being discussed here. Accordingly, the atmospheric residence time used here is based only on measurements of the air reservoir. The residence time measured by Segalstad [8,10] appears to fit that description. The relevant issue, of course, is what assumptions the models the IPCC and economists actually used were. There are three different carbon isotopes, carbon 12, 13, and 14, usually denoted as 12C, 13C, and 14C. An important characteristic of plants is that they prefer to use CO2 with lighter carbon isotopes, namely, 12C. Hence there is a difference in the isotopic composition of CO2 derived from plants living or dead compared to other sources (such as volcanic activity), and therefore in the fossil fuels derived from these plants, and in the CO2 produced by burning them in one form or another. Specifically, carbon in plants has a lower 13C/12C ratio. When fossil fuels are burned carbon from the ancient plants from which the fuels were formed is released as CO2 into and mixes with the atmosphere, thus lowering the 13C/12C ratio there. By measuring the isotopic composition of CO2 in the atmosphere it is possible to estimate the percentage of CO2 derived from plants and to infer the residence time of CO2 in the atmosphere. Most but not all anthropogenic CO2 emissions come from burning fossil fuels. The question at hand is whether these measurements correspond to the IPCC assumptions in this regard. If not, there would appear to be something of great importance wrong with their assumptions or hypotheses. The short answer to this question is that the isotopic CO2 analysis presented by Segalstad [8,10] are not consistent with the IPCC assumptions in several important regards, and