Int. J. Environ. Res. Public Health 2011, 8 1015 actually would reduce global warming or whether there might be other less expensive or more effective alternatives that would accomplish such a reduction in global warming. Speaking broadly, most environmental economists who have examined the costs of GHG mitigation have directly or indirectly assumed that (1) reducing GHG emissions will significantly reduce global warming, (2) such reductions are either the best alternative available for this purpose or at least the only one worth examining, and (3) geo-engineering approaches either are not worth examining or can be ruled out for various reasons, particularly ―tampering with nature‖ and the possibility of unintended consequences [55,60]. The major exception is Lomborg [58] and associates [61], who have examined some geoengineering approaches in great detail, and whose views will be discussed further in Section 5.2 below. The most prominent lower cost alternatives involve geoengineering, which in this case means deliberate modification of the atmosphere by humans. There are a number of varieties involving different approaches towards reducing global temperatures. One of these approaches, involving placing various types of particles into the stratosphere to increase or decrease the reflection of incoming solar radiation [60] so that they will not be washed out by rain, is sometimes called solar radiation management or SRM. Another involves placing salt molecules into air above oceans using ships [61]. Previous research [55,60] resulted in comparisons between SRM and reductions in GHG emissions (see Table 2), referred to as exclusive regulatory decarbonization (ERD). ERD is defined as a strategy for decreasing emissions of greenhouse gases as the exclusive approach used to control global warming. Most current climate change control proposals involve some form of ERD such as emission regulations, emission taxes, fuel economy, bio-fuel standards, and ―cap and trade‖ proposals. ERD assumes that if we could just reduce carbon emissions that humans are putting into the environment, the global warming problem would be solved. I have termed such attempts ERD because most, but not all, involve decreasing various forms of carbon emissions. As can be seen in the table under marginal costs, rough estimates place the equivalent cost of reducing carbon emissions as 4 to 5 orders of magnitude (that is, 10,000 to 100,000 times) less than the cost by reducing carbon emissions. As discussed in Section 3.2 the actual costs of ERD are probably another order of magnitude higher than shown. These estimates are necessarily rough given that no actual geoengineering experimentation has been done by humans. And various other forms of geoengineering probably have somewhat higher or lower costs. The differences, however, are so large that it is reasonable to assume that at least SRM and possibly some other geoengineering approaches are much less expensive. And most important, the global temperature effects of SRM have been repeatedly demonstrated by nature as a result of major volcanic eruptions in the tropics. Such is not the case for emissions reductions, which have not been demonstrated to actually achieve any temperature reductions. A very important consideration is that with adequate preparation, no actual use of SRM would be needed until there were clear and immediate risks of either decreases or increases in global temperatures that posed a serious risk to humans or the environment. Errors could be quickly corrected by altering the type, quantity, or placement of particles in the stratosphere. Emissions reductions, on the other hand, require a guess decades in advance as to when and how adverse an effect may occur and how much of a decrease/increase in GHGs might be required to mitigate these effects. If the assumed CSF or CO2 residence time in the atmosphere prove to be incorrect or people or governments do not fulfill their obligations, it would take several additional decades to correct any errors made