Int. J. Environ. Res. Public Health 2011, 8 989 There are alternative approaches that have been proposed to the use of the scientific method for determining scientific validity. An example is that proposed by Armstrong and Green [3]. Their broader approach includes many of the attributes of using the scientific method outlined above, and reaches the same conclusion as this study does as to the validity (or lack thereof) of much current research on climate change, but places somewhat less emphasis on the use of observational data and the validity of hypotheses. 1.2.1. Why Computer Models Do Not Demonstrate Scientific Validity CAGW supporters often claim that the results of climate models validate their conclusions scientifically. Although these climate general circulation models (GCMs) have many uses in scientific inquiry, they do not prove or disprove scientific validity. Models show what the model builders believe is the way some physical system, in this case climate, works. But all that they represent (at best) is the model builders‘ beliefs and judgments. Models produce scenarios, not forecasts [3], and neither prove nor disprove scientific hypotheses nor show what will happen in the future. The only way to do this is by comparison of particular scientific hypotheses with real world data other than past temperature data, which the models have usually been modified to emulate. The particular climate models used by the UN IPCC appear to be even more questionable than many since they do not even do a good job of hindsight [4], let alone foresight. 1.2.2. Uncertainties in the Scientific Data Used for Rejecting a Hypothesis Although the principle of using the most relevant observational data and the scientific method as outlined above for deciding what is and is not valid science and hence what should and should not be used in economic analyses appears inescapable, its application to particular issues is often more difficult. There will inevitably be questions about whether the correct comparisons have been made between the hypotheses to be tested and the real world data used to test them, or whether the best data have been used to make the comparisons, or whether there even is valid, unmanipulated, and adequate quality data that can reasonably be used for such comparisons, or whether the most recent research results have been used, or what constitutes research results that should be used. In my view there is no such thing as settled science because settled science is not science. After a scientific hypothesis has undergone many comparisons with real world data it may gradually assume the status of a physical law. But short of such laws, and in some cases even in their case, science only advances by constantly questioning all aspects of any hypothesis. So although I will attempt to use what I consider to be the most valid science available to me at the time this is written, and am willing to defend my judgments as such, future research may result in either further evidence confirming my judgments or even refuting them. If the risk of such changes is high and the cost of undertaking a policy change is high, the best approach is normally to do nothing until either the science becomes more settled or the costs come down. It should be the responsibility of those proposing a public policy change to make the case in this respect, not those opposed to making the change. An important footnote here is that valid science as used here is determined by whether it is based on the most relevant observational tests and the scientific method, not by whether it has or has not appeared in a peer-reviewed journal. Peer-review may play an important role in determining which