Int. J. Environ. Res. Public Health 2011, 8 987 change proposed. These general principles are illustrated by applying them to the case of climate change mitigation, one of the most interdisciplinary of public policy issues, and showing how they lead to quite different conclusions than those of most previous such economic analyses. This seems likely to be the case for other interdisciplinary public policy issues besides climate change. There is little doubt that proposals to mitigate the threat of climate change, sometimes referred to as catastrophic anthropogenic climate change (CAGW) or global warming, has presented environmental economics with its most important challenge to date in terms of providing useful advice on what if anything should be done concerning what is perhaps the major environmental public policy issue of the last decade or more. The economic implications of some prominent proposals to control climate change are large. There is currently probably no public policy issue with greater economic impact on which environmental economists might have particular expertise and therefore something useful to say. The purpose of this article is to assess whether these efforts would be more effective if economists used these principles. This article was initially motivated by personal discussions with two of the principal economic analysts on this subject concerning the validity of the physical science inputs that they had used. One said that he had never examined the physical science inputs he used from the United Nations (UN) Intergovernmental Panel on Climate Change (IPCC) efforts since he assumed that they were valid. The other said that he believed there should be a division of effort and that since his expertise was not in the physical sciences this should be left to experts in this field, which he felt included the UN IPCC. 1.1. Use of Most Relevant Observational Data and the Scientific Method as the Basis for Determining Effects This article will assume, I think reasonably, that the criterion for use of an assumption or hypothesis is whether it is supported by relevant observational tests or by the use of the scientific method. If the assumption or hypothesis is not so supported, this means that it is not supported by real world data since the role of relevant observational tests and the scientific method is to make sure that assumptions and hypotheses actually correspond with reality. Under this approach, assertions of alleged facts, model claims, and questionable arguments (such as that the authors cannot think of any alternative explanations for their conclusions) are not considered. This differs substantially from much previous physical science and economic research on climate change, but for good reason. It is important that environmental economics be based on valid science and not conjecture or religious belief if it is to be a reliable guide for policy making. There is no other way to lay a firm foundation for environmental economics so it corresponds to the real world rather than the current fashions of the day. According to my definition of valid science in Section 1.2, it is important for economists to try to clearly identify those physical science conclusions they are using that are supported by specific observational tests and the application of the scientific method and which are not. Those not supported by observable real world data or the scientific method are best identified as hypothetical calculations in my view so as not to be confused with scientific conclusions. Although such hypothetical analyses can play an important role in some cases, they need to be carefully distinguished from conclusions of more general applicability. This includes conclusions endorsed by national academies of science, scientific