Int. J. Environ. Res. Public Health 2011, 8 988 societies, peer reviewers, model outputs, or governmental organizations. Hypothetical effects can be often be usefully in analyzed, of course, but it is vital if economists are to present credible advice to policy makers that any conclusions reached using such assumptions and hypotheses be clearly labeled as being not verified scientifically so that the user keeps these qualifications in mind when using the conclusions resulting from their use. A similar approach is needed in those cases where there is insufficient evidence to determine whether assumptions or hypotheses represent valid science. 1.2. Why Science Should Be Based on Relevant Observational Data and the Scientific Method It is very important to understand what valid science is. Valid science as used here is the result of using the most relevant observational data and the scientific method to understand the world and its inhabitants. The most relevant goal needs to take into account the reliability of the observational data. The essential elements of the scientific method are characterizing the subject of inquiry, generating a theoretical, hypothetical explanation for the characterizations, making predictions based on the hypothesis, and finally experimentally determining the validity of these predictions by comparisons between the predictions and real world data. The determination of validity needs to be reproducible and independently verifiable. The assumptions and hypotheses being tested throughout this paper are those found in the various United Nations (UN) Intergovernmental Panel of Climate Change (IPCC) reports concerning climate change, reasonable reformulations of those found there, or interpretations of them by economists preparing benefit estimates. Valid science is not writing a description of the world or the opinions of world authorities on a particular subject. Valid science is not a statement of belief by scientific organizations. The question is not what someone believes but whether what he or she believes corresponds to the most relevant observational data and the scientific method. It is important to note that science evolves over time as new discoveries are made and new hypotheses are formulated and discarded. Only continuing research can insure that important new research results become available. Richard Feynman [1] expressed this as follows: ―In general, we look for a new law by the following process. First, we guess it. Then we compute the consequences of the guess to see what would be implied if this law that we guessed is right. Then we compare the result of the computation to nature, with experiment or experience; compare it directly with observation to see if it works. If it disagrees with experiment it is wrong. It‘s that simple statement that is the key to science. It does not make any difference how beautiful your guess is. It does not make any difference how smart you are, who made the guess, or what his name is—if it disagrees with experiment (observation) it is wrong.‖ According to the scientific method, a scientific hypothesis should be tested by comparing real world data with the implications of the hypothesis. This is how Albert Einstein‘s ideas on relativity were ultimately accepted. Many scientists doubted his hypothesis, and repeatedly proposed real world tests of it. But each test confirmed its validity or was shown to be based on faulty data or analysis. After a number of these tests, the opposition conceded that his hypothesis was valid. This process is described in Crelinsten [2]. The same process is needed in the case of climate change hypotheses.