The evidence is supposed to be valid for a period of more than 2000 years, even though actual temperature data are available for less then 500 of those years, but I don't think anyone seriously doubts that human activity has some effect on temperature data. Two things are in serious dispute, however. First, what is the nature and extent of that effect and, second, what normative conclusions follow from our awareness of that effect.
One is tempted to leave the first question to the scientists, but it is because of the way scientists famously mess up questions such as the second one that it is even more tempting to monitor their contributions to the first question as well. Scientists have a notoriously bad sense of what follows from what in a normative context, and very often it is what they think follows from what in a normative context that determines what they think follows from what in a purely empirical context. This is unfortunate, but it is not surprising, since this is probably true for most folks in most contexts: we are, after all, political animals.
In the case of this panel's study, some politicians have argued that the data was selected by the scientists themselves in such a way as to produce the effects that they themselves expected. Their response?
"I saw nothing that spoke to me of any manipulation," said one member, Peter Bloomfield, a statistics professor at North Carolina State University. He added that his impression was that the study was "an honest attempt to construct a data analysis procedure."What did you expect him to say? "Sure, we cooked the books. Everybody does." He wouldn't have said that even if he had believed it. But of course he didn't believe that--he believed that what he did say was true. However, his mere belief doesn't actually make it true, and the selection and analysis of data in a scientific context is precisely one of those areas where scientists themselves are the least competent to make judgments such as the one this guy made. It is not really clear whether an outside observer could make such judgments either: science is one of those areas of human knowledge that fools us into thinking that there is such a thing as objectivity about observational data and the theories that attempt to explain them, but in this particular instance, Nietzsche was actually right about something--he famously wrote, in Beyond Good and Evil, that science is nothing more than one more interpretation, among many possible interpretations, of the world we observe. All interpretations are open to revision, especially those about which we are so sure as to be unable to see possible alternatives.
So it's worth keeping an eye on these reports--in some cases they may actually come close to the truth, but if the history of science teaches us anything at all, it teaches us that science itself is just a long sequence of theories and explanations being proved false. Why should any of these theories about the climate be exceptions to that long and invariable history?