I've been thinking about this issue for a while, and this quote sort of spurred me to post something:
The debate is over. The consensus is in.
That's hardly the way that science works. The debate is never over, and consensuses (consensi?) are often wrong, especially scientific consensuses.
I can give several examples in the history of science where the consensus has been wrong, but I'm sure you've heard it all before...
The thing that's more interesting to me is that this ``consensus'' sets major policy decisions, for the good or bad. Clearly, the science isn't settled now, and it will be a long time before it is. Even predictions made based on this science are suspect, at best---for example, the climate simulations on computer don't take into account the presence of water vapor in the atmosphere. Water vapor is a greenhouse gas, too, and there is a LOT of water vapor in the atmosphere. It is currently not known how the presence of water vapor effects the climate predictions, so people guess.
I will give one concrete example. If you, as a layperson, ask me if the
higgs boson exists, I will say, with great certainty, yes. Of course, I don't know if the higgs exists or not---it is possible, however unlikely, that the higgs doesn't exist. Now, the statement that the higgs exists is partly based on the evidence---there aren't any convincing theories that don't have some type of higgs boson. However, if the higgs isn't discovered, and you confront me with that, I will say something along the lines of ``Well, we were wrong, and this is why...''
I wonder, how much of this is going on in the field of climate science? On the one hand, it is very hard to separate personal biases from scientific data. For example, Milikan in his famous oil drop experiment, is known to have disregarded data which didn't support his conclusion. Likewise, Eddington's famous confirmation of the GR prediction that light bends in a gravity field was probably fudged as well. Of course, both of these conclusions were later confirmed by other experiments, so the fudging happened to work out. So if a climate science has some data that doesn't seem to support his bias, it seems likely that he might disregard it altogether, and look for other data.
Of course, on the other hand, scientists are having to relate their conclusions to non-scientists. It is very difficult job to do, because science never really proves anything---the best we can do is set limits. But this statement isn't good enough for popular audiences, who want ``scientific proof'' of something. So it is also quite easy for scientists to overstate the results of their research.
I will give one final example: in the 1980s and 1990s, string theorists claimed to be on the verge of a ``theory of everything''. But 20 years later, the state of string theory is much less secure. At one time, scientists thought that the equations of string theory uniquely predicted our universe, but today most of us will tell you that that just simply isn't the case. String theory itself has changed, and so our conclusions are different now. Were string theorists wrong in the 1980s to say that we were on the verge of understanding Nature at it's most fundamental level? No---that was a prediction that turned out to be wrong.
The current status of climate science, I would be willing to bet, is very similar. Scientists, when talking to the public, will say ``We know for sure that...'', but when talking among themselves, they may be much more reluctant to make the same statements. There are a variety of reasons: sometimes we're lazy and don't want to deal with questions, sometimes there's money at stake (funding), and sometimes we don't understand all the science completely. For example, it's easier for me to say ``The higgs definitely exists'' than to explain the other theories that people work on. I
believe that the higgs exists, because I've looked at the data and have made a judgment. But that judgment carries with it an inherent bias. I think this is the culture which was revealed with the emails.
The problem with this is that, as DH rightly points out, the economic ramifications are quite severe.