This word is used to mean someone who refuses to accept the clear line of the scientific community. It is quite common to hear people named as evolution denialists, anthropogenic global warming denialists, AIDs denialists etc etc. and it is common to hear these people being attacked for the reasons that make them a denialist. This is starting to scare me. I think a bit of separation might help here before we turn science into an unquestionable religious dogma and we start burning heretics.
The point of science is to subject every hypothesis to rigorous empirical testing, to trust no authority and to believe exactly what the evidence says. Science is the best, if not the only, possible way to understand the physical world and needs masses and masses of respect. But it needs respect as a process, not as a set of results.
When we attack someone or call them deluded for not believing that all organisms alive today share common ancestors whose offspring differentiated and changed by means of natural selection before becoming distinct non-interbreeding species, we are not (or at least shouldn't be) attacking the lack of belief itself. Many intelligent people lived before there was good evidence for, or a consistent theory of, evolution. They did not believe in evolution and this is no reason to dismiss them intellectually. What people need to be opposed to is wilful ignorance and refusal to accept fact. To fail to believe in the theory of evolution today means one of two things: Either that you haven't heard the endless arguments to support it, heard about the endless fossils, DNA analyses, geographical comparisons, real world real time examples etc etc which have been all over the TV, internet and bookshops ad nausiem, in which case you are wilfully ignorant and I would ask that you please never ever vote, not even for x-factor, your opinion really doesn't count, nobody cares what you think, because you dont think. OR you have looked at all the evidence and have said “…. nah, my 4000 year old book of Jewish fairytales is better than all this science rubbish”, in which case you simply are not connected to the physical world the rest of us share, this is genuinely a form of psychosis and again, please dont vote, for the sake of the rest of us, I'm begging you, seek medical help. In the case of evolution the evidence is so strong that no rational couter-arguments exist, we must be open the the possibility that they might, but the first person to find me such an argument can have my right leg as a prize.
It is important to distinguish these two cases, one is simply an act of laziness, which can be remedied by better public outreach from science and by simple effort on the part of the denier. The second case is someone who says “if all the evidence in the world is placed against my pre-existing belief I will not change my beliefs”. This second case is a really dangerous sort of person: their certainty in spite of, not because of, the evidence means they can very easily be made to believe very bad things, or when they do believe such things it is hard to correct them. Someone who doesn't accept that his senses and the evidence provided indirectly by them provide a real way of analysing the world are simply beyond rational discourse and persuasion, so if they get it into their heads that they want to do something stupid there is no easy way to talk them out of it.
So it is right that we complain long and hard at and generally disrespect this second type of person, it's bad enough to have one person who is disconnected from reality, it's a problem of a different order if people start listening to them. The pope himself is not all that dangerous, his legions of gay hating followers on the other hand are. The more a given proto-pope's arguments are refuted and their idiocies opposed the better for the rest of us.
I have a problem with how this is done in the liberal blogosphere, and it's climate change. I read blogs and newspaper columns from every respected scientist in the world and I hear unambiguously that the climate is changing and that human CO2 is responsible and we need massive cuts in everything to save our asses. And my soft-hearted liberal side says yeh, lets listen to the scientists, Sarah Palin disagrees so they must be right, people opposed to this are all doing it for the oil. And when people start disagreeing on blogs I think, well you're just running scared and you're using bad arguments to prop up your need for petrol. And then I see others on the same blogs wanting to shut these people up, or calling them denialists, and saying they have the blood of polar bears on their hands and other reckless stupid things. I see all criticism of the genuinely bad science of the guys at UEA ignored or shouted down by people assuming bad faith. I hear people arguing by presenting a list of names of scientists who agree, I hear people shouted at for asking to see the raw data. And it makes my blood chill.
Science is not a religion, you get no points at all by having the president of the royal society on your side, you get points for having evidence, evidence that others can see and examine, with clear methodology and error bars. The motto of the royal society, which I have adopted as my own, is nullius in verba, on no-one's word. When arguments in public go along the lines of “scientists say x, those who doubt x are evil and wrong” then something has gone very badly wrong.
And in climate change this isn't just a matter of pedantry about exactly what we are attacking. I have no reason at all to doubt that all the competent scientist studying the climate have good evidence to suggest that CO2 humans put in the atmosphere has caused a raise in global average temperature. I have no reason at all to doubt this evidence is complex and detailed, and very compelling. But yet, when I go looking around for it, can I find it? Damned if I can. A good proxy of what the public knows is wikipedia, for many people no other source of information exists. If I go on wikipedia and look for evidence of climate change, what do I find? Stupid Al Gore style graphs. Charts of CO2 and temperature which prove nothing about climate change because correlation does not imply causality, but which do prove that the various proxy measurements we have of past temperature disagree with each other by huge amounts, often more than the range of temperature. We learn from wikipedia then that the science behind global warming is not only shockingly inaccurate, it also uses logical fallacies, all the time.
I say again, my liberal conscience compels me to say that scientists say the evidence is good, and anything that gets through peer review must be good, peer review is tough. But it's really not right to attack people for doubting the word of the all high science. If they doubt you, and they are playing by the rules of only looking at what the evidence says and they aren't purely driven by an ideological agenda, then the problem might not be with them. The problem might be that the way the evidence is presented to the public on this is shockingly bad, really awful. And if that is the evidence in the public conciousness I'm glad the public dont believe in global warming, I'm glad they dont just take the word of science as revealed unquestionable truth. Let's have some of the vast body of evidence (which really does exist and is of good quality) out in the public domain and more importantly in the public conciousness and in a language people can understand without having to struggle. And even more importantly, let's have a differentiation between people who reject the scientific method and who must be attacked and opposed, and those who reject some common belief without being shown some good evidence for it, such people are quite the opposite of a denialist.