One of the complaints against those who deny, or doubt, anthropogenic global warming, is that they are mistaken. That must be the fundamental complaint. We won't do the right things, unless we get the facts right.
Another complaint, less fundamental but more interesting ethically, is that by opposing the scientific consensus, they get in the way of action. This complaint is sometimes mentioned, for example in paragraph 7 in this piece by James Garvey:
There will always be some politicians who will oppose action on global warming, and if some people offer arguments that action would be wasteful and pointless, they will give those politicians material to use in their own campaigns. This will reduce the probability of action at the level of governments, whether they would work singly, or in co-operation at the international level.
My question here is, is there a specific ethical failing of opposing a consensus, where the result is likely to be to delay action, or to reduce the scale of action, to solve a problem that is real and pressing if the opponents of consensus are indeed mistaken?
If the opponents know that the evidence for their position is of poor quality, or if they are reckless as to whether it is of good or poor quality, there is a failing, but it is of a rather more general sort than any failing of opposing a consensus. The failing is that of not being sufficiently careful to put forward true claims, and to avoid making false claims, in a situation in which one ought to take more than usual care, because the consequences of people being misled are so serious. William Kingdon Clifford set a high standard here, in his essay "The Ethics of Belief". His standard is conveniently summed up in his words towards the end of its first section: "It is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence". We might debate whether this was an appropriate standard when the stakes were very low. But with global warming, the stakes are very high, and the standard is undoubtedly apt.
Not all global warming sceptics are reckless as to the quality of the evidence for their position. Some would be too ill-informed to be aware that the quality of their evidence needed to be reviewed. But the corporate lobbyists, and their think tank pals, who push the sceptical position, cannot plead that kind of innocence. They know about the evidence. They study it when formulating their positions. They also know about the need to look critically at the quality of evidence. They spend a lot of time criticizing the evidence on the consensus side. Given that the overwhelming majority of scientific, as distinct from political, commentary, is on the side that anthropogenic global warming is real, it must be reckless for them to take a contrary position, unless they have a large and thoroughly-analysed body of evidence to support that contrary position. The corporate lobbyists and think tanks that take up global warming scepticism have no reason to suppose that they have such a body of evidence. They are, therefore, at best reckless, and they stand condemned by Clifford's standard.
This does not mean that there is anything wrong with their challenging the detail of the scientific claims that are made, searching for errors, and highlighting alternative interpretations of the data. It does, however, mean that they should not crow that there is no sound support for the consensus scientific view, when they have no good reason to think that they have established that the consensus scientific view is baseless. One can be reckless, not only in taking up an overall position, but in making claims about how much one has shown, or about the extent to which the things one has shown undermine the other side's view.
Now let us leave the example of global warming, and ask whether there is a specific ethical failing of opposing a consensus, assuming that:
(1) the opponent of the consensus is not reckless as to the quality of the evidence for his position;
(2) if the consensus is correct, failure to act as the consensus view would recommend would have serious consequences;
(3) if the consensus is mistaken, there would be significant waste in acting as the consensus view would recommend.
It seems to me that there is no ethical failing here, unless, perhaps, the opponent of the consensus is in a position of exceptional power, so that his position is very likely to prevail, whatever the merits of the arguments on his side and on the consensus side. Absent such power, the opponent is acting in a way that is, in general, likely to lead to the advancement of our knowledge. The opponent is simply engaged in subjecting other people's claims to criticism. Criticism exposes error, and can strengthen correct positions that show themselves able to withstand the criticism, as Mill remarked (On Liberty, chapter 2).
What if we leave out condition (3), and suppose that while serious loss would follow from failing to act on the consensus view if it were correct, there would be no serious loss from acting on the consensus view if it were incorrect?
We might take the attitude of Pascal's Wager: we might as well act on the consensus view anyway. Then it might seem that opposition to the consensus view would be reprehensible, if it would be likely to hinder that desirable course of action.
However, even assuming that action on the consensus view would be the only sensible thing to do, and that opposition might hinder that course of action, it would not follow that opposition would be reprehensible. Opposition could still fortify belief in the consensus view, as suggested by Mill, if the opposer's arguments were found wanting. And if we were to adopt Clifford's attitude, we would want opponents of the consensus to be as active as they could be, in order to root out any mistaken beliefs that might happen to be held by the majority. That would be so, even if retention of the beliefs in question would not lead to any immediate adverse consequences, whether they were mistaken or correct. It would be so, both because an accumulation of erroneous beliefs, individually harmless, might have adverse consequences long after the beliefs were acquired, and because truth is something that is to be valued in itself.