Climate sceptics resent being called deniers because of the odium associated with Holocaust revisionism.
Even critics of the sceptics are careful to distance themselves from the implication that they are comparing climate denialism with Holocaust denialism for fear of being seen to trivialise the Holocaust by suggesting some sort of moral equivalence.
Judgments about moral equivalence depend on the ethical standpoint one adopts.
For consequentialists the morality of an action is judged by its outcomes. For those who adopt this ethical standpoint, any assessment of the consequences of the two forms of truth-rejection would conclude that climate deniers deserve greater moral censure than Holocaust deniers because their activities are more dangerous.
If the David Irvings of the world were to succeed, and the public rejected the mountain of evidence for the Holocaust, then the consequences would be a rewriting of history and a probable increase in anti-Semitism.
If the climate deniers were to succeed, and stopped the world responding to the mountain of evidence for human-induced global warming, then hundreds of millions of mostly impoverished people around the world would die from the effects of climate change.
They will die from famine, flood and disease caused by our unwillingness to act. The Stern report provides some sobering estimates: an additional 30-200 million people at risk of hunger with warming of only 2-3°C; an additional 250-500 million at risk if temperatures rise above 3°C; some 70-80 million more Africans exposed to malaria; and an additional 1.5 billion exposed to dengue fever.
Instead of dishonouring the deaths of six million in the past, climate deniers risk the lives of hundreds of millions in the future. Holocaust deniers are not responsible for the Holocaust, but climate deniers, if they were to succeed, would share responsibility for the enormous suffering caused by global warming.
It is a ghastly calculus, yet it is worth making because the hundreds of millions of dead are not abstractions, mere chimera until they happen. We know with a high degree of certainty that if we do nothing they will die.
But not everyone adopts a consequentialist ethic. An alternative ethical stance is to judge climate deniers not by the effects of what they do but by the rightness of their activities (a so-called duty ethic) or by their character and motives (a virtue ethic).
From a duty ethic position, the moral obligation climate deniers are violating is to the truth. Here there is a moral difference between denying the commission of a great crime, for which there are whole libraries of documentation, and rejecting the overwhelming evidence from science in which uncertainties nevertheless persist. This suggests that climate deniers are less culpable.
From a virtue ethic standpoint, moral culpability depends on motives. Attempting in good faith to uncover the facts is a good thing, which is why we regard genuine scepticism as healthy. Denialism is not scepticism but a refusal to accept the facts, the rejection of all of the evidence.
We think of Holocaust deniers as being immoral because we suspect them of being motivated by anti-Semitism or a desire for political advancement through stirring up racial hatred.
We think of climate deniers as being immoral because we suspect them of being motivated, not by truth-seeking, but by political goals, a desire for funds from fossil-fuel companies or personal aggrandisement.
Those who adopt a duty or virtue ethic would probably feel more personal antipathy towards a David Irving than towards an Ian Plimer or Andrew Bolt. There is something especially repugnant, even evil, about Holocaust denial. Denying or covering up a monstrous crime makes Holocaust deniers somehow complicit in it.
Better to have your daughter marry a climate sceptic, who is perhaps motivated by contrarianism, foolishness or self-importance rather than wickedness.
If, like me, you adopt a virtue or duty ethic, but one tempered by consideration of the consequences of an act, climate deniers are less immoral than Holocaust deniers, although they are undoubtedly more dangerous.
However, as the casualties from a warming world mount over the next decades, the denialism of those who continue to reject the scientific evidence will come to be seen as more and more iniquitous. So the answer to the question of whether climate denialism is morally worse than Holocaust denialism is no, at least, not yet.
Clive Hamilton is the Greens candidate in the Higgins by-election.