According to Wikipedia, a climate change denier is a person who does not accept “the scientific consensus on the rate and extent of global warming, its significance, or its connection to human behavior, in whole or in part.” Now what does that mean?
Let’s start with the “scientific consensus”. Per Cook et al (2013), the scientific consensus is that the earth's climate is warming and human activity is largely responsible: a very general statement. Once you get to the particulars, like how much the earth has warmed so far, how fast it is currently warming, and how much it will continue to warm, there is no consensus. There is a range of opinion within the scientific community. And that is how it should be – this is a complicated field of study.
So the scientific consensus is that the climate is warming and it’s mostly because of human activity. But then the Wikipedia definition of climate change denier indicates there is a consensus on the rate, extent, and significance of global warming. To repeat: there is not. There are different estimates and projections on the rate and magnitude of climate change. There are ranges with varying degrees of confidence. The models are still being worked out. This is not settled science.
As noted, the Wikipedia definition of denier also encompasses anyone who doesn't accept the consensus on the "significance" of climate change. I’m guessing significance refers to predicted effects of global warming and what actions must be taken to mitigate or adapt to those effects. Thus, if you think the impact will be harmless or even beneficial, that puts you in the denier camp. If you take a “wait-and-see” attitude to global warming, confident that “a technological fix is bound to come along when we really need it, you’re a denier. Ditto anyone who advocates incremental and/or purely market-based approaches to climate change, because these approaches are just too wimpy given the enormity of the threat.
The Wikipedia definition of climate change denier ends with “in whole or in part”. It follows that a person is a denier if she disagrees with any part of the list (global warming rate, extent, significance, or human connection), According to this definition, then, a denier could actually believe in anthropogenic climate change but doubt its significance –that is, inevitable crisis and catastrophe unless humanity gets its act together, starting now.
That’s an awfully wide net.
And note its haul is much larger than the totality of those who deny the actual scientific consensus, which is simply that “humans are causing global warming” (Cook et al, 2003)
Reference:
Cook, J., Nuccitelli, D., Green, S.A., Richardson, M., Winkler, B., Painting, R., Skuce, A., 2013. Quantifying the consensus on anthropogenic global warming in the scientific literature. Environmental Research Letters 8 (2), 024024.