Believing that it's work 1-5 trillion dollars by 2050 to reduce 2100 temperatures by 0.05C?
Believing that climate models which fail to match past or current trends are accurate enough to restructure the global economy? (Compare modeled and measured temperature trends of early 20th and early 21st centuries)
Both sides are guilty of talking past each other, ignoring ("denying") evidence that doesnt agree with their previous opinions, and overstating the strength of the evidence they see.
(I tend to fit in the "lukewarm" camp - my reading of the evidence and scientific research is that the earth has warmed, continues to warm, is affected by human activities, but that the warming and the human impact are routinely overstated or ignored by the warring advocates. Much less so by the actual scientists doing the research)