There seems to be unanimous consent that there are a handful of topics or behaviors that will get you banned and we've had a few reminders over the years. I have no reason to believe that the poster involved wouldn't be welcomed back if it was clear that he understood that climate denialism wasn't welcome.

This is, after all, DU. When we say that we're discussing things "related to environmental issues and energy policy", it goes without saying that the reason we're talking about them is because of climate change. It isn't unreasonable to be "skeptical" of some aspects, particularly as prior predictions turn out to be wrong... but outright denialism is over the line.

I don't know the poster's history well enough to judge how "outright" the denialism is, but I'm not aware of any elected/prominent Democrats who deny that mankind is having a negative impact on our climate... so I think it's reasonable to question why that kind of poster survives the frozen pizza on DU at all... let alone in an environmental group.