I'm not a global warming denier; I accept the consensus view of climatologists that anthropogenic global warming is real. I am even willing to accept that it's worth doing something about.
It is my understanding, however, that the earth has been in a lengthy period of remarkably stable climate for some time, and that sometime "soon" (I'm not sure what that means in climatological terms--it could mean a thousand years from now as far as I know) we're due for some a cooling cycle. I can't find any nice summaries of the science on this, so let's just ignore my ignorance and suppose that we found out that global cooling was about to happen in the near future. Suppose also that this cooling would have serious implications for humans and for species around the world.
What would environmentalists want to do in this case? Would they be in favor of letting the world cool down, possibly leading to human suffering, and the extinction of many species? Would they be in favor of trying to warm the world somehow (not necessarily by increasing CO2 emissions--they might not like ocean acidification or some other side effect)? I don't know what their answer would be. Maybe different environmentalists would answer differently. I think the answer says something important about the opinions of the person answering, though.
I think economists would be in favor of doing whatever cost-benefit analysis supports, even if that means trying to manipulate global temperatures to avoid the costs of accommodating rapid climate change.