Objection: The kind of drastic actions required to mitigate global warming risk the destruction of the global economy and the deaths of potentially billions of people.
Answer: Is this supposed to mean the theory of anthropogenic global warming must be wrong? You can not come to a rational decision about the reality of a danger by considering how hard it might be to avoid. First things first: understand that the problem is real and present.
Once you acknowledge the necessity of addressing the problem, taking action suddenly become less daunting. There is no point in discussing the best solutions or the cost of those solutions with someone who does not yet acknowledge the problem.
But even if mitigating global warming would be harmful, given that famine, droughts, disease, loss of major coastal cities, and a tremendous mass extinction event are on the table as possible consequences of doing nothing, it may well be we are faced with a choice between the lesser of two evils. I challenge anyone to conclusively demonstrate that such catastrophes as listed above await us if we try to reduce fossil fuel use.
Now, in terms of conservation and a global switch over to alternative fuels, the people who oppose doing this for climate change mitigation are forgetting something rather important. Fossil fuels are a non-renewable resource, and as such we have to make this global economic transformation regardless, whether now or a bit later. Many bright minds inside the industry think we are already at peak oil. So even if it turned out that climate mitigation was unnecessary, we would still be in a better place as a global society by making the coming switch sooner rather than later.
Seems like a win-win situation to me.