So if we stand back, as Al Gore asks us to do, and look at it from the coming generation's point of view, they are going to ask 'what were they thinking?' They tried to do a tiny little bit about climate change at a fairly high cost, but have done very little good, whereas there are many other problems that they could have tackled that would have left a much better world behind.
I'm firmly in Lomborg's camp here. Sure, people should do whatever they're passionate about to reduce human suffering. But some things are more effective than others, and pointing that out does not amount to discouraging people from acting at all.