Many people have pointed out that the right to free speech doesn't mean that you are allowed to shout "Fire!" in a crowded theater. I suppose that what Lomborg is doing isn't quite as bad; the theater is on fire, and he's shouting "Sit down, there's nothing to worry about!" That may put him just on the right side of the line, but I still wish he wouldn't do it.
As everyone who's worked with science knows, if you cherry-pick your facts to favor only the most extreme interpretations, you can support almost any position, and make it sound superficially plausible. That's what Lomborg has done here with the skeptical position on global warming. Unfortunately, it's not easy to refute him out of hand, unless you are yourself an expert; I had a long argument with a friend a couple of years ago, and spent several hours searching the web. The problem is that we don't yet know for sure what's going on, the models are all very complicated, and even the experts disagree. Lomborg is exploiting that disagreement, and making it sound like they think there is room for doubt; it's rather like the way that Intelligent Design people look at disagreements between evolutionary biologists, and then tell you that evolution's just a theory. This is disingenuous. They disagree over many details; they don't disagree about the basic fact of evolution.
The bottom line for me, is simple. Why ever should Lomborg, who has had no formal training in this subject, be able to get it right, when the very responsible IPCC (Intergovernmental Panel on Climate Change) are supposed to have got it wrong? They have hundreds of people working for them, they take all the competing viewpoints into consideration, and they are careful to give only the most conservative projections, the ones that everyone is prepared to sign up to. You have to postulate some kind of conspiracy among most of the world's scientists, which is frankly absurd. They don't have sufficient motivation, and science doesn't work that way.