Reddit reviews Global Warring: How Environmental, Economic, and Political Crises Will Redraw the World Map

Since they believe there is a scientific conspiracy, giving them hard data probably isn't going to be that helpful. Perhaps the most effective way to reach them would be to find a practical middle-ground which you can agree with them on.

With that in mind, I'd recommend Global Warring, by Cleo Paskal. It brushes the blame-game aside, and asks practical questions about specific issues we're going to face as the climate changes. For example, India's whole infrastructure is built around a very specific monsoon season happening, and that cycle is being disrupted. Another example is the recent opening of the northwest passage to ships. Hurricane Katrina is another big one.

The book looks at things like this that show how fragile our infrastructure is, and how finely tuned it is to one specific climate configuration. The argument is that no matter what you think about AGW, we have a whole lot of work ahead of us to adapt to the changing climate, and we have to do it now. The message I took away was that we have to start thinking about the climate as an object in motion, not as a fixed background condition.

Once our climate denier has dealt with that, maybe they'll be more willing to evaluate the evidence for a human hand in the process.