Business as usual costs nothing extra but leads to a 6°C increase in temperatures (kinda apocalyptic; watch this). Spend $4.1 trillion (but save $7 trillion) to limit the increase to 3°C. Spend $9.3 trillion (but save $5.8 trillion) to limit it to 2°C. Although it looks like we can keep temperatures in the range of survivable at a net savings, it's the distribution of costs and benefits that's mucking things up. Unless we get a world dictator, we need some serious cooperation.