The 5th assessment report of the IPCC is drawing a lot of attention for its claim that reducing greenhouse gas emissions would only have small costs in terms of gross social product, that “saving the planet” is cheaper than we might think. I have not read the report in detail and I am in no position to say whether the IPCC got it right or wrong.
However, it occurred to me that the IPCC is asking the wrong question. They ask how much mitigation policies would cost in terms of (world) GDP. But climate policy is a classic case where GDP is a very bad indicator of economic welfare. Greenhouse gas emissions are a byproduct of producing goods and services and they cause global warming, which is arguably a bad thing, i.e. a negative externality. Individual consumers and producers do not take account of the negative effects of their consumption/production decisions on society and therefore consume and produce too much.
Here is how that works in a simple model. (For the visual types, here is how it works graphically: climate policy graph.)