You can read his detailed thoughts here. The bottom line is:
- "...there is a broad consensus that the cost of climate inaction would greatly exceed the cost of climate action—it's cheaper to act than not to act. Reducing greenhouse gas emissions by moving to alternative energy sources and capturing carbon from coal-fired power plants will cost less in the long run than dealing with the effect of rising sea levels, drought, famine, wildfire, pestilence, and millions of climate refugees."
- the cost of acting is less than 1% of GDP through the middle of the century,
Is this affordable? It's about 1/4th of what we spend on the military, and about 1/2 what we spend on servicing our debt.
It is, of course, only a fraction of what we're currently spending to bail out banks and stimulate the economy. Last quarter the US GDP fell at an annual rate of 3.8% -- four times more than solving the climate problem entails.