Vast subject, but the most immediate item is simple.  Anyone talking about the costs -- "can we afford the cost of making things efficient? can we afford renewable energies?" -- should remember two things, in my view:

1. Making things efficient and developing renewables are in themselves economic activities. In fact, it would seem that state money invested in subsidizing renewables adds a multiple of itself to the economy of a country, at least a developed one. This is obviously a result of supply and demand, i.e. enough people and organizations are convinced of the utility that they are prepared to pay good money for it (thank goodness).

2. Not paying these costs actually costs more -- viz. natural disasters, cost of making emissions tolerable for human health (a non-optional cost), etc.