You mean geo-engineering specifically, or just a transition to a renewable economy? Olivine weathering and ocean cloud seeding both seem like reasonably likely

You mean geo-engineering specifically, or just a transition to a renewable economy? Olivine weathering and ocean cloud seeding both seem like reasonably likely solutions that each cost on the order of 10's of billions. Major doubts that addressing carbon will be a big money maker. But obviously the carbon economy is huge, so the renewable economy will at least that big.