20 years is an incredibly quick timespan in which to do this. If we can become carbon neutral by 2040 and carbon negative after, we can stave off many of the worst parts of climate change.
Cleaning up 150 years of emissions in 20 or 40 or even 60 years seems like a miracle on par with the moon landing or eradicating smallpox.
In front of me right now I have the June issue of the IEEE Spectrum special report, titled "Blueprints for a Miracle: 10 Technologies for a More Livable Planet". Its highlights include carbon-eating fuel cells; turbines driven by super-critical CO2 (nearly "free" carbon capture, courtesy the Allam cycle); a plant in Iceland producing methanol fuel from renewable (geothermal) energy and waste CO2; a pure-electric airplane; plant-based and in-vitro meats; and a sodium-cooled travelling-wave nuclear reactor with decent performance.... It seems to me like we're coming to the fight awfully late, but we may also have more weapons than we'd realized.
Also, let's not forget that our emissions are still significant (and growing) and that feedbacks will keep on warming the planet regardless of what we do.
The cost of doing something is not the same as an economic loss from doing it.