There are natural carbon sinks which will reduce CO2 levels. The problem is we are digging up carbon vastly faster than they operate. So a 50% reduction would push things off by more than that. Also, their is a lot of inertia in having a carbon based economy, so a larger change at the start can be much cheaper long term.

Consider an electrified road would be cheaper than gas and no need to refill the tank, but we have ~zero cars that could use it right now. However, start building them and economics will finish the job. How to pay up to that tipping point? Well a new tax of some type...