Why not both - our current "forward" momentum is increasing greenhouse gases and, even if we switch fully to renewables with minimal carbon footprint in a very short time-frame, that momentum will continue to have effects. If we apply the "backward" momentum and start removing carbon while also switcing production to green, it should stop the negative effects more quickly.

Of course, I'm all for thorough planning and extensive, well-funded and organized research first - unfortunately, we can't seem to start doing one of these things seriously, let alone all three...