Off the top of my head, if levels were to drop below 120 ppm, we would see mass plant die off. Pre-industrial levels were already around 180 to 200 ppm. So even if we can stabilize right now around 400 ppm with renewables, AND start to sequester back to pre-industrial levels, we would need to also make sure we can turn that sequestration OFF at some point. And what is that point exactly? Optimal ppm CO2 level is actually very debatable. The dirty secret is that plant growth is most optimal around 1200 - 1500 ppm CO2. Likely because plants evolved in epochs of much higher levels. Stabilizing at todays 400 ppm is probably optimal.
We could very well get into a situation in the future where we are begging industry to STOP pulling CO2 out of the atmosphere for fear or crop failure, instead of pumping it out. Industry in China and India that might be outside of our sovereignty. It may take 1000 years, but levels will come down naturally if we can stabilize at 400 ppm. That stability will only come from technological advances like electric cars, planes, trucks and lab grown meats.
Given the state of the world right now, this just seems like a very unlikely scenario relative to the reverse of too much carbon.
I'm not saying we shouldn't consider it per se, but let's prioritize our energy where it matters.
- Remove CO2 using less energy than other methods
- Change it into C and CO
- Make best-class supercapacitors for energy storage from C.
In the case of the horse→car substitution in NY in the late 19th century, the optimal situation was that noone worries too much, and the car is invented, and all is well.
But they could not have known that it was a horse substitute which would be invented. Imagine if 19th century Londoners, faced with a bulging, over-capacity sewerage system, had rested easy in the knowledge that a breakthrough would sooner or later come along to save them.
It would have been catastrophic. No such breakthrough occurred and they did need to worry. In reality, they worried enough that they build a whole new system and there was never a crisis.
We should not make the mistake of hoping that the right technology will come along to fix Global warming.
Technological process is almost assured. But in any given area we have no idea how much will occur, and how soon.
If everyone hopes for such technology and no one tries to invent it, as you say, it won't come along.