I'm not sure what you are trying to recall, but this is incorrect as written. The atmosphere stays well-mixed up to about 100 kilometers:
https://en.wikipedia.org/wiki/Homosphere
Any silicate weathering process that binds CO2 at the Earth's surface will also remove atmospheric CO2 that exists at 30 km from the surface.
The atmospheric CO2 excess was created by generations of humans all over the world. Barring breakthroughs like self-replicating machinery, fixing it will also require global scale efforts over multiple generations. I am not optimistic about people even starting serious efforts toward that end in my lifetime. My pessimism is more because of the misaligned incentives than the huge numbers involved.
If you told an engineer 150 years ago that the world would burn over 4 billion tons of coal in 2013, and asked them to plan how that would happen, it would seem impossible too. Nobody could really plan an endeavor of such scale. And in fact it didn't come from one globally coordinated plan, just a set of incentives that diffused globally. Sadly for the climate, the incentives to stop burning fossils do not spread as easily as the incentives to start.