I'm seriously considering doing this myself. I've got some money set aside, and a smallish group (~20) people who are interested in contributing time, smaller amounts, etc. I'm no tech billionaire, but there's definitely areas where I could buy a few dozen acres of land.

I've been researching what's involved in reforesting, and it looks like a ton of work and a non-trivial cost. And maybe not the most efficient dollar / CO2 ratio, but also something that has the nice side effect of having a living forest around. (And also the side effect of providing exercise, access to the outdoors, etc.)