Comment by internet_points

Yes, the Nature Conservancy is a large nonprofit that buys lands to hold in its natural state, albeit not at the scale needed to offset industrial activities. They tend to focus more on qualities like undisturbed ecosystems, or biodiversity, than climate change.

And in the U.S. at least, many states have a concept of a conservation easement where you get a tax advantage by promising not to disturb or develop land you own. This is used by some wealthy individuals to lock up a bunch of land undisturbed. But again, so far it is not remotely close to offsetting the overall human behaviors that are forcing warming. (As evidenced by the directly measured rising CO2 levels and temperature anomalies.)

Not exactly what you're asking for, but [Ecologi](https://ecologi.com/) is doing lots of work on the tree-planting front, but also doing other work that helps with climate change, like solar panel setups in Morocco, wind farms in the US, methane emissions in Brazil, and more.

Search for „rewilding“. It’s a popular approach in the UK but you’ll find projects in other countries, too.