Imagine Earth

Because I skimmed through the tutorials, I wasn't sure about this but it's very easy.

Edit: I assumed it wasn't a factor but I did this on a mission world I had already completed, using the Gaian Temples. Per the comments from Draco719, it appears this will not unlock in a new world. Be sure to revisit a completed campaign world to do this.

Gaian Temples can create forests. Charge them with goods. I think they accepted mainly food, so be sure to build plenty of greenhouses and organic farms. Once you have some power, select the option at 9 o'clock in the temple. Now select a vacant tile and a forest will be built. Rinse and repeat.

Alternatively, your Park residential areas will create Seed Bombs overtime (this might be an upgrade?). They work in the same way.

The achievement didn't pop for me until I left the game. Be sure to count how many forest areas you have built and once you pass 20, quit out and it should unlock immediately.