I live in Washington, and travel around the Pacific Northwest often. It's a beautiful environment, full of huge plains of grass with (virtually) nothing in them.
I was wondering if, in order to help combat climate change, I (and a lot of other people) engaged in "Guerilla Gardening". According to these pages, one tree sucks up ~12 tons of CO2 over its lifetime, and 1 ppm is equal to 2B tons.
By this logic, in order to go from 400 ppm -> 300 ppm, we would need to plant about 16B trees. While this seems like a very large number, you could have large numbers counteract it (e.g. rally 1M people over 20 years). The problem is where to go about it - this is where guerilla gardening would come in.
What if whenever I saw one of these huge plains, I stopped, jumped out of my car, threw 20 (locally flora appropriate) seeds in the ground, and drove on. Would that work? Would that violate some property/state/federal issue? Would it help?