Why bet the bank on us humans collectively being able to reduce all our emissions? There are problems will all approaches, a quick Google search pokes some holes in why planting trees is not a good idea: https://earthscience.stackexchange.com/questions/2639/how-ma...

So the billion dollar reward would be quite small sum in the grand scheme of things. And for example with a carbon tax, where collectors could offset emitters, it might also become a quite lucrative business.