“Over the years, I’ve built up a network of people in the different collecting areas who helped me with seed collection,” Swift said.But while there’s a huge focus on planting trees, there’s little on where those seedlings will come from.

“You don't need to plant anything, you just stop whacking the system,” says Bronson Griscom, director of forest carbon science at the Nature Conservancy.

To calculate the ROI of tree cover, Kroeger and colleagues from the Conservancy and NASA, set out to identify where it would provide the largest health benefits – the greatest reduction of respirable particulate matter and heat for the highest number of people — and then measure how those benefits compare to the costs of the trees.