On the eve of year-end climate talks in Madrid, I revisit my 2017 conversation with Bronson Griscom, Director of Forest Carbon Science for the Nature Conservancy.  He headed up a team of three dozen researchers from almost two dozen institutions tasked with identifying once and for all the realistic potential of using nature as a bulwark against climate change.  The result is a report called "Natural Climate Solutions", which identifies 20 low-cost, natural "pathways" that can get us 37 percent of the way to meeting the Paris Climate Agreement targets -- sometimes at no cost, sometimes at just $10 per ton, and often while increasing food yields and reducing the cost of farming.