Under the Paris Climate Agreement, countries must document all of their greenhouse gas emissions and sinks, and that means measuring changes in tree cover. NASA Senior Scientist Sassan Saatchi has spent 30 years helping the world's space agencies and foresters do just that by blending the newest science of the space age with centuries-old practices from ship-building and forestry. Thanks, to him, countries will soon be able track the growth and decay of every tree within their borders.