Ebola, HIV, SARS, Nipah virus, Lassa fever, Covid-19—what do they have in common? They’re deadly, of course, and economy-wrecking, of course—and all emerged in regions of increasing deforestation. What would the world have paid for Covid-19 alone to have remained dormant and undisturbed in the forest? It may feel cold-blooded to put a dollar value on nature, but without cold hard numbers it’s almost impossible to make policy. That’s why economist Stephen Polasky has dedicated his life to finding those numbers. Polasky, a pillar of the Stanford University-based Natural Capital Project and InVEST, NCP’s open-source calculator for various values of nature, believes you can only save nature—and the world—by literally putting a price on it.