Joel Sartore photographs like he's running out of time. The man behind National Geographic Photo Ark, an extraordinary collection of 18,000 animal portraits and counting, is 20 years into a project that stretches beyond the horizon. Four years ago, when CNN last interviewed Sartore, he'd produced 13,000 portraits. Since then he's added another 5,000. The 64-year-old is speeding up. "The clock," he said, "is ticking." Sartore's introspection is, perhaps, unavoidable. "I could see the end of my life at the start of the project," he said. With well over a million species formally described by science, documenting even a fraction is a colossal feat. Plus, many of his dream subjects teeter on the brink of extinction. Taking their picture is, for both animal and portraitist, a sprint. Continue reading at the link in cnn's bio. 📸: Cole Sartore/National Geographic Photo Ark; Joel Sartore/National Geographic Photo Ark #calltoearth