Hollywood legend Robert Downey Jr. is putting down his Iron Man suit and taking up the bigger challenge of cleaning up the Earth.

Iron Man’s on-screen nemesis Thanos had a radical way of reducing humanity’s impact on the Earth, but Downey Jr. hopes to use a more humane solution.

Appearing on-stage at Amazon’s new re:MARS (Machine Learning, Automation, Robotics and Space) conference in Las Vegas, Downey Jr. launched an initiative called Footprint...