In the third year of the sweeping global PBS series “ Changing Planet ,” Conservation International CEO M. Sanjayan explores how climate change is affecting some of Earth’s most vulnerable ecosystems — and the groundbreaking science that’s offering hope. The newest installment of this seven-year series takes Sanjayan to the Maldives and the Florida Keys, where he follows pioneering scientists for an up-close look at their efforts to protect coral reefs. Known as the “rainforests of the sea,” coral reefs support a quarter of all marine species and are the cornerstone of healthy ocean ecosystems. Not only do they provide habitats for more than a million plants and animals , they also help protect coastlines from storm surges, and are closely connected to seagrass beds and mangroves — both of which are powerhouses when it comes to sequestering planet-heating carbon. Globally, more than 500 million people rely on coral reefs for food and livelihoods. But they’re quickly vanishing .…