Every day we hear stories of global warming – melting glaciers, wildfires and drought. But there’s another problem, which may be even more dangerous to sea life, yet many people have never heard of it. It’s ocean acidification: the gradual change in pH as CO2 dissolves in the seas. The frightening thing is that it may already be too late to do much about it. There’s a lag in time between CO2 building up in the atmosphere and its effect on the sea. Current atmospheric levels are high enough to slow the growth of corals, and stop other animals from building shells. To learn more about the problem – and get a clear sense of what we are about to lose – John Pickrell joined Australian scientists on a marine survey vessel over the Great Barrier Reef. Here he learned that entire ecosystems could collapse along with fisheries and tourism. This story follows the desperate struggle of scientists to document changes and find solutions before it’s too late.