Emissions of carbon dioxide are not just warming the world; they are also turning the oceans more acidic. The United States' Secretary of State, John Kerry, will warn at an oceans summit in the coming weeks that CO2 pollution may drive countless marine species to extinction. The BBC's Environment Analyst, Roger Harrabin, reports for Our World on the dangers facing some of the planet's most spectacular underwater environments, off the coasts of Papua New Guinea and Australia.