The more humanity acidifies and warms the world's oceans with carbon emissions, the harder we will have to work to save our coral reefs. That’s the blunt message from a major new study by an international scientific team, which finds that ocean acidification and global warming will combine with local impacts like over fishing and nutrient runoff to weaken the world's coral reefs right when they are struggling to survive. It is found that reefs already over fished and affected by land runoff are likely to be more vulnerable to increasing CO2 in the atmosphere caused by the burning of fossil fuels. Warmer conditions cause periodic mass coral deaths by bleaching, while acidifying sea water - due to CO2 dissolving out of the atmosphere - weakens the corals by interfering with their ability to form their skeletons, making them more vulnerable to impact by storms. If the corals are also affected by heavy nutrient runoff from the land - which fertilizes the algae - and over fishing of parrot fishes and others that keep the reefs clear of weed, then corals can struggle to re-establish after a setback. Safeguarding coral reefs in the 21st century will require urgent solutions to the global carbon problem as well as strong management of local disturbances.