Read here. Research finds more CO2 enhances coral growth.
"Many are the people who have predicted that rates of coral calcification, as well as the photosynthetic rates of their symbiotic algae, will dramatically decline in response to what they typically refer to as an acidification of the world's oceans, as the atmosphere's CO2 concentration continues to rise in the years, decades and centuries to come. As ever more pertinent evidence accumulates, however, the true story appears to be just the opposite of what these climate alarmists continue to tell us."