Laboratory experiments reveal calcification and extension rates of corals are strongly correlated to seawater aragonite saturation state. Predictions of reduced coral calcification rates, due to ocean acidification, suggest that coral reef communities will undergo ecological phase shifts as calcifying organisms are negatively impacted by changing seawater carbonate chemistry. This project will assess the effects of changes in seawater chemistry and temperature on calcification and extension rates of Porites spp. corals. This project furthers an existing partnership between CRED and Dr. Anne Cohen (WHOI), who has developed CAT scanning techniques to measure coral growth rates, evaluate gradients in skeletal density, and identify structural bioerosion rates. Dr. Cohen has developed a state of the art non-destructive, fully three-dimensional, CAT scan and image analysis technique that quantifies the size and density of annual growth bands in coral skeletons far more reliably that prior methods constrained by core axis. The coral cores were collected by SCUBA divers during Pacific RAMP cruises, stored in accordance with Dr. Cohen protocol, and sent to WHOI for analysis. Through comparative analyses across natural gradients, this project will assist efforts determining whether key reef-building taxa can acclimatize to changing carbonate chemistry environments. These data will have immediate, direct impacts on predictions of reef resilience in a higher CO2 world and on the design of reef management strategies.