We investigate the calcification rate of reef corals from an upwelling zone, where low seawater pH and high nutrient concentrations represent a recent analogue for the future ocean. Calcification rate of the corals largely relies on extension growth. Variable responses of extension growth to nutrients either compensate or exacerbate negative effects of weak skeletal thickening associated with low seawater pH – a mechanism that is critical for the persistence of coral reefs under global change.