Rates of ocean acidification not seen for the past 65 million years could result in unprecedented pressure on marine organisms with carbonate shells and body parts, according to a study published online this week in Nature Geoscience.

Andy Ridgwell and colleagues used an Earth-system model to compare past and future changes in ocean acidity. According to their simulations, the surface of the ocean is set to acidify more quickly than it did during a well-documented greenhouse warming event 55.5 million years ago. Equally unprecedented changes in the chemistry of the deep ocean are also expected. The scientists warn that future changes in ocean chemistry threaten both surface- and bottom-dwelling organisms.