A new assessment finds that the world’s oceans crossed the safe threshold for acidification in 2020, breaching a key planetary boundary and posing serious threats to marine life. Ocean acidification is caused when excess atmospheric carbon dioxide, resulting from human activities like burning fossil fuels, dissolves in seawater, forming carbonic acid that increases the water’s acidity. The reduced availability of carbonate ions can affect the survival of marine species that build calcium carbonate shells and skeletons, including coral, shellfish and crustaceans. For this study, researchers looked at a key indicator of ocean acidification called aragonite saturation state, a measure of how well seawater supports the formation of aragonite, a form of calcium carbonate. They estimated aragonite saturation state over time at different depths in the ocean. They also compared that information with biological tolerance thresholds for species like coral and sea snails, or the levels of aragonite saturation below which the marine animals experience stress. Previously, scientists established that a 20% drop in aragonite saturation, compared with preindustrial levels, was the threshold for breaching the ocean acidification planetary boundary. The last global assessment in 2023, led by Katherine Richardson with the Globe Institute at the University of Copenhagen, Denmark, found a 19% decrease. The new study’s findings confirm that this boundary has now been crossed. Richardson, who wasn’t involved with the latest research, told Mongabay by email she was “not at all surprised” by the new finding. “We said it was on the edge in our last assessment and, as…This article was originally published on Mongabay