A critical measure of the ocean’s health suggests that the world’s marine systems are in greater peril than scientists had previously realized and that parts of the ocean have already reached dangerous tipping points.

A study, published Monday in the journal Global Change Biology, found that ocean acidification—the process in which the world’s oceans absorb excess carbon dioxide from the atmosphere, becoming more acidic—crossed a “planetary boundary” five years ago.

“A lot of people think it’s not so bad,” said Nina Bednaršek, one of the study’s authors and a senior researcher at Oregon State University. “But what we’re showing is that all of the changes that were projected, and even more so, are already happening—in all corners of the world, from the most pristine to the little corner you care about. We have not changed just one bay, we have changed the whole ocean on a global level.”

The new study, also authored by researchers at the UK’s Plymouth Marine Laboratory and the National Oceanic and Atmospheric Administration (NOAA), finds that by 2020 the world’s oceans were already very close to the “danger zone” for ocean acidity, and in some regions had already crossed into it.

First proposed in 2009 (Rockström et al. 2009), the planetary boundaries assessment defines nine large scale Earth-system processes and associated boundaries that, if crossed, could generate unacceptable environmental change. These nine processes are: climate change, rate of biodiversity loss (terrestrial and marine), interference with the nitrogen and phosphorus cycles, stratospheric ozone depletion, ocean acidification, global freshwater use, change in land use, chemical pollution and atmospheric aerosol loading. Three boundaries had been crossed in 2009 (Rockström et al. 2009), increasing to four in 2015 (Steffen et al. 2015) and six in 2023 (Richardson et al. 2023). Ocean acidification (OA) was assessed as not yet having crossed the boundary, but lies at the margin of the safe operating space (Richardson et al. 2023). This remained the same conclusion in the Planetary Health Check published in 2024 (https://www.planetaryhealthcheck.org/).