How we track ocean acidification

What the dashboard shows, where the numbers come from, how current they are, and what they can’t tell you.

The short answer

The ocean absorbs about 30% of the carbon dioxide released into the air. In seawater it forms carbonic acid, so as CO₂ rises the ocean’s pH falls. Seawater is still basic, above 7, but it is becoming less so. Worldwide, surface pH has fallen by about 0.1 since the industrial revolution, roughly 30% more acidic (NOAA). This dashboard shows the cause, CO₂ measured at Mauna Loa, and the effect, pH measured at sea off Hawaii since 1988.

How we know

CO₂ in the air. NOAA has measured carbon dioxide at Mauna Loa Observatory in Hawaii since 1958, the record known as the Keeling Curve. We download NOAA’s daily and monthly files every night.

pH in the sea. The Hawaii Ocean Time-series has sampled Station ALOHA, about 100 km north of Oʻahu, roughly once a month since October 1988. Its scientists measure the carbon dissolved in seawater and its alkalinity, and from those calculate the pH at sea temperature and the aragonite saturation; since 1992 they also measure pH directly, and both give the same trend. The surface data are published as a public file, updated about once a year. Over 1988–2024 the pH there fell by about 0.019 units per decade, within the range the IPCC gives for the open ocean since the late 1980s (0.017 to 0.027 per decade).

Shell builders. Aragonite saturation says how readily aragonite, the form of calcium carbonate corals and many shells are made of, forms in seawater. Below 1 it dissolves. Off Hawaii it is still well above 1, but it has fallen steadily.

The future. The IPCC’s Sixth Assessment Report gives the projected fall in global surface pH from 1870–1899 to 2080–2099: about 0.16 under low emissions and 0.44 under very high emissions. Under the lowest-emission scenarios pH falls until about 2070 and then rises slightly; otherwise it keeps falling all century.

How current each part is

• CO₂ at Mauna Loa: NOAA updates daily; we download every night.
• pH and saturation off Hawaii: the file is updated about once a year (latest: 1 January 2026, with cruises through December 2024); we check it every night.
• Worldwide figures and projections: NOAA and the IPCC, updated when they publish new assessments.

What the dashboard can’t tell you

Station ALOHA is warm, open, subtropical ocean. Coastal waters, upwelling regions and polar seas acidify differently: rivers, upwelled deep water and cold temperatures can make them far more corrosive, and that is where shellfish farms and pteropods are hit first. One station is a reliable record of the open-ocean trend, not a map of the world’s seas.

It cannot say how a particular species or fishery will fare. Some animals cope with lower pH better than others, and acidification acts together with warming and oxygen loss.

Further reading

• Dore et al. (2009), PNAS: physical and biogeochemical modulation of ocean acidification in the central North Pacific, the Station ALOHA record. doi.org/10.1073/pnas.0906044106
• Doney et al. (2009), Annual Review of Marine Science: ocean acidification, the other CO₂ problem. doi.org/10.1146/annurev.marine.010908.163834
• Feely et al. (2008), Science: evidence for upwelling of corrosive, acidified water onto the continental shelf. doi.org/10.1126/science.1155676
• Bates et al. (2014), Oceanography: a time-series view of changing ocean chemistry, including the Bermuda record. doi.org/10.5670/oceanog.2014.16
• Bednaršek et al. (2012), Nature Geoscience: extensive dissolution of live pteropods in the Southern Ocean. doi.org/10.1038/ngeo1635
• Barton et al. (2012), Limnology and Oceanography: Pacific oyster larvae and naturally elevated CO₂ at an Oregon hatchery. doi.org/10.4319/lo.2012.57.3.0698
• IPCC (2019), Special Report on the Ocean and Cryosphere, Summary for Policymakers. ipcc.ch/srocc

Credits

The CO₂ record comes from NOAA’s Global Monitoring Laboratory, continuing the measurements Charles David Keeling started in 1958. The ocean record comes from the Hawaii Ocean Time-series at the University of Hawaiʻi, whose surface carbon data product is maintained by John Dore, funded by the US National Science Foundation. We only download and present their public files.