Ocean acidification measurements show change at several scales: long-running Pacific time series, recent coral-reef surveys, modeled estuary records, Arctic observations, and global climatologies. The most consistent signals are rising dissolved inorganic carbon, falling pH, and declining aragonite saturation, although local conditions can vary sharply by season, depth, and geography.
Contents
- Long-term Pacific measurements
- Recent Pacific-island chemistry
- Coastal and estuary measurements
- Arctic and glacial-estuary variation
- Global reference ranges
- Regional indicator results
Long-term Pacific measurements
The California Current Ecosystem shipboard record provides a useful coastal-Pacific benchmark. From 2007–2021, dissolved inorganic carbon increased by 0.9 ± 0.1 µmol kg−1 yr−1, while pH declined by 0.002 ± 0.0005 units yr−1. Over the same period, aragonite saturation declined by 0.007 ± 0.002 yr−1. These are rates measured across the 2007–2021 observation period, not a single-year snapshot. The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America (s1).
A longer California Current System record, covering 1983–2021, found dissolved inorganic carbon increasing by 0.7 ± 0.05 µmol kg−1 yr−1. Its pH trend was a decline of 0.0015 ± 0.0001 units yr−1, and aragonite saturation declined by 0.0062 ± 0.0008 yr−1. The longer period gives a different rate from the 2007–2021 record, so the two estimates should be read as period-specific results rather than combined into one average. The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America (s1).
The Northeast Pacific Subarctic Gyre record from 1990–2020 showed dissolved inorganic carbon increasing by 0.6 ± 0.3 µmol kg−1 yr−1. pH declined by 0.0014 ± 0.0004 units yr−1, and aragonite saturation declined by 0.005 ± 0.002 yr−1. Compared with the California Current results, these estimates describe a different region and time window. The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America (s1).
Several fixed stations show why location matters. At Ocean Station Papa, 50°N and 145°W, the 2007–2014 mooring record measured a dissolved inorganic carbon increase of 1.0 ± 0.3 µmol kg−1 yr−1 and a pH decline of 0.001 ± 0.0005 units yr−1. Aragonite saturation changed by 0.01 ± 0.004 yr−1 in that record. At K2, 47°N and 160°E, the 1999–2015 shipboard time series measured a pH decline of 0.0025 ± 0.001 units yr−1 and an aragonite-saturation decline of 0.012 ± 0.005 yr−1. The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America (s1).
The Kuroshio Extension Observatory at 32°N and 144°E recorded, during 2007–2014, a dissolved inorganic carbon increase of 1.3 ± 0.2 µmol kg−1 yr−1, a pH decline of 0.001 ± 0.0005 units yr−1, and an aragonite-saturation decline of 0.010 ± 0.002 yr−1. At the HOT station, 22.75°N and 158°W, the 1988–2011 record measured dissolved inorganic carbon increasing by 1.78 ± 0.12 µmol kg−1 yr−1, pH declining by 0.0016 ± 0.0001 units yr−1, and aragonite saturation declining by 0.0084 ± 0.0011 yr−1. The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America (s1).
Recent Pacific-island chemistry
The National Coral Reef Monitoring Program reported calculated carbonate parameters from in-situ total alkalinity and dissolved inorganic carbon for surveys conducted during 2021–2023. At Rose Atoll in American Samoa, mean aragonite saturation was 4.0 ± 0.1, and mean pH was 8.06 ± 0.01. Across Taʻū, Ofu, Olosega, and Tutuila, mean aragonite saturation was 3.8 ± 0.2–0.3, while mean pH ranged from 8.03 ± 0.02 to 8.03 ± 0.06. Summary of ocean acidification data collected by the National Coral Reef Monitoring Program, 2021—2023 (s2).
The Mariana Archipelago survey found aragonite saturation from 3.6 to 3.7, with standard deviation ±0.1, across the surveyed islands. Mean pH ranged from 8.01 to 8.04. Around Oʻahu, mean aragonite saturation was 3.3 ± 0.2, and mean pH was 8.02 ± 0.04. Summary of ocean acidification data collected by the National Coral Reef Monitoring Program, 2021—2023 (s2).
The lowest Pacific-island values in this set were reported from Howland Island and Baker Island. At Howland Island, mean aragonite saturation was 3.3 ± 0.1, with mean pH 7.96 ± 0.02. At Baker Island, mean aragonite saturation was 3.1 ± 0.2, with mean pH 7.95 ± 0.04. These are survey means for 2021–2023, not long-term trend rates. Summary of ocean acidification data collected by the National Coral Reef Monitoring Program, 2021—2023 (s2).
| Survey location or group | Period | Mean or reported range |
|---|---|---|
| Rose Atoll | 2021–2023 | Aragonite saturation 4.0 ± 0.1; pH 8.06 ± 0.01 |
| Taʻū, Ofu, Olosega, Tutuila | 2021–2023 | Aragonite saturation 3.8 ± 0.2–0.3; pH 8.03 ± 0.02–0.06 |
| Mariana Archipelago | 2021–2023 | Aragonite saturation 3.6–3.7; pH 8.01–8.04 |
| Oʻahu | 2021–2023 | Aragonite saturation 3.3 ± 0.2; pH 8.02 ± 0.04 |
| Howland Island | 2021–2023 | Aragonite saturation 3.3 ± 0.1; pH 7.96 ± 0.02 |
| Baker Island | 2021–2023 | Aragonite saturation 3.1 ± 0.2; pH 7.95 ± 0.04 |
Coastal and estuary measurements
Coastal waters can show changes that differ from open-ocean trend records. In the middle mesohaline Chesapeake Bay, a modeled 1986–2015 record found that summer surface pH declined by 0.24 units per 30 years. Summer surface aragonite saturation declined by 0.9 units per 30 years. The same modeled record estimated that the annual duration of bottom water with pH below 7.5 increased by about 2 days yr−1. Mechanisms Driving Decadal Changes in the Carbonate System of a Coastal Plain Estuary (s5).
The Chesapeake Bay model also estimated that the aragonite saturation horizon became shallower by about 0.1 m yr−1 over 1986–2015. A shallower saturation horizon means the modeled depth boundary associated with aragonite conditions moved upward during that period. This result is specific to the modeled bay record and should not be treated as a direct measurement from every estuary. Mechanisms Driving Decadal Changes in the Carbonate System of a Coastal Plain Estuary (s5).
For the Salish Sea, a biogeochemical model estimated an average aragonite-saturation decrease of about 0.11 from preindustrial conditions in the upper 100 m. It also estimated an average pH decrease of about 0.06 over the same preindustrial-to-model-present comparison. These are model scenario results, not a dated monitoring-series trend. Chemical Exposure Due to Anthropogenic Ocean Acidification Increases Risks for Estuarine Calcifiers in the Salish Sea: Biogeochemical Model Scenarios (s6).
Arctic and glacial-estuary variation
The 2010–2011 Western Arctic baseline found that about 20% of surface waters in the combined Canada and Makarov basins were undersaturated with respect to aragonite. The percentage refers to the surface waters covered by that baseline and period, not to all Arctic waters in every year. Baseline Monitoring of the Western Arctic Ocean Estimates 20% of Canadian Basin Surface Waters Are Undersaturated with Respect to Aragonite (s3).
Glacier Bay illustrates the size of seasonal variation in a glacially dominated estuary. Surface aragonite saturation reached a minimum of 0.40 during summer 2011 and rebounded to a maximum of 3.26 in spring 2012. The two values come from different seasons and years, so they describe observed variability rather than a single annual average. The dynamic controls on carbonate mineral saturation states and ocean acidification in a glacially dominated estuary (s4).
Global reference ranges
The NDP-094 global surface-ocean climatology reported pH values from 7.9 to 8.2 outside coastal areas. In subtropical oceans, seasonal pH was about 8.05 in warmer months and 8.15 in colder months. These are climatological reference values, not measurements from one station or one date. OCADS - NDP-094 (s7).
The same climatology reported aragonite saturation up to about 4.2 and calcite saturation up to about 6.3 in warm tropical and subtropical waters. In cold subpolar and polar waters, aragonite saturation was as low as about 1.2, and calcite saturation was as low as about 2.0. These ranges provide geographic context for interpreting local observations. OCADS - NDP-094 (s7).
NOAA reported a 2024 global-average atmospheric pCO2 of 422.7 ppm for its ocean-acidification indicator context. A NOAA Science on a Sphere model projected an approximately 0.3-unit surface-ocean pH decrease during the 21st century under its emissions scenario. The projection is not a measurement and should be kept separate from the observed records above. OA Indicators Explained (s9); Ocean Acidification: Saturation State (s10).
Regional indicator results
NOAA’s regional indicators compare a recent five-year window with each region’s historical series. The results are percentile positions, not absolute pH or saturation values. In the Gulf of America, recent pH was below the historical 10th percentile, recent pCO2 was above the historical 90th percentile, and recent aragonite saturation was below the historical 10th percentile. OA Indicators Explained (s9).
The Hawaiʻi and Pacific Islands indicator region showed the same three-direction pattern: recent pH below the historical 10th percentile, pCO2 above the historical 90th percentile, and aragonite saturation below the historical 10th percentile. This comparison identifies unusual recent conditions relative to that region’s own historical series; it does not state the absolute pH or saturation level. OA Indicators Explained (s9).
In the Northeast U.S. indicator region, recent pH was below the historical 10th percentile and pCO2 was above the historical 90th percentile, while recent aragonite saturation remained between the historical 10th and 90th percentiles. In the Southeast U.S. indicator region, recent pH was below the historical 10th percentile, pCO2 was above the historical 90th percentile, and aragonite saturation was below the historical 10th percentile. OA Indicators Explained (s9).
Together, these statistics show why ocean acidification measurement needs multiple indicators. Long-term records quantify rates of change, surveys describe present carbonate chemistry, models estimate historical or future conditions, climatologies establish broad reference ranges, and percentile indicators show whether recent regional values are unusual within their own records.