Climate Change Indicators: Ocean Acidity | US EPA (2022)

This indicator describes changes in the chemistry of the ocean thatrelate to the amount of carbon dioxide dissolved in the water.

Key Points | Background | About the Indicator | About the Data | Technical Documentation

Key Points

  • Measurements made over the last few decades have demonstrated that ocean carbon dioxide levels have risen in response to increased carbon dioxide in the atmosphere, leading to an increase in acidity (that is, a decrease in pH) (see Figure 1).
  • Historical modeling suggests that since the 1880s, increased carbon dioxide has led to lower aragonite saturation levels in the oceans around the world, which makes it more difficult for certain organisms to build and maintain their skeletons and shells (see Figure 2).
  • The largest decreases in aragonite saturation have occurred in tropical waters (see Figure 2); however, decreases in cold areas may be of greater concern because colder waters typically have lower aragonite saturation levels to begin with.4

Background

The ocean plays an important role in regulating the amount of carbon dioxide in the atmosphere. As atmospheric concentrations of carbon dioxide rise (see the Atmospheric Concentrations of Greenhouse Gases indicator), the ocean absorbs more carbon dioxide. Because of the slow mixing time between surface waters and deeper waters, it can take hundreds to thousands of years to establish this balance. Over the past 250 years, oceans have absorbed about 28 percent of the carbon dioxide produced by human activities.1

Although the ocean’s ability to take up carbon dioxide prevents atmospheric levels from climbing even higher, rising levels of carbon dioxide dissolved in the ocean can have a negative effect on some marine life. Carbon dioxide reacts with sea water to produce carbonic acid. The resulting increase in acidity (measured by lower pH values) changes the balance of minerals in the water. This makes it more difficult for corals, some types of plankton, and other creatures to produce a mineral called calcium carbonate, which is the main ingredient in their hard skeletons or shells. Thus, declining pH can make it more difficult for these animals to thrive. This can lead to broader changes in the overall structure of ocean and coastal ecosystems, and can ultimately affect fish populations and the people who depend on them.2 Signs of damage are already starting to appear in certain areas.3

While changes in ocean pH and mineral saturation caused by the uptake of atmospheric carbon dioxide generally occur over many decades, these properties can fluctuate over shorter periods, especially in coastal and surface waters. For example, increased photosynthesis during the day and during the summer leads to natural fluctuations in pH. Acidity also varies with water temperature.

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About the Indicator

This indicator describes trends in pH and related properties of ocean water, based on a combination of direct observations, calculations, and modeling.

Figure 1 shows pH values and levels of dissolved carbon dioxide at four locations that have collected measurements consistently over the last few decades. These data have been either measured directly or calculated from related measurements, such as dissolved inorganic carbon and alkalinity. Data come from two stations in the Atlantic Ocean (Bermuda and the Canary Islands), one in the Caribbean Sea (Cariaco Basin), and one in the Pacific (Hawaii).

The global map in Figure 2 shows changes over time in aragonite saturation level. Aragonite is a specific form of calcium carbonate that many organisms produce and use to build their skeletons and shells, and the saturation state is a measure of how easily aragonite can dissolve in the water. The lower the saturation level, the more difficult it is for organisms to build and maintain their protective skeletons and shells. This map was created by comparing average conditions during the 1880s with average conditions during the most recent 10 years (2006–2015). Aragonite saturation has only been measured at selected locations during the last few decades, but it can be calculated reliably for different times and locations based on the relationships scientists have observed among aragonite saturation, pH, dissolved carbon, water temperature, concentrations of carbon dioxide in the atmosphere, and other factors that can be measured. Thus, while Figure 2 was created using a computer model, it is based on measurements.

About the Data

Indicator Notes

This indicator focuses on surface waters, which can absorb carbon dioxide from the atmosphere within a few months.12It can take much longer for changes in pH and mineral saturation to spread to deeper waters, so the full effect of increased atmospheric carbon dioxide concentrations on ocean acidity may not be seen for many decades, if not centuries. Studies suggest that the impacts of ocean acidification may be greater at depth, because the aragonite saturation level is naturally lower in deeper waters.13

Ocean chemistry is not uniform around the world, so local conditions can cause pH or aragonite saturation measurements to differ from the global average. For example, carbon dioxide dissolves more readily in cold water than in warm water, so colder regions could experience greater impacts from acidity than warmer regions. Air and water pollution also lead to increased acidity in some areas.

Data Sources

Data for Figure 1 came from four studies: the Bermuda Atlantic Time-Series Study, the European Station for Time-Series in the Ocean (Canary Islands), the Carbon Retention in a Colored Ocean (CARIACO) program (Cariaco Basin), and the Hawaii Ocean Time-Series. Bermuda data are available at: http://bats.bios.edu. Canary Islands data are available at: www.plocan.eu/en/open-ocean-observatory. Cariaco Basin data are available at: www.imars.usf.edu/cariaco. Hawaii data are available at: https://hahana.soest.hawaii.edu/hot/.

The map in Figure 2 was created by the National Oceanic and Atmospheric Administration and the Woods Hole Oceanographic Institution using Community Earth System Model data. Related information can be found at: https://sos.noaa.gov/catalog/datasets/ocean-acidification-saturation-state/.

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Technical Documentation

  • Download related technical information PDF

References

1Calculated from numbers in the IPCC Fifth Assessment Report. From 1750 to present: total human emissions of 545 Pg C and ocean uptake of 155 Pg C. Source: IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg1.

2Wootton, J.T., C.A. Pfister, and J.D. Forester. 2008. Dynamic patterns and ecological impacts of declining ocean pH in a high-resolution multi-year dataset. P. Natl. Acad. Sci. USA 105(48):18848–18853.

3Bednaršek, N., G.A. Tarling, D.C.E. Bakker, S. Fielding, E.M. Jones, H.J. Venables, P. Ward, A. Kuzirian, B. Lézé, R.A. Feely, and E.J. Murphy. 2012. Extensive dissolution of live pteropods in the Southern Ocean. Nat. Geosci. 5:881–885.

(Video) Ocean Acidity

4Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

5Recreated from Environment Canada. 2008. The pH scale.

6IPCC (Intergovernmental Panel on Climate Change). 2014. Climate change 2014: Impacts, adaptation, and vulnerability. Working Group II contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/ar5/wg2.

7Bates, N.R. 2016 update to data originally published in: Bates, N.R., M.H. Best, K. Neely, R. Garley, A.G. Dickson, and R.J. Johnson. 2012. Indicators of anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean. Biogeosciences 9:2509–2522.

8González-Dávila, M. 2012 update to data originally published in: González-Dávila, M., J.M. Santana-Casiano, M.J. Rueda, and O. Llinás. 2010. The water column distribution of carbonate system variables at the ESTOC site from 1995 to 2004. Biogeosciences 7:3067–3081.

9University of South Florida. 2021. Carbon Retention in a Colored Ocean (CARIACO) Ocean Time-Series Program. Accessed March 2021. https://imars.usf.edu/pages/projects/cariaco.

10University of Hawaii. 2021. Hawaii Ocean Time-series (HOT). Accessed February 2021. https://hahana.soest.hawaii.edu/hot.

11Woods Hole Oceanographic Institution. 2016 update to data originally published in: Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

(Video) EPA: Coastal acidification

12Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

13Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a high-CO2 world. Oceanography 22(4):36–47.

FAQs

What is a good indicator for ocean acidification? ›

Thus, several key water quality parameters (alkalinity, partial pressure of CO2, concentration of dissolved inorganic carbon – DIC, and the seawater pH) serve as environmental indicators for ocean acidification.

How does climate change affect ocean acidity? ›

Because of human-driven increased levels of carbon dioxide in the atmosphere, there is more CO2 dissolving into the ocean. The ocean's average pH is now around 8.1 , which is basic (or alkaline), but as the ocean continues to absorb more CO2, the pH decreases and the ocean becomes more acidic.

What are the 3 indicators of climate change? ›

Geneva, 18 May 2022 (WMO) - Four key climate change indicators – greenhouse gas concentrations, sea level rise, ocean heat and ocean acidification – set new records in 2021.

How do you measure the acidity of the ocean? ›

Acidity is commonly measured using the pH scale. Pure water has a pH of about 7, which is considered neutral. A substance with a pH less than 7 is considered to be acidic, while a substance with a pH greater than 7 is considered to be basic or alkaline. The lower the pH, the more acidic the substance.

How can acidity be measured? ›

The pH scale is commonly used to represent hydrogen ion activity. On the pH scale, pH values below 7 represent acidic solutions (hydrogen ion activity greater than hydroxide ion activity) while values above 7 represent basic solutions. At pH = 7, hydrogen ion and hydroxide ion activity are equal.

Does climate change affect pH? ›

Key Points. Measurements made over the last few decades have demonstrated that ocean carbon dioxide levels have risen in response to increased carbon dioxide in the atmosphere, leading to an increase in acidity (that is, a decrease in pH) (see Figure 1).

How does CO2 make the ocean more acidic? ›

As the amount of carbon dioxide in the atmosphere rises, the oceans absorb a lot of it. In the ocean, carbon dioxide reacts with seawater to form carbonic acid. This causes the acidity of seawater to increase.

What is the main cause of ocean acidification? ›

Ocean acidification refers to a reduction in the pH of the ocean over an extended period of time, caused primarily by uptake of carbon dioxide (CO2) from the atmosphere.

What are the 10 indicators of climate change? ›

Ten Signs of Global Warming
  • Arctic sea ice extent is diminishing.
  • Ocean heat content is increasing.
  • Air temperature over ocean is increasing.
  • Sea surface temperature is increasing.
  • Global sea level is rising.
  • Humidity is increasing.
  • Temperature of the lower atmosphere is increasing.
  • Air temperature over land is increasing.
4 Apr 2017

What is the largest indicator of climate change? ›

1. Air temperatures over land are increasing. It's clear that weather stations on land show average air temperatures are rising, and as a result, the frequency and severity of droughts and heat waves are increasing.

What are climate indicators? ›

The Global Climate Indicators are a set of parameters that describe the changing climate without reducing climate change to only temperature. They comprise key information for the most relevant domains of climate change: temperature and energy, atmospheric composition, ocean and water as well as the cryosphere.

What determines the acidity or alkalinity of seawater? ›

Acidity or alkalinity of a solution is determined by the amount of hydrogen ions [H+], using the pH scale (pH = -log[H+]). Pure water has pH 7; solutions below pH 7 are acidic, and above pH 7 are alkaline, or basic. More hydrogen ions lower the pH and increase acidity (decrease alkalinity).

Are oceans becoming more acidic? ›

Even though the ocean is immense, enough carbon dioxide can have a major impact. In the past 200 years alone, ocean water has become 30 percent more acidic—faster than any known change in ocean chemistry in the last 50 million years.

How do you measure acidity or alkalinity? ›

Basically, acidity is determined by titrating the sample with sodium hydroxide to a pH of 8.3 (often called the phenolphthalein acidity – this term dates back to the time before electronic pH meters). Alkalinity is determined by titration with sulfuric acid to a pH of 4.5.

What are the two terms used to measure acidity? ›

pH is a measure of how acidic/basic water is. The range goes from 0 - 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water.

Why do we measure the pH of seawater? ›

pH is an important property of seawater because it affects a wide range of chemical and biogeochemical processes in the ocean, such as chemical reactions, equilibrium condi- tions, and biological toxicity (Dickson 1993; Millero et al.

What would happen if ocean becomes too acidic? ›

Marine life uses carbonate from the water to build shells and skeletons. As seawater becomes more acidic, carbonate is less available for animals to build shells and skeletons. Under conditions of severe acidification, shells and skeletons can dissolve.

Does temperature affect ocean pH? ›

pH is based on the concentration of free H+ in solution. As the temperature increases the bonds holding the protons are broken and the pH increases. As Neal said, an increase in seawater temperature would lead to a decrease in seawater CO2 concentration.

Can we reverse ocean acidification? ›

Cutting carbon emissions is the only way for oceans to recover from the devastating effects of climate change, according to the new research published in Nature Climate Change.

Why do we measure pH of sea water? ›

High-quality carbonate-chemistry measurements are required in order to understand fully the dynamics of the oceanic carbonate system [8]. The precision of seawater-pH measurements needs to be better than 0.002 pH units in order to allow detection of the average annual decrease in surface-ocean pH [3], [4], [5].

What determines the acidity or alkalinity of seawater quizlet? ›

When CO2 is dissolved in seawater, it forms carbonic acid, which releases hydrogen ions (H+) into solution. Acidity is a measure of the hydrogen ions in solution, measure on the pH scale.

How do scientists measure pH? ›

The most common method is a pH meter, which involves a pH-sensitive electrode (usually made of glass) and a reference electrode. Acid-base indicators change color in response to different pH values. Litmus paper and pH paper are used for quick, relatively imprecise measurements.

What is the chemistry behind ocean acidification? ›

Ocean acidification occurs when carbon dioxide (CO2) is absorbed rapidly into the ocean. It reacts with water molecules (H2O) to form carbonic acid (H2CO3). This compound then breaks down into a hydrogen ion (H+) and bicarbonate (HCO3-). These hydrogen ions decrease seawater pH.

How does CO2 affect pH in water? ›

When CO2 is dissolved in water, a part of it reacts with water to become carbonic acid (H2CO3). It is the hydrogen ions present in carbonic acid that make water acidic, lowering the pH.

What determines the acidity or alkalinity of seawater? ›

Acidity or alkalinity of a solution is determined by the amount of hydrogen ions [H+], using the pH scale (pH = -log[H+]). Pure water has pH 7; solutions below pH 7 are acidic, and above pH 7 are alkaline, or basic. More hydrogen ions lower the pH and increase acidity (decrease alkalinity).

What happens if the ocean becomes too acidic? ›

Ocean acidification reduces the amount of carbonate, a key building block in seawater. This makes it more difficult for marine organisms, such as coral and some plankton, to form their shells and skeletons, and existing shells may begin to dissolve.

How does CO2 make the ocean more acidic? ›

As the amount of carbon dioxide in the atmosphere rises, the oceans absorb a lot of it. In the ocean, carbon dioxide reacts with seawater to form carbonic acid. This causes the acidity of seawater to increase.

What species is the most at risk in acidic water? ›

Generally, shelled animals—including mussels, clams, urchins and starfish—are going to have trouble building their shells in more acidic water, just like the corals. Mussels and oysters are expected to grow less shell by 25 percent and 10 percent respectively by the end of the century.

How is pH related to acidity? ›

pH is a measure of how acidic/basic water is. The range goes from 0 - 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water.

What is universal pH indicator? ›

Universal indicator is a mixture of dyes that changes colour gradually over a range of pH from 4-14. The colour is used to indicate pH directly. The main components of a Universal indicator, in the form of a solution, are thymol blue, methyl red, bromothymol blue and phenolphthalein.

Does CO2 make water acidic? ›

Carbon dioxide can make water more acidic which is causing a big problem in the oceans. The excess acid in ocean water, called ocean acidification, makes it difficult for some organisms to form shells and is especially damaging to coral.

How does climate change influence ocean chemistry? ›

> Massive emissions of carbon dioxide into the atmosphere have an impact on the chemical and biological processes in the ocean. The warming of ocean water could lead to a destabilization of solid methane deposits on the sea floor. Because of the excess CO2, the oceans are becoming more acidic.

How much has the pH of the ocean changed? ›

Ocean surface pH has declined from 8.2 to below 8.1 over the industrial era as a result of the increase in atmospheric CO2 concentrations. This decline corresponds to an increase in oceanic acidity of about 30 %.

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