Ocean Acidity

Climate Change Indicators in the United States: Ocean Acidity
www.epa.gov/climatechange/indicators - Updated May 2014

Ocean Acidity

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

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 that burn fossil fuels.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 and 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.

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 three 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) 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 skeletons and shells.

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Climate Change Indicators in the United States: Ocean Acidity
www.epa.gov/climatechange/indicators - Updated May 2014

This map was created by comparing average conditions during the 1880s with average conditions during
the most recent 10 years (2004-2013). 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.

•	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

Key Points

2

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Climate Change Indicators in the United States: Ocean Acidity
www.epa.gov/climatechange/indicators - Updated May 2014

pH Scale

t |

Increasing 3
acidity

Neutral

Increasing
alkalinity

I

1

2
I

4

5

6

7

8

9

10

11

12
M
14

Battery add
Lemon juke
Vinegar

Add rain

Milk

Baking soda
Sea water

Adult fish die
Fish reproduction affected
Normal range of precipitation pH

Normal range of stream pH

Milk of magnesia

Ammonia

Lye

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. Like the
well-known Richter scale for measuring earthquakes, the pH scale is based on powers of 10, which
means a substance with a pH of 3 is 10 times more acidic than a substance with a pH of 4. For more
information about pH, visit: www.epa.aov/acidrain/measure/ph.html.

Source: Environment Canada, 2008s

3

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Climate Change Indicators in the United States: Ocean Acidity

www.epa.gov/climatechange/indicators - Updated May 2014

Figure 1. Ocean Carbon Dioxide Levels and Acidity, 1983-2012

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450
400
350
300

Bermuda

250

500
450
400
350
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Canary Islands

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450
400
350
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Hawaii

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1980 1990 2000 2010 2020 ~

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Figure 2. Changes in Aragonite Saturation of the World's Oceans,
1880-2013

Climate Change Indicators in the United States: Ocean Acidity

www.epa.gov/climatechange/indicators - Updated May 2014

Change In aragonite saturation at the ocean surface

This map shows changes in the aragonite saturation level of ocean surface waters between the 1880s
and the most recent decade (2004-2013). Aragonite is a form of calcium carbonate that many marine
animals use to build their skeletons and shells. The lower the saturation level, the more difficult it is for
organisms to build and maintain their skeletons and shells. A negative change represents a decrease in
saturation.

Data source: Woods Hole Gceariographic Institution, 20149

Indicator Notes

This indicator focuses on surface waters, which can absorb carbon dioxide from the atmosphere within a
few months.10 It 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.11

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.

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Climate Change Indicators in the United States: Ocean Acidity
www.epa.gov/climatechange/indicators - Updated May 2014

Data Sources

Data for Figure 1 came from three studies: the Bermuda Atlantic Time-Series Study, the European
Station for Time-Series in the Ocean (Canary Islands), and the Hawaii Ocean Time-Series. Bermuda data
are available at: http://bats.bios.edu. Canary Islands data are available
at: www.eurosites.info/estoc/data.php. Hawaii data are available
at: http://hahana.soest.hawaii.edu/hot/products/products.html.

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: http://sos.noaa.gov/Datasets/list.php?category=Ocean.

1 Calculated 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/wgl.

2 Wootton, 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.

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

4 Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a
high-C02 world. Oceanography 22(4):36-47.

5 Recreated from Environment Canada. 2008. The pH scale, www.ec.gc.ca/eau-
water/default.asp?lang=En&n=FDF30C16-l.

6 Bermuda Institute of Ocean Sciences. 2014 update to data originally published in: Bates, N.R., M.H.P. Best, K.
Neely, R. Garley, A.G. Dickson, and R.J. Johnson. 2012. Detecting anthropogenic carbon dioxide uptake and
ocean acidification in the North Atlantic Ocean. Biogeosciences 9:2509-2522.

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

8 Dore, J. 2014 update to data originally published in: Dore, J.E., R. Lukas, D.W. Sadler, M.J. Church, and D.M.

Karl. 2009. Physical and biogeochemical modulation of ocean acidification in the central North Pacific. Proc Natl
Acad Sci USA 106:12235-12240.

9 Woods Hole Oceanographic Institution. 2014 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-C02 world.
Oceanography 22(4):36-47.

10 Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a
high-C02 world. Oceanography 22(4):36-47.

11 Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present conditions and future changes in a
high-C02 world. Oceanography 22(4):36-47.

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