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 Climate Change Indicators: Ocean
Acidity
This indicator describes changes in the chemistry of the
ocean that relate to the amount of carbon dioxide dissolved
in the water.
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. |
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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.
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. Signs of damage are already starting to
appear in certain areas.
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 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 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.
Indicator Notes
This indicator focuses on surface waters, which can absorb
carbon dioxide from the atmosphere within a few months. 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.
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 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:
https://bats.bios.edu.
Canary Islands data are available at:
www.eurosites.info/estoc/data.php. Hawaii data are
available at:
https://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:
https://sos.noaa.gov/Datasets/list.php?category=Ocean.
Technical Documentation
Download related technical information PDF |
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 Figure
1. Ocean Carbon Dioxide Levels and Acidity, 1983–2015
This figure shows the relationship between changes in ocean
carbon dioxide levels (measured in the left column as a
partial pressure—a common way of measuring the amount of a
gas) and acidity (measured as pH in the right column). The
data come from two observation stations in the North
Atlantic Ocean (Canary Islands and Bermuda) and one in the
Pacific (Hawaii). The up-and-down pattern shows the
influence of seasonal variations.
Data sources: Bates, 2016;5 González-Dávila, 2012;6 Dore,
20157 |
 Figure
2. Changes in Aragonite Saturation of the World’s
Oceans, 1880–2015
This map shows changes in the aragonite saturation level of
ocean surface waters between the 1880s and the most recent
decade (2006–2015). 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 Oceanographic Institution, 20168 |
 Figure
3. pH Scale
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.gov/acidrain/what-acid-rain.
Sources: Environment Canada, 2008,9 with additional data
from IPCC, 201410 |
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EPA Page |
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This is the
EPA page for this topic. To see if the Trump
administration has changed the EPA page, simply click the
link and compare the information with this page. If you
notice changes were made to the EPA page, please post a
comment. Thanks. |
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Additional Climate Change Information |
Climate Change and Carbon Dioxide
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change
and carbon dioxide.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice. |
Carbon Dioxide and Climate Change
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of carbon dioxide
and climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice. |
Environmental Group Warns Earth's Health at Risk
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
A report by the World Wildlife Fund looked at thousands of animal populations
and found they have dropped significantly in 40 years. |
Sea Levels Rising at Fastest Rate in 3,000 years
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
A group of scientists say sea levels are rising at record rates. Another group
found that January temperatures in the Arctic reached a record high. |
Capturing CO2 Gas Is Not Easy
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Most scientists agree that carbon-dioxide gas is partly to blame for climate
change: rising global temperatures. But capturing the CO2 gas released by power
stations is costly and difficult. |
Growth, Climate Change Threaten African Plants and
Animals
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Researchers believe Africa may lose as much as 30 percent of its animal and
plant species by the end of this century. |
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