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Our Changing Ocean

What is Ocean Acidification?

The ocean is a great sponge for absorbing carbon dioxide from the atmosphere, causing ocean acidification.


The ocean is a great carbon sponge
The ocean is a great sponge for the increasing carbon dioxide from the atmosphere. When the ocean absorbs this carbon dioxide, it changes the chemistry of the ocean. While the ocean itself is not acidic, the absorption of carbon dioxide increases the ocean's acidity. This can impact some marine life and the people who depend on them.
A Timeline of Evidence
Since the Industrial Revolution, the atmospheric concentration of carbon dioxide has increased - almost 50% - from 280 to over 400 parts per million. This increase is from human activities like the burning of fossil fuels such as coal, gas, and oil, along with land-use change. Ocean acidification (OA) refers to a change in ocean chemistry in response to the uptake of increasing carbon dioxide (CO2) in the atmosphere.
Chemistry of Seawater
Increases in carbon dioxide (known as CO2) in the atmosphere drive corresponding increases in dissolved CO2 within the surface waters of our ocean. This dissolved CO2 reacts with seawater to form carbonic acid (H2CO3). Carbonic acid breaks apart to form bicarbonate ions (HCO3-) and hydrogen ions (H+). Hydrogen ions (H+) act like free agents. And while the ocean is not acidic, these free agent hydrogen ions cause the seawater to become more acidic. We measure this using pH (H represents hydrogen ions). The free agent hydrogen ions also react with carbonate ions (CO32-) to form bicarbonate (HCO3-), making carbonate ions relatively less abundant. Next, find out why this matters next.
Bicarbonate as a Buffer
This is how carbonate ions help to buffer seawater against large changes in pH by reacting with some of the excess hydrogen ions and forming bicarbonate. However, carbonate ions (CO32-) are an important part of calcium carbonate (CaCO3). Calcium carbonate ions are the building blocks of shells and skeletons of many types of marine life. Therefore, decreases in seawater carbonate ions can make building and maintaining shells and other calcium carbonate structures difficult for calcifying marine organisms such as coral, plankton, and shellfish.
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Monitoring "the Big four"

Learn about what we measure, and why. Download the infographic for even more. 


When the ocean absorbs carbon dioxide, chemical reactions create hydrogen ions that act like free agents, able to react with other compounds. Two ways we track ocean acidification are through pH and total alkalinity (TA). pH is a measure of how many free hydrogen ions are in the seawater. The more carbon dioxide in the ocean, the more these free agents are created, causing lower pH (more acidic).


The partial pressure of CO2 (pCO2) tells us how much carbon dioxide is in seawater. This information helps us understand ocean carbonate chemistry and biological productivity in the region. pCO2 increases when the ocean absorbs more CO2 from the atmosphere with elevated emissions.


Alkalinity is the ocean’s buffering system against increasing acidity. Total alkalinity is a measure of the concentration of buffering molecules like carbonate and bicarbonate in the seawater that can neutralize acid.


Dissolved inorganic carbon (DIC) tells us how much non-biological carbon is in seawater. Inorganic carbon comes in three main forms that we measure for DIC: carbon dioxide (CO2), bicarbonate (HCO3-), and carbonate (CO32-). Understanding DIC can help us determine the balance of carbonate forms in the ocean and the likelihood of ocean acidification.

Nutrient runoff and pollution threaten ecological productivity. 

Coastal acidification impacts are changes in water chemistry caused mostly from human activities on land. They have local effects and can threaten the health of coastal residents and ecosystems.

Coastal acidification includes local changes in water chemistry from freshwater river inputs and excess nutrient run-off (e.g. nitrogen and organic carbon) from land. Excess nutrients from runoff and fertilizers can cause increases in algal growth. When these algal blooms die, they consume oxygen and release carbon dioxide.

The ability of an ecosystem to cope with acidification is influenced by the amount of local stressors it needs to contend with, such as high nutrient input or changes in temperature or salinity. By minimizing local stresses, some ecosystems may prove more resilient to ocean acidification.

Aqua MODIS images courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC. Caption by Kathryn Hansen.
Aqua MODIS images courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC. Caption by Kathryn Hansen.

Freshwater bodies like the Great Lakes also experience acidification. Researchers project that pH, the measure of how acidic or alkaline the water body is, will decline at a rate similar to that of the oceans in response to increasing atmospheric carbon dioxide.

Absorption of carbon dioxide from the atmosphere isn’t the only source of acidification. The Great Lakes are also recovering from acid deposition. The Midwestern and Northeastern United States experienced an increase in deposition of sulfuric and nitric acids from the early 20th century until air-quality regulations mitigated this trend.

Present-day mean pH and alkalinity vary according to the geology of each lake basin, with Superior having the most acidified waters and Michigan the least acidified. In addition, considerable short-term spatial and temporal variability in pH occurs, driven largely by varying rates of photosynthesis, respiration, and seasonal mixing. There has not been any long-term robust monitoring of acidification in the Great Lakes system until recently.

Be the Change

Take Action within Your Community

What’s at stake from ocean acidification may be different depending on where you live. As a community member, you can take a larger role in educating the public about ocean acidification.

Reduce Your Footprint

There are many actions you can take to reduce your personal carbon footprint. From helping to reduce oil consumption with petrol products to making your home more energy efficient.

Adapt to Our Changing Ocean

The NOAA Ocean Acidification Program (OAP) works to build knowledge about how to adapt to the consequences of ocean acidification and conserve marine ecosystems as acidification occurs.

Addressing Ocean Acidification

The OAP works closely with coastal state governments, on-the-ground networks, impacted industries, and NGOs to develop their responses to ocean acidification. See how we take action by supporting legislation development and reporting for ocean acidification research. 

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The NOAA Ocean Acidification Program (OAP) works to prepare society to adapt to the consequences of ocean acidification and conserve marine ecosystems as acidification occurs. Learn more about the human connections and adaptation strategies from these efforts.

Adaptation approaches fostered by the OAP include:


Using models and research to understand the sensitivity of organisms and ecosystems to ocean acidification to make predictions about the future, allowing communities and industries to prepare


Using these models and predictions as tools to facilitate management strategies that will protect marine resources and communities from future changes


Developing innovative tools to help monitor ocean acidification and mitigate changing ocean chemistry locally


On the Road

Drive fuel-efficient vehicles or choose public transportation. Choose your bike or walk! Don't sit idle for more than 30 seconds. Keep your tires properly inflated.

With your Food Choices

Eat local- this helps cut down on production and transport! Reduce your meat and dairy. Compost to avoid food waste ending up in the landfill

With your Food Choices

Make energy-efficient choices for your appliances and lighting. Heat and cool efficiently! Change your air filters and program your thermostat, seal and insulate your home, and support clean energy sources

By Reducing Coastal Acidification

Reduce your use of fertilizers, Improve sewage treatment and run off, and Protect and restore coastal habitats

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You've taken the first step to learn more about ocean acidification - why not spread this knowledge to your community?

Every community has their unique culture, economy and ecology and what’s at stake from ocean acidification may be different depending on where you live.  As a community member, you can take a larger role in educating the public about ocean acidification. Creating awareness is the first step to taking action.  As communities gain traction, neighboring regions that share marine resources can build larger coalitions to address ocean acidification.  Here are some ideas to get started:

  1. Work with informal educators, such as aquarium outreach programs and local non-profits, to teach the public about ocean acidification. Visit our Education & Outreach page to find the newest tools!
  2. Participate in habitat restoration efforts to restore habitats that help mitigate the effects of coastal acidification
  3. Facilitate conversations with local businesses that might be affected by ocean acidification, building a plan for the future.
  4. Partner with local community efforts to mitigate the driver behind ocean acidification  – excess CO2 – such as community supported agriculture, bike & car shares and other public transportation options.
  5. Contact your regional Coastal Acidification Network (CAN) to learn how OA is affecting your region and more ideas about how you can get involved in your community
       More for Taking Community Action