Our Changing Ocean

What is Ocean Acidification?

Our ocean’s chemistry is changing, causing impacts to marine life and livelihoods.

Overview
Monitoring “The Big Four”
Coastal Acidification
Freshwater Acidification
What we can do

Ocean acidification occurs when the ocean absorbs carbon dioxide. This causes a fundamental and global change in the chemistry of the ocean.

The Ocean is a Great Carbon Sponge

The ocean is a great sponge for carbon dioxide. When our 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.
AI generated - earth as a sponge
AI-Generated image by Canva

A Timeline of Evidence

Air and ocean observing provides critical information for understanding how carbon changes in the air and sea, and in turn how acidity tracks carbon in our ocean. Over the past 250 years, the ocean has become about 26% more acidic on average globally. Ocean acidification (OA) refers to a change in ocean chemistry in response to the uptake of increasing carbon dioxide (CO2).
a figure shows increasing atmospheric carbon dioxide from the 1950's to 2023. Also increasing from the late 1980's is ocean carbon dioxide with seasonal variability and decreasing ocean pH for the same time period.
Ocean CO2 rises with increasing atmospheric CO2. In turn, the pH of the ocean declines, reflecting increased acidity. Credit: NOAA Pacific Marine Environmental Laboratory Carbon Group

Seawater chemistry

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 that 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.

Basic chemistry of ocean acidification and it's effects on shelled organism. Credit: NOAA Pacific Marine Environmental Laboratory
Basic chemistry of ocean acidification and it's effects on shelled organism. Credit: NOAA Pacific Marine Environmental Laboratory

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).
Image Credit: NOAA AFSC

Monitoring "the Big four"

There are four ocean chemistry parameters we measure to understand ocean chemistry related to acidification. Researchers need to measure two of these parameters to be able to calculate the others and characterize conditions.

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

pH

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).

pCO2

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.

TA

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.

DIC

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.

Coastal Acidification

Local coastal processes can cause acidification.

Coastal acidification occurs from local processes that affect water chemistry. They have local effects and can threaten coastal ecosystems and communities.

  • Monitoring and modeling conditions in coastal zones is critical for tracking and predicting coastal acidification.
  • Minimizing other local stresses can help ecosystems and people manage the challenges of coastal acidification better.
  • Science-based management of coastal ecosystems can help support fisheries and coastal economies and cultures.

Coastal acidification includes local changes in water chemistry that can arise from human inputs on land as well as natural processes. 

Coastal upwelling is a natural process that brings deeper,
more acidic waters to the surface ocean in the coastal zone. Upwelling occurs primarily
on the western boundaries of continents, but the timing, duration and severity of upwelling events may increase with climate change.

Human impacts include excess nutrient run-off (e.g. nitrogen and organic carbon) from land. This runoff can cause increases in algal growth, commonly known as ‘blooms’. When these algal blooms die, they consume oxygen and release carbon dioxide and increasing acidity.

Coastal acidification can also occur with changes in water column circulation occur. Changes in wind, temperature, or salinity changes can have both natural and human causes.

The impacts of ocean acidification depend on the timing, duration, and severity of acidification, how marine life and ecosystems respond, and the effects on people who depend on healthy coastal ecosystems.

Freshwater 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.

Ocean Acidification Solutions

Engage 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.

  • Share what you know - this is an action everyone can take.
  • Join your regional coastal acidification network
  • Participate in community science for directly monitoring ocean chemistry or other activities that reduce other stressors to our marine ecosystems.
  • Help restore or protect important marine ecosystems that we rely on for food, safety and the blue economy.

Use Your Tools

The NOAA Ocean Acidification Program aims to provide information to help people adapt to the consequences of ocean acidification. Check out our near real-time data and support tools that include:

  • Maps and visualization tools to explore ocean chemistry in your area
  • Forecasts, hindcasts, and nowcasts that help you assess what's happening and what is coming
  • Multimedia resources to better understand changing ocean chemistry and share with others

Build Adaptive Capacity

NOAA works to better understand how to mitigate and adapt to acidification on local, regional and national scales. Continued research that uses innovative technologies and approaches provides better information faster.

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. 

Legislation & Reporting