Ocean Acidification: What CO2 Does to Seawater
Ocean acidification is a change in the chemistry of seawater. The National Oceanic and Atmospheric Administration (NOAA) and the Environmental Protection Agency (EPA) each explain it, from carbon dioxide in the air to the shells and skeletons of sea animals.
NOAA says that when CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions. NOAA puts the ocean’s average pH at around 8.1 now, and EPA puts it at about 8.1 today. EPA says each decrease of one pH unit is a ten-fold increase in acidity.
What Happens When Carbon Dioxide Meets Seawater
NOAA traces the path from air to water. Atmospheric CO2 levels rise from human activity such as burning fossil fuels (e.g., car emissions) and changing land use (e.g., deforestation). As those levels increase, the amount of carbon dioxide absorbed by the ocean also increases.
The ocean’s share, by NOAA’s count, is about 30% of the carbon dioxide that is released in the atmosphere. Carbon dioxide, which is naturally in the atmosphere, dissolves into seawater, where water and carbon dioxide combine to form carbonic acid, a weak acid.
That acid breaks, or “dissociates,” into hydrogen ions and bicarbonate ions. Because of human-driven increased levels of carbon dioxide in the atmosphere, NOAA says, there is more CO2 dissolving into the ocean.
EPA gives its own figure. The ocean surface layer absorbs about one third of human-released CO2. The ocean readily absorbs dissolved carbon dioxide, which in turn increases the acidity.
Once carbon dioxide dissolves in water, it reacts with water molecules to form carbonic acid. Carbonic acid can be further transformed to bicarbonate and carbonate ions.
Four different forms of carbon exist in balanced proportions in seawater: dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate. As more carbon dioxide is added to seawater, the balance shifts. The carbonate ion concentration decreases as it is transformed to bicarbonate due to increasing acidity.
Increasing CO2 uptake by the ocean leads to a suite of changes that are altering the chemistry of the ocean and increasing its acidity.
EPA also compares today’s levels with the distant past. The post-1700s to current day concentrations of CO2 have exceeded known amounts of atmospheric carbon dioxide from the past 800,000 years. The acidity of the ocean is greater than any point in the past two million years.
Reading the pH Numbers
The scale itself comes first. NOAA says the pH scale runs from 0 to 14, with 7 being a neutral pH; anything higher than 7 is basic (or alkaline) and anything lower than 7 is acidic.
The pH scale is an inverse of hydrogen ion concentration, NOAA says. More hydrogen ions translates to higher acidity and a lower pH.
EPA says the acidity of a liquid is reported as pH, and that the lower the pH value, the higher the acidity of a liquid. Solutions with low pH are acidic and solutions with high pH are basic, also known as alkaline.
Two ocean readings from EPA. Prior to the 1700’s, average ocean pH was about 8.2. Today, average ocean pH is about 8.1.
This might not seem like much of a difference, but the relationship between pH and acidity is not direct. Each decrease of one pH unit is a ten-fold increase in acidity. This means that the acidity of the ocean today, on average, is about 25% greater than it was prior to the 1700’s.
NOAA gives its own figures. In the 200-plus years since the industrial revolution began, the concentration of carbon dioxide in the atmosphere has increased due to human actions. During this time, the pH of surface ocean waters has fallen by 0.1 pH units.
This might not sound like much, NOAA says, but the pH scale is logarithmic. Over those 200-plus years, that change represents approximately a 30 percent increase in acidity.
Ocean pH and carbon dioxide, in NOAA’s and EPA’s words
| What each agency says | NOAA | EPA |
|---|---|---|
| Average ocean pH, as each agency states it now | The ocean’s average pH is now around 8.1 | Today, average ocean pH is about 8.1 |
| pH change and period | The pH of surface ocean waters has fallen by 0.1 pH units in the 200-plus years since the industrial revolution began | Prior to the 1700’s, average ocean pH was about 8.2 |
| Increase in acidity | This change represents approximately a 30 percent increase in acidity | About 25% greater than it was prior to the 1700’s |
| Carbon dioxide the ocean absorbs | About 30% of the carbon dioxide (CO2) that is released in the atmosphere | The ocean surface layer absorbs about one third of human-released CO2 |
Shells, Skeletons, and Carbonate
NOAA calls carbonate ions an important building block of structures such as sea shells and coral skeletons. Decreases in carbonate ions can make building and maintaining shells and other calcium carbonate structures difficult for calcifying organisms — organisms such as oysters, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton.
Ocean acidification is already impacting many ocean species, NOAA says. Especially affected are organisms like oysters and corals that make hard shells and skeletons by combining calcium and carbonate from seawater.
However, as ocean acidification increases, available carbonate ions bond with excess hydrogen. The result is fewer carbonate ions available for calcifying organisms to build and maintain their shells, skeletons, and other calcium carbonate structures. If the pH gets too low, NOAA says, shells and skeletons can even begin to dissolve.
EPA explains how shells are built. Marine life uses carbonate from the water to build shells and skeletons, and 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. Many ocean plants and animals build shells and skeletons out of two chemicals that exist in seawater, calcium and carbonate, which they combine to form hard shells and skeletons out of the mineral calcium carbonate.
The plants and animals that use calcium carbonate for structure and protection are called calcifying organisms. Increased acidity slows the growth of calcium carbonate structures. Under severe conditions, it can dissolve structures faster than they form.
NOAA describes one small animal. The pteropod, or “sea butterfly,” is a tiny sea snail about the size of a small pea. Pteropods are an important part of many food webs and eaten by organisms ranging in size from tiny krill to whales.
Because of acidification, EPA says, marine life face a two-fold challenge: decreased carbonate availability and increased acidity.
What Extra Acidity Costs Marine Animals
EPA starts inside the animal’s body. If the acidity of seawater is beyond the optimum range for that organism, its body must use more energy to maintain healthy body fluid chemistry.
Organisms can often compensate when faced with increased acidity. That compensation comes at the expense of using energy to grow critical body parts like muscle or shell.
Scientists have found that mussels, sea urchins, and crabs start to dissolve their protective shells to counter elevated acidity in their body fluids. So even if an organism can adjust to survive increasing acidity its overall health can be impaired.
Many marine fish and invertebrates have complex life cycles, and larvae are very small, which makes them especially vulnerable to increased acidity.
That vulnerability means organisms may be able to reproduce while their offspring may not reach adulthood.
Ocean life also has a great deal of variation, and increased acidity can harm or help individual plant and animal species in different ways.
NOAA cites a study from the West Coast. A NOAA-funded study has documented that acidification along the U.S. Pacific Northwest coast is impacting the shells and sensory organs of some young Dungeness crab.
EPA also notes what is not yet known. “However, we do not yet know exactly how ecosystems will be impacted,” the agency writes.
Acidification Closer to Shore
Near the coast, EPA adds a second source. In addition to ocean acidification from carbon dioxide in the atmosphere, land-based sources of pollution can contribute to localized acidification of coastal waters.
Changing atmospheric composition, acid rain, and activities which cause nutrient pollution contribute to acidification in coastal waters. Excess nutrients contribute to acidification in coastal waters when algal blooms peak and die.
Water can carry excess nutrients down streams and into coastal waters. Agricultural activities are a major source of nutrients to coastal waters. Other sources include sewage, wastewater treatment plant effluent, and nitrogen oxide air pollution.
In coastal waters excess nutrients stimulate the growth of algae, and algae multiply rapidly under ideal growing conditions. Algal blooms can impair water quality by causing hypoxia, foul odors and even toxins.
When algae die their decomposing tissue releases carbon dioxide directly into the water, resulting in acidification.
Reefs, Food Webs, and the People Who Rely on Them
EPA’s account of ecosystems begins with food webs. Life in the ocean is always food for something else. Any increase or decrease in the abundance of a species can have a ripple effect on other species.
Changes in the number of one type of animal can affect the number of animals that feed on it, and so forth. Coral reefs host an abundant and wide array of marine life, and corals themselves are living organisms on which the whole ecosystem depends.
Coral reefs provide habitat and food for a large variety of marine life. That life includes various sponges, oysters, clams, crabs, starfish, sea urchins, and many species of fish.
An estimated 25 percent of all marine life are dependent on coral reefs at some point in their life cycle, including over 4,000 species of fish. Corals may not form calcium carbonate under increased acidity, and under severe acidity the coral’s skeleton can dissolve.
EPA lists some ecosystem services provided by coral reefs. They are fish to eat, protection of coastal areas from storms, and tourism activities like snorkeling and scuba diving.
Fisheries are not only essential to the physical well-being and cultural identity of many Native Americans. In some cases they are protected by treaty with the United States government.
Shellfish and salmon in Puget Sound in Washington State are cornerstones of the Suquamish Tribe’s economy and culture.
EPA says the complexity of ocean food webs and other ongoing environmental problems make it difficult to predict how ecosystems will reorganize under increased seawater acidity. NOAA says many jobs and economies in the U.S. and around the world depend on the fish and shellfish that live in the ocean.