Ocean Acidification, A Global Threat To Marine Life

Smithsonian, The Ocean Portal Team

Reviewed by Jennifer Bennett (NOAA)

Introduction

is sometimes called “climate change’s equally evil twin,” and for good reason: it's a significant and harmful consequence of in the atmosphere that we don't see or feel because its effects are happening underwater. At least one-quarter of the the ocean. Since the beginning of the , the ocean has absorbed some 525 billion tons of CO2 from the atmosphere, presently around 22 million tons per day.

ocean acidification : Ocean water becoming more acidic from greater concentration of hydrogen ions (H+). Carbon dioxide (CO2) combines with water (H2O) to create carbonic acid (H2CO3), which then releases H+ ions to make the ocean more acidic.

industrial era : The transition to new manufacturing process that began between 1760 to 1840. The transition replaced hand or manual production to using machines, chemicals, steam, and coals.

Excess carbon dioxide : 30 billion tons per year, 84%, of CO2 are related to human acitivity . Natural processes remove some CO2; however if too much CO2 is released into the air, CO2 in our atmosphere increases.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

At first, scientists thought that this might be a good thing because it leaves less carbon dioxide in the air to warm the planet. But in the past decade, they’ve realized that this slowed warming has come at the cost of changing the ocean’s chemistry. , the water becomes more acidic and the (a measure of how acidic or basic the ocean is) drops. Even though the ocean is immense, enough can have a major impact. In the past 200 years alone, ocean water has become 30 percent more acidic—faster than any known change in ocean chemistry in the last 50 million years.

ocean pH : Concentration of hydrogen ion (H+) in the ocean. It is a measure of how acidic or basic the ocean is. Lower pH is more acidic meaning greater concentration of positive H+ ions. Higher pH means the opposite, lower concentration H+ ions. Stable pH of the ocean is around 8.1.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

Scientists formerly didn’t worry about this process because they always assumed that rivers carried enough to the stable. (Scientists call this stabilizing effect “buffering.”) But so much carbon dioxide is dissolving into the in surface waters. As those surface layers gradually mix into deep water, the entire ocean is affected.

dissolved chemicals from rocks : weathering and erosions dissolves rocks and minerals, which are then transported to river and eventually into ocean, impacting ocean chemistry

Such a relatively quick change in , much time to adapt. life. Overall, it's expected to have dramatic and mostly negative impacts on ocean ecosystems—although some species (especially those that live in ) are finding ways to adapt to the changing conditions.

shells already dissolving in the more acidic seawater : In a lab expirement, a sea butterfly shell placed in seawater with increased acidity was found to slowly dissolve over a 45 day period.

estuaries : estuaries and their surrounding wetlands are bodies of water usually found where rivers meet the sea.

However, while the chemistry is predictable, the details of the biological impacts are not. Although scientists have been tracking (link is external). What we do know is that things are going to look different, and we can't predict in any detail how they will look. Some organisms will survive or even thrive under the more acidic conditions while others will struggle to adapt, and may even go extinct. Beyond lost biodiversity, acidification will affect fisheries and aquaculture, threatening food security for millions of people, as well as tourism and other sea-related economies.

the term ocean acidification was first coined : The 2003 article published in Nature finds that CO2 from fossil fuels may result in larger pH changes than any geological record of the past 300 million years.

Acidification Chemistry

At its core, the issue of is simple chemistry. There are two important things to remember about what happens when . First, the as it becomes more acidic. Second, this process binds up carbonate ions and makes them less abundant—ions that corals, oysters, mussels, and many other shelled organisms need to build shells and skeletons.

lower pH : The uptake of CO2 in the ocean decreases pH levels, making the ocean more acidic and effecting marine life.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

A More Acidic Ocean

Two bright orange anemonefish poke their heads between anemone tentacles.

This graph shows rising levels of carbon dioxide (CO2) in the atmosphere, rising CO2 levels in the ocean, and decreasing pH in the water

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

off the coast of Hawaii. (NOAA PMEL Carbon Program

NOAA PMEL Carbon Program : Government researchers whose mission is to understand the changing chemistry of the oceans and impact on marine ecosystems.

(Link(link is external)))

NOAA PMEL Carbon Program : Government researchers whose mission is to understand the changing chemistry of the oceans and impact on marine ecosystems.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

Carbon dioxide is naturally in the air: plants need it to grow, and animals exhale it when they breathe. But, thanks to people burning fuels, there is now more carbon dioxide in the atmosphere than anytime in the past 15 million years. Most of this CO2 collects in the atmosphere and, because it absorbs heat from the sun, creates a blanket around the planet, warming its temperature. But some 30 percent of this , where it doesn't remain as floating CO2 molecules. A series of chemical changes break down the CO2 molecules and recombine them with others.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

When water (H2O) and ic acid (H2CO3). Carbonic acid is weak compared to some of the well-known acids that break down solids, such as hydrochloric acid (the main ingredient in gastric acid, which digests food in your stomach) and sulfuric acid (the main ingredient in car batteries, which can burn your skin with just a drop). The weaker carbonic acid may not act as quickly, but it works the same way as all acids: it releases hydrogen ions (H ⁺), which bond with other molecules in the area.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

Seawater that has more hydrogen ions is more acidic by definition, and it also has a (link is external). In fact, the definitions of acidification terms—acidity, H ⁺, pH —are interlinked: acidity describes how many H ⁺ions are in a solution; an acid is a substance that releases H ⁺ions; and pH is the scale used to measure the concentration of H+ ions.

lower pH : The uptake of CO2 in the ocean decreases pH levels, making the ocean more acidic and effecting marine life.

Smithsonian Institution

The , the more acidic the solution. The pH scale goes from extremely basic at 14 (lye has a pH of 13) to extremely acidic at 1 (lemon juice has a pH of 2), with a pH of 7 being neutral (neither acidic or basic). The less than 7, and it won’t become acidic even with all the CO2 that is dissolving into the ocean. But the changes in the direction of increasing acidity are still dramatic.

lower pH : The uptake of CO2 in the ocean decreases pH levels, making the ocean more acidic and effecting marine life.

So far, has dropped from 8.2 to 8.1 since the industrial revolution, and is expected by fall another 0.3 to 0.4 pH units by the end of the century. A drop in pH of 0.1 might not seem like a lot, but the pH scale, like the Richter scale for measuring earthquakes, is mic. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. If we more acidic than any seen for the past 20 million years or more.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

logarithm : logarithm counts the number of occurrences of the same factor in repeated multiplication; e.g. since 1000 = 10 × 10 × 10 = 10^3, the "logarithm base 10" of 1000 is 3, or log10 (1000) = 3. Similarly, log10 (10,000) = 4. The difference between 3 and 4 is a factor of 10 times the amount.

Why Acidity Matters

The acidic waters from the CO2 seeps can dissolve shells and also make it harder for shells to grow in the first place. (Laetitia Plaisance)

Many chemical reactions, including those that are essential for life, are sensitive to small changes in pH. In humans, for example, normal blood pH ranges between 7.35 and 7.45. A drop in blood pH of 0.2-0.3 can cause seizures, comas, and even death. Similarly, a small change in the pH of seawater can have harmful effects on marine life, impacting chemical communication, reproduction, and growth.

The building of skeletons in marine creatures is particularly sensitive to acidity. One of the molecules that (CO3-2 ), a key component of (CaCO3) shells. To make , shell-building marine animals such as corals and oysters combine a calcium ion (Ca ⁺²) with carbonate (CO3-2 ) from surrounding and water in the process.

calcium carbonate : A thin layer of crystals that forms the basis of coral's skeleton.

hydrogen ions bond with carbonate : hydrogen ions steal carbonate (CO3--) to form bircarbonate (HCO3-). That leaves fewer carbonate (CO3--) to react with Calcium to create shells (calcium carbonate, CaCO3) for reefs.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

Like —but they have a greater attraction to carbonate than calcium. When a hydrogen bonds with carbonate, a ion (HCO3-) is formed. Shell-building organisms can't extract the carbonate ion they need from , preventing them from using that carbonate to grow new shell. In this way, the hydrogen essentially binds up the carbonate ions, making it harder for shelled animals to build their homes. Even if animals are able to build skeletons in more acidic water, they may have to spend more energy to do so, taking away resources from other activities like reproduction. If there are too many molecules apart—dissolving shells that already exist.

calcium carbonate : A thin layer of crystals that forms the basis of coral's skeleton.

bicarbonate : Anion with formula HCO3-. Bicarbonates are less stable than carbonate ion (CO3--). Ocean acidifcation's H+ ions steals carbonate ion (CO3--) to create bicarbonate(HCO3-). Carbonate ions ((CO3--) are needed by corals to form their skeleton. The key difference between carbonate and bicarbonate is that the carbonate ion has -2 electrical charge whereas, the bicarbonate has -1 electrical charge. Few corals can build skeleton using bicarbonate instead of carbonate ion; however, vast majority of corals use carbonate ions, not bicarbonate.

This is just one process that extra hydrogen ions—caused by dissolving —may interfere with in the ocean. Organisms in the water, thus, have to learn to survive as the water around them has an increasing concentration of carbonate-hogging hydrogen ions.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.

Impacts on Ocean Life

The fluctuates within limits as a result of natural processes, and ocean organisms are well-adapted to survive the changes that they normally experience. Some marine species may be able to adapt to more extreme changes—but many will suffer, and there will likely be extinctions. We can't know this for sure, but during the last great acidification event 55 million years ago, there were mass extinctions in some species including deep sea invertebrates. A more acidic ocean won’t destroy all marine life in the sea, but the rise in seawater acidity of 30 percent that we have already seen is already affecting some ocean organisms.

Coral Reefs

Branching corals, because of their more fragile structure, struggle to live in acidified waters around natural carbon dioxide seeps, a model for a more acidic future ocean

model for a more acidic future ocean : Normanby Island in Papua New Guinea shows that under acidic condition massive brownish boulder corals colonize the sea floor instead of the usually beautiful, lively, colorful and complex corals.

. (Laetitia Plaisance)

model for a more acidic future ocean : Normanby Island in Papua New Guinea shows that under acidic condition massive brownish boulder corals colonize the sea floor instead of the usually beautiful, lively, colorful and complex corals.

Reef-building corals craft their own homes from , that house the coral animals themselves and provide habitat for many other organisms. Acidification may (link is external) by corroding pre-existing coral skeletons while simultaneously slowing the growth of new ones, and the weaker reefs that result will be more vulnerable to erosion. This erosion will come not only from storm waves, but also from or eat coral. A recent study(link is external) predicts that by roughly 2080 ocean conditions will be so acidic that even otherwise healthy coral reefs will be eroding more quickly than they can rebuild.

calcium carbonate : A thin layer of crystals that forms the basis of coral's skeleton.

forming complex reefs : Coral reefs are the most diverse of all marine ecosystems. They teem with life, with perhaps 25% of all ocean species depending on reefs for food and shelter. They are often called the rainforests of the sea.

limit coral growth : Lower ocean pH (increased H+ ions) decrease coral calcification rates significantly. The acidification lowers the concentration of carbonate ions in seawater, making it more difficult for corals to build their calcium carbonate skeletons.

animals that drill into : Ocean acidification provides advantages to ocean sponges that breaks down coral reefs. The acidification and increased temperature help sponges degrade coral skeleton faster.

Acidification may also impact corals before they even begin constructing their homes. The eggs and (link is external) while they were still in the . However, (link is external), preventing them from reaching adulthood.

larvae in acidic water had more trouble finding a good place to settle : Increasingly acidic conditions in the ocean appear to have a dramatic effect on the ability of baby corals to sense their surroundings.

coral larvae development : “The coral larvae normally have this amazing ability to settle on one particular type of rock-like seaweed called Titanoderma. This stony seaweed is a safe haven for young corals...yet, as levels of ocean acidification increased, the coral larvae avoided this seaweed and started to settle absolutely anywhere.” says research scientist Christopher Doropoulos

plankton : The word “plankton” comes from the Greek for “drifter” or “wanderer.” An organism is considered plankton if it is carried by tides and currents, and cannot swim well enough to move against these forces. Plankton are usually microscopic, often less than one inch in length, but they also include larger species like some crustaceans and jellyfish.

How much trouble corals run into will vary by species. Some types of coral can use (link is external) instead of carbonate ions to build their skeletons, which gives them more options in an acidifying ocean. (link is external) once the water returns to a more comfortable pH. Others can handle a wider pH range.

return to normal skeleton building activities : Some corals can survive acidic condition and resume growth when normal conditions are reintroduced

Nonetheless, in the next century we will see the common types of coral found in reefs shifting—though we can't be entirely certain what that change will look like. that are affected by natural carbon dioxide seeps, big boulder colonies have taken over and the delicately branching forms have disappeared, probably because their thin branches are more susceptible to dissolving. This change is also likely to affect the many thousands of organisms that live among the coral, including those that people fish and eat, in unpredictable ways. In addition, acidification gets piled on top of all the other stresses that reefs have ), pollution, and overfishing.

On reefs in Papua New Guinea : Normanby Island in Papua New Guinea shows that under acidic condition massive brownish boulder corals colonize the sea floor Instead of the usually beautiful, lively, colorful and complex corals.

coral bleaching : Tiny plant-like organisms called zooxanthellae live in the tissues of some corals. These microscopic algae capture sunlight and convert it into energy, just like plants, to provide essential nutrients to the corals. But under high temperature conditions, they will die or leave their host—a process known as bleaching.

Looking to the Future

If the amount of carbon dioxide in the atmosphere stabilizes, eventually buffering (or neutralizing) will occur and pH will return to normal. This is why there are periods in the past with much higher levels of carbon dioxide but no evidence of : the rate of carbon dioxide increase was slower, so the ocean had time to buffer and adapt. But this time, pH is dropping too quickly. Buffering will take thousands of years, which is way too long a period of time for the ocean organisms affected now and in the near future.

So far, the signs of acidification visible to humans are few. But they will only increase as more over time.

CO2 carbon dioxide dissolves into seawater : Carbon dioxide (CO2) mixes with water (H2O) to form carbonic acid. H2CO3, which is a weak acid.