Introduction

Ocean Acidification (OA) is a term used to describe significant changes to the chemistry of the ocean.

It occurs when carbon dioxide gas (or CO2) is absorbed by the ocean and reacts with seawater to produce acid. Although CO2 gas naturally moves between the atmosphere and the oceans, the increased amounts of CO2 gas emitted into the atmosphere, mainly as a result of human activities (e.g. burning fossil fuels), has been increasing the amount of CO2 absorbed by the ocean, which results in seawater that is more acidic. 

OA Diagram

The pH scale (“power of hydrogen”), which spans from 0 to 14, is used to measure how acidic or basic a substance is.

Acidic substances, such as vinegar, lemon juice, and battery acid have lower pH values (less than 7) while substances that are basic such ammonia and baking soda have higher pH values (greater than 7). The pH scale is based on a logarithmic index, just like the Richter scale for earthquake magnitudes. This means that a very small decrease in pH actually represents a large increase in acidity.

How is CO2 related to pH?

As CO2 is absorbed, it reacts with the water to form carbonic acid. This leads to the release of hydrogen ions - charged particles that are derived from the hydrogen in the water. The greater the hydrogen ion concentration, the more acidic the substance is.

Although it is unlikely that the ocean will ever become a true acid, the term ocean acidification refers to the process of the ocean becoming gradually more acidic over time. Scientists have projected that if the current levels of CO2 emissions continue, the pH in the ocean could fall by another 0.3 to 0.4 units by the end of this century. This drop in pH would be equivalent to increasing the acidity  of the oceans between 100 and 150%, or increasing the concentration of H+ ions to 2-2.5 times its current concentration.

Another substance found in the ocean is carbonate. Carbonate is very important because many marine creatures use carbonate to make the shells, skeletons, and other hard parts of their bodies.

OA can limit the availability of dissolved carbonate in the ocean. The combination of increasing acidity along with a decrease in the amount of available carbonate for use by marine organisms raises significant concerns for the future health and stability of ocean ecosystems.

In a lab experiment, a sea butterfly (pteropod) shell placed in seawater with increased acidity slowly dissolves over 45 days. Image courtesy of R. Feely, PMEL.

Historically, the pH of the world’s oceans has remained relatively stable with only gradual fluctuations. This is mainly due to the natural buffering activity that occurs in seawater. However, the increase in anthropogenic - or human caused - atmospheric CO2 that has occurred over the last century has led to significant increases in the amount of CO2 now being absorbed by the world’s oceans. Consequently, the acidity of seawater has been increasing at an unprecedented rate and this dramatic alteration to the chemistry of the oceans has already proven harmful to the overall health of the marine environment. In particular, the ability of shellfish, corals, and other marine creatures to produce and maintain their shells and skeletons has been significantly compromised.

To better understand the impact that OA will likely have on the coastal marine environment, scientists have begun using sophisticated computer software to construct models that predict future ocean pH values. This time-lapse animation depicts changes over time (from 2007 through 2050) in both the levels of dissolved carbonate (i.e. the saturation level of Aragonite – a form of dissolved carbonate) and pH off the west coast of the United States. 

OA Diagram
  • Increased amounts of CO2 gas are present in the atmosphere. As a result, more CO2 gets absorbed by the oceans.

  • CO2 then reacts with seawater to produce carbonic acid. Carbonic acid breaks down, resulting in a decrease in the pH of seawater.

  • Increases in ocean acidity lead to a decline in the availability of carbonate which is needed by marine organisms to build and maintain the hard parts of their bodies.