Ocean acidification is occurring because our ocean is absorbing carbon dioxide from the atmosphere, leading to lower pH and greater acidity. This is literally causing a fundamental change in the chemistry of the ocean from pole to pole.
Since the industrial revolution, the atmospheric concentration of carbon dioxide has increased from 280 to over 400 parts per million due to the burning of fossil fuels such as coal, gas, and oil along with land use change (for instance, conversion of natural forest into crop production). Ocean acidification (OA) refers to a change in ocean chemistry in response to the uptake of increasing carbon dioxide (CO2) in the atmosphere. The world’s surface ocean is tightly linked with the atmosphere and absorbs huge amounts of carbon dioxide each year. This exchange, in part, helps to regulate the planet’s atmospheric CO2 concentrations, but comes at a cost for the oceans and life within it; from the smallest, single celled algae to the largest whales. Were it not for ocean uptake of CO2, atmospheric CO2 levels would be increasing at an even greater rate than they are now.
Anthropogenic ocean acidification refers to the component of chemical change that is caused by human activity (IPCC: Workshop on Impacts of Ocean Acidification on Marine Biology and Ecosystems). Roughly a third of all carbon dioxide emissions related to human activities since the 1700’s have been absorbed by the oceans (Sabine et al., 2004). Each year, approximately 2.5 billion metric tonnes of additional carbon enters the ocean in the form of CO2; That’s equivalent to 11 million railroad hopper cars filled with coal. Such a train would encircle the earth 14 times! Ocean pH has already declined globally by about 0.1 units and current emissions trajectories suggest it could decrease 0.7 units by the year 2300 (Zeebe et al, 2008).
Graphic courtesy of NOAA Pacific Marine Environmental Laboratory
Increases in carbon dioxide (CO2) in the atmosphere drive corresponding increases in dissolved CO2 within the surface waters of our ocean. This dissolved CO2, sometimes referred to as an “acid gas,” reacts with seawater to form carbonic acid (H2CO3). Carbonic acid almost completely dissociates to form bicarbonate ions (HCO3-) and hydrogen ions (H+). The increase in the concentration of hydrogen ions (H+) from these reactions causes the seawater to become more acidic (and is what the “H” in “pH” represents), hence the term “ocean acidification” and link to pH. The increase in hydrogen ions (H+) produced by the uptake of CO2 reacts with carbonate ions (CO32-) to form bicarbonate (HCO3-), making carbonate ions relatively less abundant. This is how carbonate ions help to buffer seawater against large changes in pH by reacting with some of the excess hydrogen ions. However, these carbonate ions (CO32-) are an important part of calcium carbonate (CaCO3) structures, such as sea shells and coral skeletons. 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.
Near-shore, estuarine, and intertidal environments are experiencing acidification, often referred to as coastal acidification. These highly productive coastal habitats provide essential nursery ground for many important fisheries, shellfisheries, and coral reef ecosystems. Coastal systems are highly susceptible to acidification because of the many anthropogenic impacts in these regions. The chemistry of these near-shore regions is strongly influenced by rivers that bring freshwater and nutrients into the coastal waters. Nutrients from runoff and fertilizers can cause increases in algae growth, a process called eutrophication. As other organisms consume these algal blooms and the blooms die, they consume oxygen and release carbon dioxide. Furthermore, increases in freshwater flow from changes to land use or precipitation can change the ability of coastal water to buffer against changes in the chemistry. These types of changes in coastal chemistry are called coastal acidification. Multiple stressors can enhance coastal acidification effects to marine life and threaten human our interactions with our coastlines.