The ocean plays a fundamental role in regulating Earth’s climate and supporting diverse life. Its chemistry, measured by pH level, is balanced. This measure indicates a solution’s acidity or alkalinity. Over time, changes in ocean pH have emerged as a significant environmental concern, impacting marine organisms and ecosystems globally.
Understanding Ocean pH
The pH scale ranges from 0 to 14, with 7 representing a neutral solution. Values below 7 indicate increasing acidity, while values above 7 signify increasing alkalinity. Ocean water is naturally slightly alkaline, with a typical surface pH around 8.1 or 8.2. The ocean also acts as a large carbon sink, absorbing carbon dioxide (CO2) from the atmosphere. This absorption is part of a natural buffering system, where seawater components help to stabilize pH by neutralizing acids.
Historical pH Trends
Scientists determine past ocean pH levels through ice cores, sediment records, and direct measurements in recent decades. These studies reveal ocean pH has decreased since the beginning of the Industrial Revolution. The average pH of surface ocean waters has fallen by approximately 0.1 pH units from a pre-industrial baseline of about 8.2 to around 8.1 today. While a 0.1 unit drop might seem small, the pH scale is logarithmic, meaning this change represents a 25 to 30 percent increase in ocean acidity. This rate of change is faster than any known change in ocean chemistry over the last 50 to 300 million years.
Drivers of Ocean Acidification
The primary driver of decreasing ocean pH is the absorption of atmospheric carbon dioxide (CO2) by the oceans, which have absorbed about 30 percent of the CO2 released by human activities since the Industrial Revolution. When CO2 dissolves in seawater, it combines with water (H2O) to form carbonic acid (H2CO3). Carbonic acid quickly dissociates, releasing hydrogen ions (H+) and bicarbonate ions (HCO3-) into the seawater. The increased concentration of these hydrogen ions causes the ocean’s pH to decrease, making it more acidic. These excess hydrogen ions also react with naturally present carbonate ions (CO3^2-), converting them into bicarbonate, which reduces the availability of carbonate ions fundamental for many marine organisms.
Impacts on Marine Ecosystems
Declining ocean pH impacts marine organisms and their ecosystems. Calcifying organisms, which rely on carbonate ions to build and maintain their shells and skeletons, are particularly affected. This group includes corals, shellfish like oysters, mussels, clams, and scallops, as well as pteropods and some plankton. Reduced carbonate availability makes it harder for these creatures to form and maintain their structures, leading to thinner, weaker, or even dissolving shells and skeletons.
These impacts can lead to slower growth rates and increased mortality, especially in larval stages. For example, coral reefs, which provide habitat for numerous species, may erode faster than they grow, impacting their ability to support diverse marine life. Pteropods, often called “potato chips of the sea,” are a critical food source in Arctic food webs, and their decline could disrupt entire food chains. Ocean chemistry changes can also affect fish behavior, such as impairing their sense of smell, and alter their internal blood chemistry. Such disruptions can ripple upwards, affecting larger predators and leading to shifts in biodiversity and ecosystem composition, impacting coastal economies dependent on shellfish.
Addressing Ocean pH Changes
Addressing the changes in ocean pH primarily involves reducing global carbon dioxide emissions. This is the most direct approach to mitigating ocean acidification. Beyond emission reductions, other strategies are being explored and developed to help manage ocean pH.
One such approach is ocean alkalinity enhancement (OAE), which involves adding alkaline substances like minerals to seawater. This process aims to accelerate the ocean’s natural carbon sink capacity, converting dissolved CO2 into more stable forms like bicarbonates and carbonates, thereby increasing pH. This can potentially remove gigatons of carbon from the atmosphere. Restoring coastal habitats like seagrass beds can also contribute, as seagrasses naturally sequester carbon dioxide at high rates. Continued research and global collaboration are important for implementing these solutions.