The pH of the ocean indicates its acidity or alkalinity, a fundamental measure of its chemical state. Over time, this measure has shown significant changes, prompting scientific inquiry into its causes and consequences. Understanding these shifts directly impacts marine ecosystems and the services they provide.
Understanding Ocean Acidity
The pH scale is a logarithmic measurement ranging from 0 to 14, where 7 indicates neutrality. Values below 7 denote acidity, while values above 7 signify alkalinity. Naturally, seawater is slightly alkaline, with a pre-industrial average pH of approximately 8.2.
Small alterations in pH values represent substantial changes in acidity due to the logarithmic nature of the scale. For instance, a decrease of one pH unit signifies a tenfold increase in acidity.
Historical Trends in Ocean pH
Scientists track ocean pH over time using various methods, including direct measurements from research vessels and stationary moorings, as well as chemical proxies from deep ocean sediments and ice cores. These records help reconstruct historical ocean chemistry.
Since the Industrial Revolution, around the early 1800s, the average pH of surface ocean waters has declined by approximately 0.1 pH units. This represents about a 30% increase in ocean acidity. This rate of change is faster than any known in Earth’s geologic past, estimated at 10 times faster than any period in the last 300 million years. Projections indicate that if current carbon dioxide emission rates continue, ocean pH could decrease by an additional 0.3 to 0.4 units by the end of this century, reaching levels not seen for at least 20 million years.
Drivers of pH Change
The primary cause of decreasing ocean pH is the substantial absorption of excess carbon dioxide (CO2) from the atmosphere. Since the Industrial Revolution, human activities like burning fossil fuels and changes in land use have significantly increased atmospheric CO2 concentrations. The ocean acts as a large carbon sink, absorbing approximately 25-30% of the CO2 released into the atmosphere.
When CO2 dissolves in seawater, a series of chemical reactions occur. CO2 reacts with water (H2O) to form carbonic acid (H2CO3). This carbonic acid then dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+). The increased concentration of these hydrogen ions directly leads to a decrease in the ocean’s pH, making it more acidic. This process also reduces the availability of carbonate ions (CO32-), which are important building blocks for many marine organisms.
Consequences for Marine Life
The decrease in ocean pH has widespread impacts on marine organisms and ecosystems. Organisms that build shells and skeletons from calcium carbonate, such as corals, shellfish (oysters, mussels, clams), sea urchins, and various types of plankton, are particularly vulnerable. As seawater becomes more acidic, carbonate ions become less available, making it harder for these calcifying organisms to form and maintain their structures. In severe cases, existing shells and skeletons can even begin to dissolve.
The effects extend beyond calcification, impacting broader ecosystem functions and food webs. For example, some fish species, like clownfish, experience a decreased ability to detect predators in more acidic waters, and their larvae may struggle to locate suitable habitats.
Changes in the nutritional quality of primary producers, such as coccolithophores, can also occur, affecting organisms higher up the food chain. Disruptions at the base of the food web, such as declines in plankton populations, can have cascading effects, impacting larger marine animals, including fish and marine mammals. This ultimately threatens human food security and economies reliant on seafood.
Addressing Ocean Acidification
Addressing ocean acidification requires a concerted global effort. The most direct solution is a substantial reduction in global carbon dioxide emissions, involving a transition away from fossil fuels across all sectors, including transportation, energy production, agriculture, and manufacturing.
Local mitigation efforts can complement global emission reductions in protecting coastal ecosystems. These efforts include reducing land-based pollution, such as nutrient and sediment runoff, which can exacerbate local acidification.
Protecting and restoring coastal habitats like seagrass meadows and mangrove forests also helps, as these “blue carbon” ecosystems absorb CO2 and act as natural buffers. Additionally, ongoing scientific research into carbon dioxide removal technologies and adaptation strategies, such as enhancing ocean alkalinity, aims to develop additional approaches to mitigate ocean pH changes.