Proceedings, Sept. 2012
Carbon dioxide . . . it’s what makes our soft drinks zesty and champagne . . . well, champagne. It is also a major greenhouse gas. Absorbed into the sea it is an essential component for growth of plankton, the first stage of marine life. By contrast, excessive amounts can make some organisms sick and possibly extinct.
The beneficial role of CO2 is in the photosynthetic process that feeds phytoplankton, the microscopic plants of the sea. Carbon dioxide, solar energy and the chlorophyll pigment combine to make food for these organisms that are the first level of life in the sea. A critical byproduct of this chemistry is the respiration of oxygen by these tiny plants. The vast surface area of the World Ocean supplies 50–80 percent of Earth’s oxygen. Indeed, it is the lungs of our planet.
The sea has an enormous capacity to absorb CO2. Since the Industrial Revolution began about 1750, man’s activities have loaded the atmosphere with great quantities of it. However the oceans have been able to absorb about 50 percent of this anthropogenic CO2. Remarkably, this percentage has remained fairly constant over the past 250 years even as increasing amounts are injected into the Earth’s atmosphere. But this “carbon sequestration” comes at a price. When atmospheric CO2 dissolves in seawater there is a chemical reaction that forms carbonic acid, a very weak acid.
Scientists use the pH numerical scale to define the relative acidity or alkalinity of a solution using a range of 0–14. Distilled water is neutral at a value of 7.0. Numbers lower than this are acidic and those above are alkaline. At present the average pH for the oceans is 8.1, so it is slightly alkaline—but this number is decreasing steadily. To be clear, it is unlikely that the oceans’ pH will ever drop below 7.0, so the term “acidification” refers to relative acidity as seawater becomes less alkaline. The accompanying graph shows the relationship between dissolved CO2 and increasing acidity in the sea from the years 1850–2100 (estimated).