Causes of ocean acidification
To address a basic misconception: pH is a measure of acidity or alkalinity of water soluble substances (pH stands for 'potential of hydrogen'). A pH value is a number from 0 to 14, with 7 as the middle (neutral) point. Values below 7 indicate acidity which increases as the number decreases, 1 being the most acidic. The pH scale is logarithmic. This means a change factor of 10-fold is behind each whole number jump.
Oceanic pH has moved in the last two hundred and fifty years from a value of around 8.25 to a current value of around 8.15. It is therefore true to say that ocean acidification could also be equally expressed as a move towards pH neutrality. That of course does not garner headlines. The disturbing fact is that whatever the appropriate label is, marine ecosystems are showing significant signs of stress as a result of increasing concentrations of carbonic acid caused by carbon dioxide storage.
Ocean acidification: the carbon cycle
The carbon cycle involves biological, geological and chemical processes - it is an amalgamation of systems through which carbon is exchanged among the biosphere (Earth's surface), pedosphere (soil strata), geosphere (atmosphere) and hydrosphere (water systems).
The global carbon cycle is usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:
The terrestrial biosphere.
The oceans, including dissolved inorganic carbon and living and non-living marine biota.
The sediments, including fossil fuels, fresh water systems and non-living organic material.
The Earth's interior, carbon from the Earth's mantle and crust. These carbon stores interact with the other components through geological processes.
The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.
The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments is fairly balanced, so that carbon levels would be roughly stable without human influence.
Ocean acidification history
While ongoing ocean acidification is man-made in origin, it has occurred previously in Earth's history.
The most notable example is the Paleocene-Eocene Thermal Maximum (PETM), which occurred approximately 56 million years ago.
Much of what’s known about corals in an acidifying ocean was discovered in the Arizona desert in the 1990s, in a sealed environment designed to mimic conditions on Earth, called Biosphere 2.
Over time, carbon dioxide levels soared and the pH of the “ocean,” simulated inside a stainless-steel tank, dropped.
A then-Columbia University scientist, Chris Langdon, tried to correct the ocean’s pH by adding baking soda and baking powder. Boosting the alkalinity not only restored pH but also restored coral growth.
To test his hypothesis, that coral reef-building depended on the saturation state of water, he spent three years measuring coral growth in varied states of saturation. His paper, published in 2000, generated such a stir that he spent another two years replicating the results.
A German marine biologist, Ulf Riebesell, documented a similar effect on coccolithophores, a species of phytoplankton covered in plate-like armor made of calcite (Kolbert, 2006).
The urgency of the problem became apparent. In 2004, scientists from around the world gathered for the first-ever symposium on ocean acidification to compare notes and discuss research priorities for the future.
The journal Nature called it, “a turning point in expanding awareness among scientists about acidification.”
Ocean acidification chemistry
When carbon dioxide (CO2) dissolves, it reacts with water to form a chemical cocktail:
Dissolved free carbon dioxide (CO2(aq)).
Carbonic acid (H2CO3).
The ratio of these elements depends on factors such as seawater temperature and alkalinity.
These different forms of dissolved inorganic carbon (inorganic carbons are derived from ores and minerals rather than from living matter) are transferred from an ocean's surface to its interior.
Dissolving carbon dioxide in seawater increases the hydrogen ion (H+
) concentration in the ocean, and thus decreases ocean pH, as follows:
CO2 (aq) + H2O <-> H2CO3 <-> HCO3− + H+ <-> CO32− + 2 H+.
Dissolved free carbon dioxide AND seawater CREATING carbonic acid CREATING bicarbonate AND hyrogen ion CREATING carbonate AND 2 hydrogen ions.
Since the industrial revolution began, it is estimated that surface ocean pH has dropped by slightly more than 0.1 units on the logarithmic scale of pH, representing about a 30% increase in H+.
It is expected to drop by a further 0.3 to 0.5 pH units (an additional doubling to tripling of today's post-industrial acid concentrations) by 2100 as the oceans absorb more man-made carbon dioxide, the impacts being most severe for coral reefs and the Southern Ocean.