The Ocean's Carbon Crisis: How Acidification and Stratification Threaten Earth's Largest Carbon Sink

It is well-known and accepted that carbon dioxide, further discussed as carbon, is the most prominent and potentially manageable greenhouse gas on Earth. Globally, carbon comes from a variety of sources that are both natural and anthropogenic (of human origin). Naturally, the Earth can sequester more carbon than is produced. This is the result of both biological productivity known as photosynthesis and carbon naturally dissolving into water. Hence, without the input from anthropogenic sources, the net amount of carbon would naturally decrease in the atmosphere. However, due to anthropogenic sources, primarily from the burning of fossilized solar energy known as fossil fuels coal, oil, and natural gas—there is an increasing glut of excess carbon in the atmosphere that must go somewhere. Additional anthropogenic impacts on the global carbon balance—urbanization, deforestation, habitat loss, decreasing global biological diversity, cement production, and Green Revolution-style agriculture—reinforce the impacts of carbon on a global scale. The only remaining natural sink for excess carbon is the Earth’s oceans. And these oceans are imperiled in their ability to absorb excess carbon, as their limits have been reached.

The global ocean plays a significant role in the Earth’s carbon cycle. On average, terrestrial or land ecosystems sequester as much carbon from the atmosphere as they release. Thus, terrestrial ecosystems tend to not have a role as a global carbon sink (a place to store excess carbon). However, the ocean is a net sink of carbon and represents the largest sink for anthropogenic and natural carbon on Earth. This means that the ocean’s role in the carbon cycle is paramount to the health and quality of all life and ecosystems on the Earth. Without the capacity of the ocean to absorb excess carbon and to sequester more carbon than is released, the Earth would warm quicker and the negative impacts of anthropogenic climate change would be extremely pronounced. Already, the global ocean is feeling the impact of excess anthropogenic carbon. This is manifested in the increasing global temperatures, resulting in increased ocean temperatures, as well as increased ocean acidity. The carbon from the atmosphere is sequestered in many forms, including sea life, phytoplankton, and buried organic solids. However, the most insidious impact of the excess carbon is the carbonation of the water. Carbonic acid is the net result of too much carbon being sequestered. A net global increase in ocean acidity from carbonation is creating a foundation for catastrophe and extinction.

Two key components of oceanic chemistry that must be addressed as they relate to excess anthropogenic carbon are the pH and the salinity of the oceans. The pH is a measure of the alkalinity or acidity of the waters, and the salinity is a measure of how much dissolved salts are in the water. Both pH and salinity play a major role in the health and abundance of ocean life and relate directly to the physical structure of the oceans and the associated natural movement of waters around the globe. Naturally, the ocean is alkaline—having a pH greater than neutral or 7—the natural pH on average is above 8.2. With the impact of excess sources of carbon from anthropogenic sources, the ocean pH is directionally changing toward more acidic levels.

This increase in acidity is having a major negative impact on all sea life that has calcium in their structures and bodies. Carbonic acid is neutralized by the presence of calcium and calcium bicarbonate, both of which are found in the bones of fishes; the shells of crabs, lobsters, clams, and oysters; and in the structures of sea life conglomerates known as corals. The increase in ocean acidity is invariably, in a very real sense, dissolving sea life and thus reducing further the ability of the oceans to sequester excess carbon. Additionally, as excess anthropogenic carbon warms the atmosphere and surrounding oceans, the cryosphere of freshwater thaws at the poles and releases abundant amounts of nonoceanic water into the global oceans.

Freshwater is less dense than seawater, which makes the oceans more strongly stratified. This, in turn, means there will be less turnover in the ocean, creating hypoxic dead zones (devoid of dissolved oxygen) and little nutrient cycling for the propagation of sea life. The stratification of the ocean will also compound the impact of acidification and further reduce the ability of the ocean to sequester atmospheric carbon. The potential for increased stochastic events (chaotic) and unmitigated harm to the planet are the end results of the oceans’ inability to absorb anthropogenic carbon. David Harper Wilson

FURTHER READING:Hoegh-Guldberg, O., et al. 2007. “Coral Reefs Under Rapid Climate Change and Ocean Acidification.” Science 318 (5857): 1737-42.

Jackson, J. B. C. 2008. “Evolution and Extinction in the Brave New Ocean.” Proceedings of the National Academy of Sciences USA 105 (Suppl. 1): 11458-65.

Doney, Scott, V. J. Fabry, R. A. Feely, and J. A. Kleypas 2009. “Ocean Acidification: The Other CO₂ Problem.” Annual Review of Marine Science 1: 169-92.