OCEANS AND THE CARBON CYCLE
A
The element carbon is found everywhere in the universe, including all living things on earth, which assimilate it in the form of carbon dioxide either by utilizing it in photosynthesis as most plants do or by consuming it in food as animals do. This is only a small part of the exchange that is constantly taking place among the four spheres of the earth—the biosphere, hydrosphere, lithosphere and atmosphere—in a process known as the carbon cycle. Carbon is never created or destroyed; it is simply cycled through the earth’s systems. Living things die and are buried in various locations, and as these decompose, carbon is released into the terrestrial and aquatic surroundings.
B
About 99 percent of the earth’s carbon is locked up in its crust. When rocks in the lithosphere erode, carbon is released into the atmosphere and hydrosphere, a transfer that can take millions of years. The same thing occurs when decaying plants and animals sink to the bottom of the ocean. The released carbon will ultimately reach the surface through a series of complex processes, although the time involved is measurable in centuries rather than millions of years. Carbon that enters the ocean from the atmosphere dissolves in the form of CO2, reacting with bicarbonate and carbonate, and is known as inorganic carbon. Of the three spheres that carry the 1 percent of earth’s carbon—oceans, atmosphere and living things—the oceans store the greatest amount.
C
In the past century or so, the oceans have been storing increasingly larger and larger quantities of carbon. Prior to the industrial revolution, carbon dioxide concentrations in the atmosphere were similar to what it was during the last ice age about 12,000 years ago. During this period, it may be said that the carbon cycle was at equilibrium. From year to year, the oceans were releasing 600 million tons of carbon into the atmosphere in exchange for equivalent amounts of organic carbon from plants and bacteria and inorganic carbon from limestone. The amounts exchanged remained relatively unchanged. However, the industrial revolution has changed all that. The burning of fossil fuels for manufacturing needs, burgeoning electricity and car use, and land use practices have increased the levels of carbon dioxide in the air surrounding the earth. Known as anthropogenic carbon dioxide, this type of CO2 is accumulating in the atmosphere at a pace that has had critical impact on the carbon cycle, resulting in altered concentrations of the element in carbon reservoirs, particularly the ocean.
D
Carbon dioxide is absorbed by the ocean when the gas migrates from the atmosphere to the water. A chemical reaction ensues, with CO2 and carbonate ions in the water molecules forming carbonic acid. This process is called carbonation, and because it takes place immediately, the oceans have a ten-fold capacity over freshwater bodies to take in carbon dioxide. Scientists call this type of assimilation a sink, a sequence of reactions that act to absorb or remove a substance from a system. The seas thus play an important role in removing excess anthropogenic CO2 from the atmosphere, which can prevent global warming. However, the buffering of excessive amounts of CO2 by reaction with carbonate takes thousands of years. CO2 emissions from human activity is so plentiful that the oceans, large though they may be, cannot absorb these gases as rapidly as they are emitted. Thus, the disequilibrium that was initially caused by the industrial revolution and is today reinforced by human activity is expected to persevere.
E
How has the excessive uptake of CO2 impacted the hydrosphere? Human activity has produced 224 billion metric tons of carbon dioxide since 1765, increasing the levels of atmospheric CO2 from 280 parts per million prior to the industrial revolution to 380 parts per million. The latter amount was only half of what scientists predicted atmospheric levels to be, and they wondered where the other half was. Studies affirmed that the “missing” half had been absorbed by the oceans.
F
The large amounts of dissolved CO2 in the ocean water have profoundly changed its chemistry, resulting in a lower Ph at the ocean surface. This has slowed the growth of marine life at the most basic levels of the ocean food chain—plankton, corals and other invertebrates. As the oceans continue to acidify, organisms with shells and algae will not be able to absorb carbonate ions to form calcium carbonate shells as these ions mainly play the role of dissolving CO2. Mass extinction of organisms could follow, with coral at particular risk because acidity dissolves its calcium carbonate skeletons. With coral dying, young fish would be deprived of the nurseries they need to help them survive. If oceans continue to take in excess atmospheric CO2, the result could alter food web structures in the ocean, and ultimately, on land.
G