Few, if any, elements are as versatile as carbon. The carbon atom has four valence electrons, which renders it capable of forming more compounds than any other element, and that fact makes it indispensable in the development of living organisms. This versatile and abundant element cycles regularly through the Earth's atmosphere, hydrosphere, geosphere and biosphere, which essentially comprise a carbon reservoirs list.
The atmosphere is particularly important in the carbon cycle because it is a carbon dioxide reservoir. Carbon dioxide is a gas, and photosynthesizing plants in the biosphere, which comprise another important reservoir in the carbon cycle, depend on it for respiration. However, the hydrosphere, which includes all the world's oceans, arguably makes a more significant impact, owing to the fact that oceans cover 70 percent of the surface area of the planet. The geosphere, for its part, locks carbon into solid structures that last for millennia and releases it through volcanic activity.
Carbon Cycle Definition
Trying to determine where the carbon cycle begins is a bit like trying to determine which came first, the chicken or the egg, but let's begin with the geosphere. Carbon that has been locked for ages in sedimentary rock gets released into the atmosphere by volcanoes as carbon dioxide. Some of it is used by plants for respiration, and some dissolves into the oceans. Some also returns back to the earth as sediment formed over eons by erosion and other natural processes.
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Living beings that excrete carbon dioxide as part of their respiratory process help maintain the concentration of carbon dioxide in the atmosphere. In addition, most – but not all – of the carbon dioxide that dissolves in sea water gets reabsorbed into the atmosphere. In this way, carbon cycles ceaselessly through the earth's ecosystems.
The Atmosphere as a Reservoir in the Carbon Cycle
Carbon dioxide only accounts for about 0.04 percent of the gases in the atmosphere. For the last 800,000 years, the concentration of carbon dioxide has remained below 300 parts per million. However, it began to rise during the Industrial Revolution, and in the past 50 years has risen an average of 0.6 ppm each year. In 2018, the scientists at Mauna Loa Observatory in Hawaii reported the concentration to be 410.79 ppm (see Resources). Scientists attribute the rise to human activity.
The fast rise upsets the carbon cycle. Some of the excess carbon dioxide gets absorbed into the oceans or used for respiration, but most of it remains in the atmosphere, where it combines with other trace gases to create a warming effect on the planet. It's a greenhouse gas, and the rapid rise in its atmospheric concentration has scientists worried.
The Oceans Are Another Key Carbon Dioxide Reservoir
The oceans absorb about 25 percent of the atmospheric carbon dioxide. Sea creatures are able to convert it into shells for their bodies which eventually fall to the ocean floor as sediment. Moreover, algae and other photosynthesizing sea flora use carbon dioxide directly for respiration.
When carbon dioxide dissolves into sea water, it produces carbonic acid. The rising amounts of atmospheric carbon dioxide thus produce a corresponding increase in ocean acidification. This has a harmful effect on sea creatures, because it makes their shells weaker and more brittle. Even worse, at some point, the oceans will become too acidic to absorb any more carbon dioxide from the atmosphere. That could kick the accelerating rise in atmospheric carbon dioxide into overdrive and cause a meteoric rise in the earth's surface temperature.