The Significance of Earth’s Carbon Cycle
The carbon atom is recognized as “the building block” of life, not only for its abundance but also for its unique chemical structure, which allows the formation of numerous and diverse stable bonds. It takes many forms and is found in all of Earth’s spheres – from nucleic acid within the DNA of all living things, to the inorganic compounds essential to all ecosystems.
Carbon is in constant flux from one form to another in an expansive network known as the carbon cycle. Atmospheric carbon in the form of carbon dioxide (CO2) is one compound that is particularly vital to Earth’s carbon cycle for its climate-regulating properties. Despite its importance to life on Earth, CO2 is also a pervasive greenhouse gas, and there is a rightful concern for its ever-increasing concentration in the atmosphere.
Oceanic Carbon
The ocean and the atmosphere are in a perpetual carbon flux wherein CO2 is exchanged between air and water. Across the globe, the concentration of atmospheric CO2 is generally higher than that of the ocean, which obligates the particles to move into the water by the principle of diffusion where they dissolve into individual ions (McKinley et al., 2017). The ocean’s uptake of atmospheric carbon through diffusion and dissolution is the first of many processes in the complex oceanic carbon cycle.
The dissolved inorganic carbon (DIC) present in seawater is essential to several life forms such as phytoplankton and corals. Near the surface, phytoplankton utilize DIC during photosynthesis, producing various fixed organic compounds as byproducts, several of which are essential for other marine organisms. Calcium carbonate (CaCO3), for example, is used by various organisms to construct the hard tissues of their body. Corals, whose skeletons consist largely of CaCO3, are a vital example. They are estimated to sink, or intake, 70 to 90 megatons of carbon annually (Kault et al., 2022). Not only are coral reefs a significant point of carbon balance with net zero emissions, but their calcareous skeletons form an ecosystem that is habitat to several thousand species – an estimated 25% of all marine life (Viau, 2020). The carbon used and distributed by phytoplankton and corals illustrates the significance of biological processes in Earth’s carbon cycle.
There are several mechanical processes throughout the carbon cycle, as well. One key aspect of ocean carbon cycling is the solubility-temperature variance of seawater. Seawater solubility varies by temperature, with cooler waters being able to absorb more carbon. As ocean currents circulate waters across the globe, the surface temperatures change, and carbon is cycled accordingly. Boundary currents, for example, push warm equatorial waters poleward by prevailing winds, where they drop in temperature and are saturated enough to uptake more CO2 (McKinley et al., 2017). With a higher concentration of DIC now in these cooler surface waters, the density increases, and they are subducted into the depths. Working in tandem, ocean temperature, and circulation are the largest drivers of carbon sink variability.
As the waters move, carrying carbon along with them, there are several direct results at the surface including changes to air pressure, humidity, and other weather patterns. Balance of oceanic processes is essential to maintaining “cool temperatures along the equator, large outgassing of natural carbon and oxygen, biological productivity, and intense heat uptake” (Delorme & Eddebbar, 2016). The ocean is thus the largest regulator of Earth’s climate.
Effects of Human Emissions
As a carbon sink, the ocean uptakes more carbon than it outgasses, and given the atmosphere’s increasing carbon concentration due to human emissions, the oceanic concentration increases, as well. The ocean is observed to be absorbing around 30% of anthropogenic emissions ("Effects of Changing," 2011). While this system manages atmospheric heat, it is not without consequences. CO2 reacts with seawater to create carbonic acid, which is exponentially increasing. As a result, the ocean’s pH has decreased by .1 in the last 200 years, a 30% change in acidity ("Effects of Changing," 2011). Ocean acidification is one of the largest threats to Earth’s waters.
Vital marine life is at risk. The skeletal structure of corals is made thin and fragile by acidic waters as there is less CaCO3 available to them ("Effects of Changing," 2011). With corals unable to grow, they are also unable to sequester carbon as usual, creating a positive feedback loop. Less carbon sequestration means higher carbon concentration. Higher carbon concentration means higher acidity that further inhibits coral sequestration abilities.
Land Carbon
Land ecosystems facilitate a myriad of carbon fluxes in both the biosphere and geosphere. Earth’s forests are the second largest carbon sink behind the ocean. The process of photosynthesis – which intakes CO2 and releases oxygen – is the primary function that sinks carbon into forests. As plants grow, the uptaken carbon is stored within their various tissues. Trees can sequester remarkable amounts of carbon, around 50 pounds annually after reaching maturity.
There are points of carbon outgassing on land, as well. Biologically stored carbon is released as CO2 through processes such as organic decomposition, animal respiration, and forest fires. Soil respiration is the largest point of outgassing, as soil’s carbon content is three times higher than that of the atmosphere. Because soil is a point of carbon efflux, biomes such as grasslands and savannas are net carbon emitters (Chen et al., 2024).
Tropical rainforests, in contrast, are often referred to as “Earth’s lungs” for their significant contribution to the global carbon cycle. They have more points of carbon flux than any other ecosystem and account for 15% of Earth’s entire land mass, nearly 50% of all Earth’s forest cover, and over 60% of terrestrial life (Chen et al., 2024). Tropical rainforests represent the largest on land carbon sink, and their warm, moist climates are essential to climate regulation across the planet.
Effects of Human Activities
Land use changes are a major threat to Earth’s vital tropical biomes. Across the globe, forest cover is lost due to the rapid increase of urbanization, forestry, and agriculture. Southeast Asia – where about 50% of all tropical peatlands are located – is a striking example of the consequences of human activity on these carbon-sinking ecosystems. “Nearly 80% of [them] are either deforested or drained for plantations and agriculture, which switches [the peatlands] from a net sink of carbon to its source” (Chen et al., 2024).
Dramatic and rapid changes are not isolated to the Asian tropics, either. Over 420 million hectares of forests globally have been destroyed within the last three decades (Chen et al., 2024). Forest coverage loss reduces rainfall and exacerbates high temperatures, resulting in drought that endangers vegetative life and alters the landscape. This is because “any changes in [precipitation or temperature] could alter the rainforest ecosystem into Savanna or semi-desert-like ecosystems…” (Nash et al., 2024). The entire biome becomes unrecognizable. The consequences of damaging “Earth’s lungs” are felt globally: longer, hotter summers in more poleward regions, increasingly frequent and intense storms and forest fires, and disruptions to the oceanic carbon cycle.
Works Cited
Chen, W., Chen, D., & Chen, R. (2024). Faster dieback of rainforests altering tropical carbon sinks under climate change. npj Climate and Atmospheric Science, 7(1). https://doi.org/10.1038/s41612-024-00793-0
Delorme, B., & Eddebbar, Y. (2016, October). Ocean Circulation and Climate: an Overview. Ocean & Climate Platform.
Kault, J., Jacob, F., & Detournay, O. (2022, February 21). Carbon balance in corals. Coral Guardian. https://www.coralguardian.org/en/carbon-balance-in-corals/
McKinley, G. A., McKinley, A. R., Lovenduski, N. S., & Pilcher, D. J. (2017). Natural variability and anthropogenic trends in the Ocean Carbon Sink. Annual Review of Marine Science, 9(1), 125–150. https://doi.org/10.1146/annurev-marine-010816-060529
NASA. (2011, June 16). The Carbon Cycle. NASA Earth Observatory. https://earthobservatory.nasa.gov/features/CarbonCycle/page5.php
Viau, E. (2020, December 3). Coral reef restoration: sexual or asexual reproduction of corals. Coral Guardian. https://www.coralguardian.org/en/coral-reef-restoration-sexual-or-asexual-reproductions-of-corals/
