Natural Cycles and Unnatural Causes of Atmospheric Carbon Dioxide Concentration
The variations in concentrations of carbon dioxide (CO2), the primary greenhouse gas affecting the atmosphere and arguably global climate change, is caused by a process called photosynthesis and the resulting carbon fixation [1]. During the growing season, plants absorb substantial amounts of CO2 from the atmosphere and store the resulting carbohydrates, in their leaves, stems, and roots. Also, the process of photosynthesis releases oxygen to the atmosphere. This facilitates the reduction of the overall amount of carbon dioxide in the atmosphere and mitigates its effects on the environment.
However, when plants die or are burned, the carbon stored in them is released back into the atmosphere, adding to the overall levels of carbon dioxide. Deforestation and other land use changes can also contribute to increased levels of atmospheric CO2 [2].
Over the millennia, the amount of CO2 in the atmosphere has varied from 180 ppm to 300 ppm [3]. However, only after the industrial revolution did CO2 levels fluctuate higher on average. Today, concentrations as high as 385 ppm are seen. This is not a natural phenomenon. This increase is due, in part, to changes in the Earth’s climate and the way that CO2 is exchanged between various parts of the carbon cycle. For example, during warm periods like the present day, there is more exchange of CO2 between the atmosphere and oceans, and less exchange between the atmosphere and land. This means that there is more CO2 in the atmosphere during warm periods.
There are other pollutants that influence the concentration of CO2 in the environment [4]. For example, methane is produced by natural sources like wetlands and termites, but it is also emitted by human activities like livestock farming and natural gas extraction. Nitrous oxide is emitted by both natural and human sources, including oceans, rainforests, and agricultural activities [5]. Chlorofluorocarbons and hydrofluorocarbons are man-made chemicals that are used in a variety of industrial applications. They are both very potent greenhouse gases, but because they do not occur naturally in the atmosphere, their concentrations are much lower than those of carbon dioxide, methane, or nitrous oxide.
Notwithstanding the foregoing, when there are fewer plants on Earth (such as during times of cooler climates), they take in less CO2 from the air through photosynthesis. This leaves more CO2 in the atmosphere, leading to an increase in atmospheric concentrations of this gas. By inference, the oxygen concentration is lower.
While much of the CO2 in the atmosphere as well as other chemicals has influenced our environment, most of the changes in natural CO2 have occurred as a consequence of man-made phenomena. The increased level of atmospheric CO2 from fossil fuel combustion has several effects on the global climate. It alters the Earth’s energy balance by trapping heat that would otherwise escape into space, high levels of atmospheric CO2 can cause ocean acidification and this process can have harmful effects on marine life, as well as coral reefs that are already struggling to cope with rising ocean temperatures, and increased levels of atmospheric CO2 can also contribute to drought conditions in some regions by altering precipitation patterns.
References:
1. Dusenge, M.E., Duarte, A.G. and Way, D.A. (2019), Plant carbon metabolism and climate change: elevated CO2 and temperature impacts on photosynthesis, photorespiration and respiration. New Phytol, 221: 32–49. https://doi.org/10.1111/nph.15283
2. G. Bala bala@llnl.gov, K. Caldeira, M. Wickett, and A. Mirin, edited by Peter Vitousek, Stanford University, Stanford, CA, and approved February 24, 2007, Combined climate and carbon-cycle effects of large-scale deforestation, Environmental Sciences, 104 (16) 6550–6555. https://doi.org/10.1073/pnas.0608998104
3. https://www.amnh.org/exhibitions/climate-change/changing-atmosphere/the-ups-and-downs-of-CO2.
4. https://www.nationalgeographic.com/environment/article/methane
