Being a grad student at an environmental school, I have a lot more exposure to climate change-related classes, and since my work focuses on greenhouse gases, I was very interested in learning more about the carbon cycle. I hadn't given it much thought before this class, but human alterations to this cycle are truly having catastrophic outcomes. I hope that this is informative!
To start, carbon is a critical element for the planet, being so much more than a little square on the periodic table: it's incorporated in fundamental earth processes at both the organismal and ecosystem level, it constitutes all organic matter, is involved in photosynthesis and respiration in primary production, is necessary for metabolism and energy transfer, constitutes body fluid components (like blood), and is involved in buffering processes [1]. However, carbon is also a component of noteworthy greenhouse gases, such as carbon dioxide (CO2) and methane (CH4), trapping heat in the atmosphere. Understanding the global carbon cycle, the sources (where carbon emits from) and sinks (what takes up and stores carbon), and the drivers of changes to the cycle is necessary for predicting the future changes in the carbon cycle and the potential impacts on the ecosystem.
So, what is the Carbon Cycle?
This is how the molecule carbon gets circulated throughout the environment. The global carbon cycle is made up of reservoirs which naturally contain a vast amount of carbon, sources which release carbon, and sinks that uptake carbon (briefly shown in Figure 1). The movement of carbon between these is a cycle that was once in equilibrium: the release would equal the uptake. Human activity has since disrupted the balance, leading to a greater net release of carbon to the atmosphere than the sinks can match in their uptake.
Figure 1. The Global Carbon Cycle. Depicted are several sources and sinks involved in the release (upward arrows) or sequestration (downward arrows) of carbon.
Where is it stored? Carbon is naturally stored in reservoirs such as the Earth’s oceans (38,400 Gt), atmosphere (720 Gt), and crust (15,000,000 Gt of carbon in kerogens and >60,000,000 Gt in sedimentary carbonate) [2]. Within the oceans, carbon is not distributed evenly: ocean currents distribute carbon heavily in the Northern Atlantic compared to other regions [3], ocean stratification occurs from a temperature-depth gradient (maintaining carbon at deep levels), ocean-atmospheric mixing occurs at the surface, and carbon is deposited and buried deep in the ocean floor [4]. Moreover, carbon is found in terrestrial ecosystems in soils and compounds from living organisms, the concentration of carbon decreases with soil depth [5], and the compression of organic matter over geologic time creates a carbon pool that later becomes fossil fuels. In the atmosphere, CO2 and CH4 are naturally present, trapping heat and allowing life on Earth to exist, but they are increasing in concentration, leading to rapid warming. Together, these reservoirs have many processes happening concurrently, cycling carbon indefinitely between sources and sinks through the Earth’s system.
What are the sources? Natural sources of carbon include plant and soil respiration, oceanic diffusion (carbon molecules naturally moving to the atmosphere from the ocean), and the microbial breakdown of organic matter in both terrestrial and aquatic systems, including the decomposition of litterfall (leaves and other tree bits that fall to the ground). Natural wetlands contribute 32% of global methane emissions due to high organic matter content and the presence of methanogens (methane producing microorganisms), while ruminants produce 8% of emissions, followed by freshwater (6%), termites (2%), and wild animals (2%) [1]. Volcanic eruptions also contribute a small fraction of carbon to the atmosphere.
What removes carbon from the atmosphere? Carbon sinks balance the natural sources of carbon. During photosynthesis, terrestrial and aquatic plants use CO2 to create glucose, and plants can store carbon for years (21 years in living plant biomass, 9 years in woody debris, and 2.2 years in roots [1]), some which can take years to be released during decomposition. Carbon harnessed by the plants is transferred to the soil through roots and becomes stored in the soil carbon pool. Moreover, the oceans also function as sinks in addition to reservoirs, extracting carbon from the atmosphere through mixing (i.e., wind and currents) and diffusion to which water and CO2 form carbonic acid or carbonate, an important component for marine organisms to create shells.
Global and temporal patterns
Spatially and temporally, carbon has not been released at an equal rate. Over the past 150 years, the flux of carbon to the atmosphere has been constantly increasing, notably since the Industrial Revolution period (circa 1760-1840) when human activities began to change (Figure 2). Land use change, such as deforestation and burning for urban development, fuel, and grazing, and coal use in factories were the main sources of CO2 emissions from 1880 until around 1950. In the 1960s, the addition of oil and gas usage saw a drastic rise in CO2 emissions [1].
Globally, emission rates have changed across countries. In the mid to late-1800s, CO2 has been released at unprecedented rates, with Asia (excluding China), China, and India comprising the top emitters in 2020 from fossil fuels alone. However, since 1750, the United States has produced the most cumulative CO2 emissions (416 billion tons), and per capita, the United States, Canada, Australia, including some oil producing countries in the East, are among the top emitters [6].
Figure 2. Global CO2 emissions from fossil fuels. [6]
Future predictions of the carbon cycle
In the coming years, human activity will cause (and are causing) changes in the carbon cycle. Since the human population is always growing, deforestation will continue to occur to make room for farming and housing, and this will both release carbon and reduce the future ability of forests to sequester carbon. The act of cultivating has caused a 20-30% of carbon loss from the soils [1] due to a loss of plant roots and increased tillage and erosion [7]. Humans also breed ruminants (resulting in 14% of global CH4 emissions) and produce profuse amounts of landfill waste (11% of global CH4 emissions) which further overloads the atmosphere with carbon [1]. Moreover, the burning of fossil fuels (14% of global CH4 emissions [1]) to sustain energy demands is increasing. As these greenhouse gases trap more heat, the temperature rises, speeding the rate of organic matter decomposition and carbon outputs. Oceans, as prominent carbon sinks, will then absorb more carbon leading to ocean acidification, causing marine organisms to die and the organic matter to breakdown. Climate change will lead to irregular rainfall patterns, leading wetter and warmer regions to have greater carbon fluxes compared to drier regions [1]. Positively, climatic warming will lengthen the growing season, allowing plants to sequester carbon for longer time periods.
While the longevity of CH4 is relatively short compared to CO2, it is more efficient at absorbing heat, having 84x more warming potential compared to CO2 in the first 20 years of emission [1]. On the other hand, CO2 lasts hundreds of years longer than CH4, so the carbon that is released into the atmosphere today will be around for generations to come.
Consequently, the growing rate of carbon entering the atmosphere from anthropogenic activity increases the atmospheric pools, leading to warming, so global policy makers need to cut back carbon emissions immediately to preserve the integrity of ecosystems in years to come, if it’s not too late already.
References
1. Driscoll, C. Biogeochemistry of Carbon. (2022). Lecture at SUNY Environmental School of Forestry. Syracuse, NY.
2. Falkowski, P. et al. The global carbon cycle: A test of our knowledge of Earth as a system. Science (80-. ). 290, 291–296 (2000).
3. Ocean Carbon Storage. Available at: https://www.pmel.noaa.gov/co2/story/Ocean+Carbon+Storage. (Accessed: 19th October 2022)
4. Hönisch, B. et al. The geological record of ocean acidification. Science (80-. ). 335, 1058–1063 (2012).
5. Minasny, B., McBratney, A. B., Malone, B. P. & Wheeler, I. Digital mapping of soil carbon. in Advances in Agronomy (ed. Sparks, D. L. B. T.-A. in A.) 118, 1–47 (Academic Press, 2013).
6. Ritchie, H. & Roser, M. CO₂ and greenhouse gas emissions. OurWorldInData (2020). Available at: https://ourworldindata.org/co2-emissions. (Accessed: 19th October 2022)
7. Lemus, R. & Lal, R. Bioenergy crops and carbon sequestration. CRC. Crit. Rev. Plant Sci. 24, 1–21 (2005).
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