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| Fig. 1: Atmospheric Concentrations of Methane and Carbon Dioxide over Time. [1] (Image Source: M. Swint) |
Colloquially, global warming is often attributed to carbon dioxide (CO2), with the species being a main focal point of decarbonization efforts. And through the sheer bulk of carbon dioxide in the atmosphere at over 591 billion tons of carbon, it is easy to use the gas as the poster child for the greenhouse effect, centering the discussion of climate change around it. [1] However, carbon dioxide is not the only greenhouse gas in the earth's atmosphere. Along with carbon dioxide, other greenhouse gases include methane, nitrous oxide, and fluorinated species (hydrofluorocarbons, perfluorocarbons, etc.).
As a quick overview of global warming - each greenhouse gas has a different energetic effect on the atmosphere referred to as radiative forcings - due to their specific chemical makeups. Radiative forcing is specifically named as the change in the net, downward minus upward, radiative flux. [1] Taking the term downward radiative flux to mean the energy coming into Earth's atmosphere from the sun and the upward radiative flux to mean the energy exiting Earth's atmosphere back to space, radiative forcing is used to describe the change in the atmospheric energy balance as the difference in the two flows of energy. If more energy is entering the earth than leaving, the energy must go somewhere - it reflects off gaseous species in the atmosphere when trying to leave the earth and remains as heat, warming the atmosphere.
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| Fig. 2: Atmospheric Methane Growth Rates over Time. [1] (Image Source: M. Swint) |
As a result of their chemical differences, twenty molecules of carbon dioxide thus do not reflect the same amount of heat as twenty molecules of methane would in the atmosphere at any one time. Like driving ten miles up a mountain road versus ten miles on a flat highway even though the distance was the same, each route had a different energetic effect on the car. Because of these differences in radiative efficiency, it can take fewer molecules of one gas versus the another to have the same effect on the atmosphere. Continuing with the automobile analogy, the wear and tear on a car from a one-mile journey on the mountain road could be equivalent to a longer journey of ten miles on the flat highway.
With that quick explanation of warming, consider the case of methane in the atmosphere. Over the past ~100 years, methane concentration has signficantly increased, from 1025 parts per billion (ppb) in 1920 to 1866 ppb in 2019, as shown in Fig. 1. [1] This change corresponded to a concentration increase of over 80%. For context, the 170 years preceding (1750 to 1920) only saw an increase of 296 ppb. [1] This increase is attributed to industrialization, where population growth and energy consumption began to grow in the late 1800s and continue more rapidly afterwards.
A notable part of this concentration change resides in the late 1970s, where the green revolution allowed for even faster industrialization and greater crop production, leading to increases in emissions-heavy activities like ruminant animal raising, waste management, and fossil fuel extraction and utilization. [1] With further industrialization over the next 50 years, it would be easily assumed that these anthropogenic sources generally maintained or continued to grow, further increasing methane concentration. However, the atmospheric methane growth rate abruptly fell off after the start of the 1980s, as shown in Fig. 2. Fluctuating oil prices led to a decrease of gas flaring emissions, and then the late 1990s saw a decrease in oil and gas emissions of over 10 teragrams per year. [1] This drop in emissions was able to temporarily stabilize the amount of methane in the early 2000s, reflected in a near-constant atmospheric concentration, but emissions began to grow again shortly thereafter and methane again increased.
With much faster rates of concentration increase in the atmosphere and knowing that methane is calculated by the IPCC to be a more effective greenhouse gas than carbon dioxide, the overall increase in methane could be considered worrisome. However, an essential difference between the two species is their respective atmospheric lifetime. Methane breaks down relatively quickly, where reactions with hydroxides and methanotrophs limit its lifetime to 11.8 years. [2] Carbon dioxide, however, has no comparable chemistries that facilitate its destruction, apart from photosynthesis, leading it to have a very long atmospheric lifetime of up to tens of thousands of years. [3]
Note how the concentration of carbon dioxide in the atmosphere increases in Fig. 1 and does not experience the fluctuations that methane does. This is because the timescale at which carbon dioxide would fluctuate is much larger as there is no quick removal process for it like methane has. Methane levels were able to remain near-constant in the early 2000s because it was breaking down at roughly the same rate as it was being produced. Carbon dioxide is not able to experience the same phenomenon, which makes it all the more worrisome as emission rates continue to increase. A large amount of methane in the atmosphere is still subpar and it will heat the earth. But in comparison, methane's part in the greenhouse effect is shorter-lived and more readily addressable than carbon dioxide.
© Maddie Swint. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] V. Masson-Delmotte et al., eds., Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, 2023).
[2] T. F. Stocker et al. eds., Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, 2014).
[3] D. Archer et al., "Atmospheric Lifetime of Fossil Fuel Carbon Dioxide," Annu. Rev. Earth Planet. Sci. 27, 117-134 (2009).