The greenhouse effect denoted by the difference between thermal infrared radiations emitted on the earth’s surface and that released into the atmosphere is a natural phenomenon that is essential in maintaining a hospitable climate on earth. The earth’s climate reflects a thermodynamic engine that is powered by solar radiation (Kweku et al., 2015). At equilibrium, the solar radiation absorbed is balanced by the same amount of infrared radiation released from the surface of the earth. The earth’s temperature is determined by this radiative equilibrium between solar radiation and the infrared radiation (IR). According to Kweku (2015), the logic of why the temperature absorbed on the surface is higher than the mean radiative temperature is elucidated by greenhouse gases (GHGs). The solar radiation hitting the surface of the earth passes through the atmosphere and remains unchanged. Nonetheless, the IR emitted from the surface is partly absorbed by the GHGs, and some of it is re-emitted downwards (Mitchell, 2005). This effect warms the earth by increasing the temperature to about 32k greater than it should be (Mitchell, 2005).
Water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are the GHGs responsible for the greenhouse effect. The most abundant natural GHG is H2O (accounting for 90% of GHG volume, 80% mass, and about 70% of the greenhouse effect). The next is CO2 (which accounts for 25% of the greenhouse effect), followed by N2O, methane, and ozone (O3) (Mitchell, 2005).
From this view, it seems that H2O is the most critical culprit in causing an increase in global temperatures. Yes, water is the most efficient GHG. Nonetheless, it cannot be considered the primary driver of temperature variations. This is because as a solid or a liquid, water is not capable of remaining in the atmosphere for a long time (it typically less than two weeks), (Hausfather, 2008). The amount of H2O in the atmosphere is dependent on the existing temperature. Under normal conditions, as explained above, IR emitted from the surface of the earth goes into the atmosphere and out into space.
Nonetheless, some of the IR is trapped by the GHGs, implying that more radiation will be maintained in the atmosphere to warm the earth (Mitchell, 2005). The rise of temperatures leads to accelerated evaporation of water from ground storage such as rivers, reservoirs, oceans, and soils. This increases the atmospheric concentration of H2O, which causes more warming—the greenhouse effect. This mechanism is known as a positive feedback loop (Hausfather, 2008). In other words, in spite of it being the highest greenhouse effect contributor, H2O can be considered as a passive actor in temperature changes and other GHGs active drivers (they can physically accelerate changes in temperature), because of their physical properties and ability to accumulate in the atmosphere.
Unlike H2O, CO2, CH4 and N2O have long lifespans in the atmosphere. CO2 can remain in the atmosphere for roughly 100 years; CH4 can stay for more than 12 years before decomposing into CO2 and H2O; and N2O can last a century in the atmosphere (Mitchell, 2005). The long lifespans of the GHGs apart from H2O produce sustained warming which drives the H2O feedback. A reduction in the concentration of GHGs causes the H2O feedback to work oppositely: lower temperatures lead to lower concentrations of atmospheric H2O, leading to further cooling of the earth. In summary, the GHGs with longer lifespans serve as the primary source of warming, while water plays a secondary role.
While GHGs occur naturally in the atmosphere, their cycles have been interrupted by human activities such as the burning of fossil fuel, mining, transport and heat production, agriculture, deforestation, and changes in land use (Mitchell, 2005). These activities not only increase the concentration of natural GHGs, they also create synthetic ones such as chloroﬂuorocarbons (CFCs) (Mitchell, 2005). Increases in the levels of GHGs at rates surpassing the earth’s ability to eliminate them from the atmosphere lead to changes in the radiative balance. This results in an increase in temperatures until a new equilibrium is reached and IR is released into the atmosphere in the same quantity as the solar radiation absorbed.
Atmospheric concentrations of human-made GHGs have been increasing at an alarming rate over the past century (Kweku et al., 2017). Each GHG has a different lifespan and radiative force. Due to its absolute concentration and long lifespan, CO2 is considered the GHG with the highest force. Human activities practically release more CO2. The current concentration of CO2 has exceeded the amount that vegetation and oceans can reabsorb. Hence, excess CO2 remains in the atmosphere where it creates a greenhouse effect and stimulates the evaporation of water. Overall, CO2 is expected to contribute to two-thirds of global warming; a quarter will be due to methane and other GHGs will contribute to the rest (Kweku et al., 2017).
Hausfather, Z (2008). The Water Vapor Feedback. Retrieved from: https://www.yaleclimateconnections.org/2008/02/common-climate-misconceptions-the- water-vapor-feedback-2/
Kweku, D. W., Bismark, O., Maxwell, A., Desmond, K. A., Danso, K. B., Oti-Mensah, E. A., & Adormaa, B. B. (2017). Greenhouse Effect: Greenhouse Gases and Their Impact on Global Warming. Journal of Scientific Research and Reports, 1-9.
Mitchell, J. (2005). Climate change and the greenhouse effect-A briefing from the Hadley Center-The Hadley Centre for Climate Prediction and Research. Met Office, United Kingdom, 69.