The “Concepts related to the climate science explained as simply as possible” series. Concept No1: The “Greenhouse gas effect”


Edited by Effie Karozou, MSc Environmental Physics

Democritus, an ancient Greek philosopher from Abdera of Thrace and a universal mind of his era (4th Century A. D.), was he who gave us the first definition of the Galaxy: “The Galaxy is a meeting of rays because of the density of many small continuous stars that give light together”.1,2 His view of the Cosmos is widely accepted today as modern astronomers define the Galaxy as an extremely large collection of millions or billions of stars, glowing nebulae, gas, dust and dark matter bound together by gravitation.3 Our solar system is just a small part of this Galaxy – which is also referred to as the “Milky Way Galaxy” (from the Latin “Via Lactea”) as it appears to form a milky, bright stripe of many stars, crossing the Earth’s visible sky from one side of the horizon to the other – with the Sun and the Earth located at the fringes of it.4 So far, and while there are eight more planets in our solar system (Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), Earth is the only one known to host life.5 But, have you ever wondered why this is happening?

To our knowledge, our planet’s habitability depends largely on the atmosphere surrounding the Earth and the natural occurring greenhouse gas effect phenomenon.5 Atmospheric air, among the major atmospheric components Nitrogen and Oxygen, contains many trace gases that are responsible for the greenhouse effect which is the ”holy grail of life” in our planet. Without these naturally occurring greenhouse gases, incoming sunlight would be radiated back into space and life on Earth would not be as we know it since its average temperature would be around -18oC.6   

Our planet receives energy from the Sun in the form of ultraviolet, visible, and near-infrared radiation. This form of radiation is called short-wave radiation. About 30% of this radiation is reflected back to space. The rest is absorbed by the earth’s atmosphere, clouds and mostly the earth’s surface. This absorbed short-wave radiation is emitted back, now in the form of long-wave infrared thermal radiation, which is responsible for heat. Unlike shortwave radiation, the atmospheric greenhouse gases have the ability to strongly absorb infrared radiation. Therefore, the released thermal energy from the earth’s surface (also clouds and other atmospheric elements but in smaller amounts), instead of escaping into space, is taken in by the greenhouse gases in the atmosphere. After that, a part is re-emitted upwards to space and the other part downwards towards the earth’s surface where it is absorbed again and re-emitted. This way thermal energy is effectively trapped between the ground and the lower 10km of the atmosphere and is responsible for keeping the earth at a habitable temperature with an average of about 15oC (Figure 1).7

However, the rapid economic growth of the early Industrial Revolution fueled human activities in such a degree that heavily impacted the climate system. Temperature data revealed that the last decade was the hottest ever recorded since 1880s, with global temperature being 0.98oC higher than the mean.8 Human derived emissions by the combustion of fossil fuels (oil, coal and natural gas), deforestation of tropical terrestrial environment and land desertification drive global warming and increase the net amount of greenhouse gases molecules contained in the atmosphere (Figure 2). By absorbing heat and re-emitting infrared radiation the increased amount of molecules of greenhouse gases traps excessive heat in the atmosphere contributing to the ongoing global warming.9 It should be mentioned that from all greenhouse gases, CO2 plays a key role in regulating temperature on Earth as it is more abundant in the atmosphere and, also, it remains in the system for a long time since it is quite stable.9 Svante Arrhenius, a Swedish Nobel Laurate in Chemistry, was the first who speculated that increases in the atmospheric concentrations of CO2 derived through the burning of fossil fuels would be key contributors in global warming.10 According to the United States Environmental Protection Agency, human emissions cause increases in atmospheric concentrations of CO2 that will last thousands of years.11 This could mean that action to mitigate global emissions is needed emergently. 

The enhanced greenhouse effect and the resulting climate change will undoubtedly have devastating consequences for natural ecosystems and influence human health in such a way that could change the practice of medicine. The era of Climate Medicine has just begun!

Figure 2: NPS, 2012: What is Climate Change? U.S. Department of the Interior, National Park Service. URL | Detail(used with permission).


1. Fortenbaugh WW, Pender EE. Heraclides of Pontus: Discussion. Rutgers University studies in classical humanities, v15. New Brunswick, NJ: Transaction Publishers. 2009.

2. Lynn WT. Democritus and Galileo on the Milky Way. The Observatory 1901;24:382-383.

3. Sparke LS, Gallagher III JS. Galaxies in the Universe: An Introduction. Cambridge University Press, 2000. ISBN 978-0-521-59740-1.

4. Richter P. Gas Accretion onto the Milky Way. In Gas Accretion onto Galaxies, eds A. Fox, R. Dave. Astrophysics and Space Science Library 2017, vol 430.

5. Olson SL, Schwieterman EW, Reinhard CT, Lyons TW. Earth: Atmospheric Evolution of a Habitable Planet. In Handbook of Exoplanets, edited by H.J. Deeg and J. A. Belmontes. Springer International Publishing, Cham. 2017, p. 1-37. Doi:

6. North GR. Climate and Climate change | Greenhouse gas effect. Encyclopedia of Atmospheric Sciences (Second edition). 2015, p. 80-86.

7. Houghton RA. Greenhouse effect. In: Environmental Geology. Encyclopedia of Earth Science. Springer, Dordrecht, 1999 Edition.

8. NOAA National Centers for Environmental Information, State of the Climate: Global Climate Report for Annual 2019, published online January 2020, retrieved on January 16, 2020 from

9. Solomon, S., D. Qin, M. Manning, R.B. Alley, T. Berntsen, N.L. Bindoff, Z. Chen, A. Chidthaisong, J.M. Gregory, G.C. Hegerl, M. Heimann, B. Hewitson, B.J. Hoskins, F. Joos, J. Jouzel, V. Kattsov, U. Lohmann, T. Matsuno, M. Molina, N. Nicholls, J. Overpeck, G. Raga, V. Ramaswamy, J. Ren, M. Rusticucci, R. Somerville, T.F. Stocker, P. Whetton, R.A. Wood and D. Wratt (2007). Technical Summary. In: Climate Change 2007: The Physical Science Basis Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

10. Arrhenius S. On the influence of carbonic acid in the air upon the temperature of the ground. Philosophical Magazine and Journal of Science, Series 5, 1896;41:p. 237-276.

11. Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland (2007). Changes in Atmospheric Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.