Incoming solar radiation
Solar radiation reaches the Earth after passing through the atmosphere. Because of the Sun’s high surface temperature, this radiation is concentrated at short wavelengths, mainly from the ultraviolet (UV) to the near-infrared range.
The red curve (Figure 1) represents the radiation emitted by the Sun approximated as blackbody radiation. In physics, a black body is an idealized object that absorbs all incoming radiation and emits radiation according to its temperature. This curve is calculated using Planck’s law for a temperature of approximately 5800 K.
Figure 1: Solar radiation spectrum. The red curve represents blackbody radiation at the Sun’s temperature (~5800 K). The yellow area corresponds to solar radiation after transmission through the atmosphere. Ozone absorbs a substantial fraction of harmful ultraviolet radiation, while water vapor absorbs part of the near-infrared spectrum.
The yellow curve represents the actual solar radiation transmitted through the atmosphere. Compared with the ideal blackbody spectrum, it shows several absorption bands caused by atmospheric gases. Ozone (O₃) strongly absorbs part of the ultraviolet radiation, which is essential for life on Earth, while water vapor (H₂O) absorbs part of the near-infrared radiation. Oxygen and carbon dioxide also contribute to absorption at specific wavelengths. As a result, the transmitted spectrum differs from the ideal solar spectrum.
At the top of the atmosphere, the Earth system receives an average incoming solar flux of about 340 W/m². However, around 30% of this incoming radiation is reflected back to space by clouds, atmospheric particles, and the Earth’s surface: this is the planetary albedo. Consequently, the Earth system absorbs on average about 240 W/m² [1]. Part of this reduction is therefore due to reflection, while another part results from atmospheric absorption. In particular, the absorption of UV radiation by ozone plays a crucial protective role for living organisms.
Outgoing terrestrial radiation
The Earth, being much colder than the Sun, emits radiation primarily in the infrared range. In a first approximation, the terrestrial emission spectrum can be compared with blackbody radiation corresponding to temperatures between about 220 K and 320 K, that is, approximately −50°C to +50°C.
In Figure 2, it was shown that, at radiative equilibrium, the Earth system must emit about 240 W/m² to balance the absorbed solar radiation. In the absence of greenhouse gases, this would correspond to an effective radiating temperature of about −18°C. However, the actual Earth system is not greenhouse-gas free, and its observed emission spectrum does not follow a single Planck curve corresponding to the mean surface temperature of 15°C (Figure 3).
Figure 2: Outgoing radiation as a function of temperature. Without greenhouse gases, an emitted flux of 240 W/m² corresponds to a temperature of −18°C. When greenhouse gases are present, they reduce radiative cooling, requiring a higher surface temperature (~15°C) to maintain energy balance.
Figure 3: Terrestrial radiation spectrum. The green area represents the infrared radiation emitted by the Earth and observed above the atmosphere. The absorption bands are caused by greenhouse gases, which make the atmosphere partially opaque at specific wavelengths. Water vapor contributes the largest share of the natural greenhouse effect, followed by carbon dioxide and other greenhouse gases.
This is because greenhouse gases absorb and emit infrared radiation selectively, at specific wavelengths. The atmosphere is therefore not equally transparent across the entire infrared spectrum. At some wavelengths, radiation escapes directly from the warm surface; at others, it escapes only from higher and colder layers of the atmosphere, where the absorbing gases become sufficiently sparse for radiation to leave the planet. This is why the observed terrestrial spectrum appears to correspond to different emitting temperatures depending on wavelength.
This point is essential: greenhouse gases do not simply “remove” part of the outgoing spectrum and force other wavelengths to compensate mechanically. Rather, they modify the altitude—and therefore the temperature—of the layers from which radiation effectively escapes to space. Since higher atmospheric layers are generally colder, the outgoing infrared flux is reduced at the affected wavelengths. To restore global radiative equilibrium, the surface and lower atmosphere must warm until the Earth system once again emits about 240 W/m² to space.
Figure 3 shows that water vapor is the dominant absorber in the present-day atmosphere and accounts for a large share of the natural greenhouse effect. Other major greenhouse gases include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃) [2]. However, it is important to note that water vapor mainly acts as a feedback in the climate system, whereas carbon dioxide and the other long-lived greenhouse gases act as the primary forcing agents driving long-term warming.
Bibliography
[1] Figure 7.2 in IPCC, 2021: Chapter 7. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Forster, P., T. Storelvmo, K. Armour, W. Collins, J.-L. Dufresne, D. Frame, D.J. Lunt, T. Mauritsen, M.D. Palmer, M. Watanabe, M. Wild, and H. Zhang, 2021: The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 923–1054, doi: 10.1017/9781009157896.009 .]
[2] Thollot, P., & Dequincey, O. (2021, June 3). Rayonnement, opacité et effet de serre — Planet-Terre. Planet-Terre.ens-Lyon.fr. https://planet-terre.ens-lyon.fr/ressource/rayonnement-effet-de-serre.xml