Earth’s Radiation Balance

by Sebastian Hettrich

Climate is driven by energy. In order to understand climate, one therefore has to first understand the basics of Earth‘s energy flows. The main energy source is the sun, which transmits energy via the incoming solar radiation onto Earth. These energy flows are usually depicted in a radiation balance diagram as shown here. The diagram and the numbers therein are taken from a study by Trenberth et al. (2009).

Averaged over the entire Earth surface, the incoming solar energy amounts to 341.3 Watts per square metre (W/m²), most of it being visible light. Of this incoming energy, 102 W/m² get directly reflected back into space. White surfaces like clouds have a higher reflectivity, scientists speak of high albedo. The same can be said about the polar ice caps which are efficient reflectors. In short, the darker a surface, the lower the reflectivity, thus the albedo. Hence, 161 W/m² get absorbed by the surface and most of it is transformed from visible light into heat, or infrared radiation, for example an asphalt street in summer absorbs light efficiently and can get really hot as result of the transformation into long-wave or infrared radiation. 

Gases, dust, and aerosols within the atmosphere absorb the remaining 78 W/m². 

Looking at the outgoing energy flows, a warm surface creates thermals which transport 17 W/m² back into the atmosphere while the evapotranspiration of water from the surface transports another 80 W/m² into the atmosphere. Together, they amount to 97 W/m², so it would perhaps make sense to think that there is only a maximum of 64 W/m² left to be emitted as infrared radiation. However, the surface radiation amounts to  a total of 396 W/m², which is almost 2.5 times the incoming solar radiation. 

This is where the greenhouse effect comes into play: a significant amount of energy is essentially trapped in the atmosphere. Certain greenhouse gases such as water vapour, carbon dioxide, methane, nitrogen oxides, and even some hydrofluorocarbons are absorbing and re-emitting infrared radiation. So they reflect a total of 333 W/m² of energy, in the form of heat, back to Earth’s surface. 

Only 40 W/m² of the surface radiation go directly into space, another 199 W/m² in infrared stem from processes within the atmosphere and emissions by greenhouse gases.

Together with the reflected  solar radiation and being a bit more precise in the numbers, a total of 340.4 W/m² gets re-emitted into space. 

The numbers show an imbalance of 0.9 W/m² between incoming and outgoing radiation. This is due to the enhanced greenhouse effect due to human-made changes in the greenhouse gas abundance. The energy that gets trapped on Earth mainly ends up warming the oceans [Hansen et al., 2005]. Resulting effects like increasing amount and strength of tropical storms, sea level rise, change in ocean currents, or coral bleaching may be addressed in more detail within a future article.

This radiation balance shows that actually only a third of the energy we receive on Earth’s surface is directly from the sun. Two thirds of the energy is from the back radiation of greenhouse gases in our atmosphere. 

We can conclude three things from this: first, without the back radiation from greenhouse gases, the source would receive only a third of the energy and therefore be quite cold; second, the relatively low abundant greenhouse gases have a higher impact on heating the surface as the direct solar radiation has, and third therefore it only requires small changes in the abundance of those greenhouse gases to have tremendous effects on the back radiation and the heating of Earth’s surface.

References:

Trenberth, K.E., Fasullo, J.T., Kiehl, J., 2009, Earth‘s global energy budget, Bulletin of the American Meteorological Society, 90, 311-324, doi.org/10.1175/200BAMS2634.1.

Hansen, J., Nazarenko, L., Ruedy, R., Sato, M., Willis, J., Del Genio, A., Koch, D., Lacis, A., Lo, K., Menon, S., Novakov, T., Perlwitz, J., Russell, G., Schmidt, G.A., Tausnev, N., 2005, Earth’s energy imbalance: Confirmation and implications, Science, 308, 1431-1435, doi.org/10.1126/science.1110252 .