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Earth's radiation budget and Greenhouse Effect

The climate system is driven by the energy of solar radiation. Only a certain fraction of this energy reaches the Earth's surface and leads to a warming. Other fractions are reflected or absorbed by the atmosphere. How exactly does the radiation system work?

 

a) With respect to radiation we can divide the system Earth into three parts:
1) the space above the atmosphere
2) the atmosphere
3) the Earth surface
If we assume a system with a quick heat exchange, we can say: In all parts the amount of energy coming in is the same than the amount of the energy going out. Otherwise one part would constantly win or loose energy and become either warmer and warmer or colder and colder. In the long term this is not the case. There is an equilibrium in all parts.
b) In such a system greenhouse gases do NOT produce energy. They help to generate an equilibrium in which the surface layer of the atmosphere is unusually warm.

 
IPCC TAR global radiation budget

1. The global radiation budget as published in IPCC TAR Chap. 1.2.1
In the following sections we try to understand the different sorts of energy transport in the system of atmosphere and Earth surface.
Please click to enlarge! (40 K)

The reality is a bit more complicated, because the oceans react very slowly to temperature changes. They are inert. While the atmosphere becomes warmer rapidly, the oceans take up the heat slowly and do not warm up immediately. Therefore, for a certain time (one turnover of the oceans takes about 1000 years) the Earth can be in a disequilibrium, before also the oceans reach the state of equilibrium. Radiation budgets as shown in the image above are therefore and also for other unknowns not exact, but have an uncertainty of 10-20%. In the following sections however, we assume an equilibrium.

 

2. Greenhouse gases keep the boundary layer warm like warm clothes in winter our body.
adapted from: fashion 3sat online

 

The role of greenhouse gases

The role of greenhouse gases is the one of a pullover in cold winter days. If we would not wear clothes in winter, our body would cool down and freeze. However, the pullover does neither make the surrounding warmer, nor makes our body produce more energy, nor produces energy itself. It simple reflects back a part of the energy of our body and causes therefore a warm layer between the pullover and the skin. This is exactly what greenhouse gases do. An increasing greenhouse effect means, that more heat accumulates above the Earth surface before it is released to the space. It does not mean, that more energy reaches the Earth surface.

 

Understanding the energy budget

We measure the energy transferred to or emitted from a part of the system in W / m2. First let us show, that in each part of the system the same amount of energy comes in and goes out (we simplify and leave out the retarding effect of the oceans) :

342 W / m2 come from the solar radiation to the outer atmosphere. 107 are directly reflected either from clouds or from the Earth surface. The fraction of the sunlight, which is directly reflected back to the space without any interaction is called the Earth's albedo. It is roughly 30%.

Definition of albedo: The ratio of the light reflected by a body to the light received by it. Albedo values range from 0 (pitch black) to 1 (perfect reflector).
energy balance

3. A separate energy balance for the space above the atmosphere, the atmosphere and the Earth surface. Everything is in equilibrium.
Solar radiation is shown in yellow, long wave infrared radiation in red. A certain fraction of the energy is also needed for evaporation of water and thermal transfer. Please note, that certainly not more energy can reach the Earth surface than the sun brings in. The 492 W /m2 count the same energy twice, first coming from the sun, then, after being emitted from the Earth again as re-emission from greenhouse gases. This allows to separate both processes.
image: Elmar Uherek, data from IPCC TAR
Please click to enlarge! (90 K)

 

The Earth's albedo is about 0,3. The best reflectors are the clouds and the polar ice shields. The remaining 235 W/m2 interact either with the atmosphere or with the Earth's surface and come back to the space as long wave radiation.

 

4. A reduced view on the Earth radiation budget (reflection excluded) and illustration of the atmospheric window.
image: Elmar Uherek

 

If we discuss the budget for the atmosphere we have to consider, that the atmosphere can either emit its energy to the space or send it back to the Earth's surface. It's the back radiation of the greenhouse gases to the Earth which leads to the fact, that the Earth's surface absorbes more energy (492 W / m2) than the sun brings in.

The atmospheric window

Only 40 W / m2 are directly emitted as long wave radiation from the Earth surface to the space.

 
Greenhouse

5. The model of a greenhouse
illustration: Elmar Uherek

 

This is, because greenhouse gases do not absorb all wavelengths. There are a few gaps in the overlapping absorption spectra of water (which absorbs roughly 60%), carbon dioxide, methane, nitrous oxide, ozone and the other greenhouse gases. In particular the most important gaps in the water and carbon dioxide absorption are called the atmospheric window. Here, the infrared radiation can disappear like through a window in the roof of a greenhouse.

 

Certainly, the analogy of the greenhouse gases to the glass of a greenhouse is not perfect. The gases interact with light, while the glass is a barrier of solid matter and prevents also convection so that heat is retained.

 
transmission in the atmosphere

6. The interaction of electromagnetic waves with the atmosphere (how much radiation and which wavelength pass the atmosphere?) leads to the fact, that certain parts of the atmosphere are opaque . In the image above these parts are shown in brown. Of special interest is the near UV-light (1), the visible light (2) and the near infrared light (3).
Ozone absorbs in the range (1) and makes the atmosphere opaque for the dangerous UV-B light. Next to it (2) visible light can pass to the ground, illuminate our days and heat the Earth's surface. In range (3) infrared light from the back radiation of the Earth (see image right) can go back to the space, but only in some areas, which are not blocked. First of all water and carbon dioxide make parts of the infrared range opaque for the radiation from the Earth (greenhouse effect). If other gases (O3, CH4, N2O) absorb in the remaining 'atmospheric window' (see spectra right), they are very efficient greenhouse gases.
Picture from NASA / IPAC.
Please click to enlarge! (80 K)
absorption of greenhouse gases

7. Only a fraction of the theoretical spectra of the Earth (so called black body radiation in red) is really emitted to the space. This fraction, shown in blue, is called the atmospheric window. The rest is absorbed primarily by water and carbon dioxide.
Image adopted from Hamburger Bildungsserver
Please click to enlarge! (70 KB)
About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz
scientific reviewing: Dr. Benedikt Steil - Max Planck Institute for Chemistry, Mainz 2004-05-16
educational proofreading: Michael Seesing - Uni Duisburg - 2003-07-02
last published: 2004-06-17

 
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