Unit 2: Atmosphere // Section 3: Radiative Balance and the Natural Greenhouse Effect
Earth's surface temperature has been remarkably constant over geologic time. Even the dramatic cooling that occurred during the most recent ice age represented a change of only 3°C in the global average surface temperature, occurring over thousands of years. Seasonal changes in temperature, although large in a particular place, correspond to very tiny changes in global mean temperature. Why have temperatures held so steady?
Earth exchanges energy with its environment primarily through transfers of electromagnetic radiation. At any time our planet is simultaneously absorbing energy from the sun and radiating energy back into space. The temperature remains stable over long periods of time because the planet radiates energy back to space at a rate that closely balances the energy input it receives from the sun (i.e., the planet is close to being in radiative energy balance).
Earth receives energy from the sun in the form of solar radiation—radiation with varying wavelengths along the electromagnetic spectrum. The sun emits strongly in the visible light range, but it also produces ultraviolet and infrared radiation. The earth radiates heat back to space mostly at much longer wavelengths than solar radiation (Fig. 2).
Figure 2. The electromagnetic spectrum
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Source: © Yochanan Kushnir.
When visible solar radiation reaches Earth, it may be absorbed by clouds, the atmosphere, or the planet's surface. Once absorbed it is transformed into heat energy, which raises Earth's surface temperature. However, not all solar radiation intercepted by the Earth is absorbed. The fraction of incoming solar radiation that is reflected back to space constitutes Earth's albedo, as shown below in Figure 3.
Figure 3. Earth-atmosphere energy balance
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Any form of matter emits radiation if its temperature is above absolute zero (zero degrees Kelvin). Incoming solar radiation warms Earth, and the planet emits infrared radiation back to outer space. Note that Earth emits radiation at a longer wavelength—i.e., a lower energy level—than the sun (Fig. 2). This difference occurs because the total energy flux from an object varies with the fourth power of the object's absolute temperature, and the sun is much hotter than the Earth.
Some outgoing infrared energy emitted from the Earth is trapped in the atmosphere and prevented from escaping to space, through a natural process called the "greenhouse effect." The most abundant gases in the atmosphere—nitrogen, oxygen, and argon—neither absorb nor emit terrestrial or solar radiation. But clouds, water vapor, and some relatively rare greenhouse gases (GHGs) such as carbon dioxide, methane, and nitrous oxide in the atmosphere can absorb long-wave radiation (terrestrial radiation, see Figure 2). Molecules that can absorb radiation of a particular wavelength can also emit that radiation, so GHGs in the atmosphere therefore will radiate energy both to space and back towards Earth. This back-radiation warms the planet's surface.
In Figure 3, 100 units of solar radiation are intercepted by the Earth each second. On average 30 units are reflected, 5 by the surface and 25 by clouds. Energy balance is achieved by Earth's emission of 70 units of infrared ("terrestrial") radiation to space. The earth's surface is warmed directly by only 45 units of solar energy, with almost twice as much energy (88 units) received from thermal radiation due to greenhouse gases and clouds in the atmosphere. Energy is removed from the surface by radiation of infrared energy back to the atmosphere and space (88 units) and by other processes such as evaporation of water and direct heat transfer (29 units).
Note that the amount of heat received by the surface is actually much larger (3x) than the amount the surface receives in solar radiation, due to the natural greenhouse effect. The result is a surface temperature on average around 15°C (60°F), as compared to temperatures colder than –18°C (0°F) if there were no greenhouse effect.