Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30
Lecture 2: The Earth's atmosphere and the greenhouse effect.
What determines the height of the atmosphere?
We know that on the surface of the Earth, gravity pulls objects to the ground. Why does the air not fall to the ground?
Pressure determines the height of the atmosphere. The gravity force is balanced by pressure. As we go higher in the atmosphere, there is progressively less air, so pressure decreases with height. Air tends to go to where pressure is lower, and the force caused by this change in pressure with height balances gravity.
The Earth's atmosphere is divided into four regions at increasing heights, in which the temperature changes differently (see the figure in page 3 of Graedel and Crutzen, or here to see a plot .
Why? The reason why temperature drops with height lies in where the energy from the Sun is absorbed: most sunlight is absorbed on the ground, and so it is the surface that gets heated. The air tends to scatter some of the light, but scattering does not cause any heat: the energy of the light is just sent back to space if the light is reflected out. The atmosphere also absorbs some light, but relatively little of it (as we see later when we talk about the energy budget).
The air, on the other hand, can cool by emitting infrared radiation out to space. With little heating from the Sun, that is why the air at high altitude gets so cold. In fact, air in the troposphere would be much colder if it were only heated by sunlight. Actually, the troposphere also gets heated by transport of heat from the hotter surface of the Earth. This transport of heat occurs in two ways:
The reason is that the stratosphere contains ozone molecules that absorb ultraviolet radiation from the Sun. In fact, this ozone layer in the stratosphere protects us from the damaging effects of ultraviolet radiation on our skin and eyes. The energy of this ultraviolet light heats the stratosphere. At higher altitude in the stratosphere, cooling by infrared emission decreases but heating by ozone is still strong, and that is why temperature increases with altitude.
When warmer air lies above colder air, there can be no convection, so the stratosphere is a very quiet layer of the atmosphere, with very little vertical mixing. All weather phenomena occur in the troposphere only.
The top of the troposphere, which separates it from the stratosphere, is marked by the maximum height reached by convective movements, and is called the tropopause.
About 70% of the sunlight reaching Earth is absorbed, the other 30% is reflected back to space. Of the 70% absorbed, 50% is absorbed in the ground, the other 20% is absorbed in the atmosphere and clouds.
The surface cools by convection and latent heat, transmitting this heat to the atmosphere. It also cools by emitting infrared radiation, but only a small fraction of this radiation escapes directly to space. Most of the infrared radiation from the surface is absorbed in the atmosphere. The atmosphere can then return a lot of infrared radiation back to the surface, keeping the surface much warmer than it would be if no infrared radiation were returned. This warming by the return of infrared radiation from the atmosphere to the ground is called the greenhouse effect. Click here for a diagram showing the Earth energy budget, and see also page 17 of Graedel and Crutzen.
The greenhouse effect is made possible by two important properties of the atmosphere:
If the atmosphere were everywhere at the same temperature as the surface, then the top of the atmosphere would emit as much radiation to space as the emission from the surface of the Earth. So, no matter how opaque the atmosphere was, there would be no greenhouse effect.
Of all the infrared radiation emitted from the surface of the Earth, only 10% escapes directly to space. The other 90% is absorbed by the atmosphere, and 80% of it is emitted back downwards from the atmosphere to the ground. The other 10% is used to heat the atmosphere, adding to the heat provided by convection and latent heat, and absorption of sunlight in the atmosphere. This heating of the atmosphere is balanced by the cooling due to the infrared emission from the atmosphere to space. This emission from the atmosphere to space is only about half of what was originally emitted by the surface.
The rate at which energy is emitted in infrared radiation from the surface is larger than the rate at which the surface absorbs energy from the Sun. The reason why the surface can stay at its warm temperature despite of this is because it gets additional energy from the infrared radiation emitted downwards by the atmosphere.
Therefore, you can say the atmosphere acts like a blanket that keeps the Earth surface warmer. The way that a blanket helps your body stay warm when you are in a cold place is analogous: the best blankets conduct the least possible heat (which is analogous to the atmosphere creating a greater greenhouse effect if it is more opaque, because infrared radiation will be less able to go through the atmosphere). At the same time, the reason the blanket keeps you warm is because its outer surface is colder than the surface of your body, so less radiation is emitted outwards from the outer surface of the blanket than the radiation that is emitted from your body surface to the inner surface of the blanket. The inner surface of the blanket returns a lot of the infrared radiation back to your body, keeping it warm.
Note an important difference between this blanket analogy and the Earth atmosphere. When you keep your body warm with a blanket, the energy keeping your body warm comes from inside the body (from your biological metabolism). For the Earth, the energy keeping it warm comes in externally with the sunlight. Greenhouse gases in the atmosphere are opaque to long-wave radiation emitted by the Earth surface (longer than 5 micrometers). But they are highly transparent to short-wave radiation (shorter than about 1 micrometer). Most of the energy from sunlight comes in at short wavelengths, so it is not blocked by greenhouse gases.
The gases in the Earth atmosphere that are opaque to long-wave infrared radiation, and therefore contribute a greenhouse effect, are water vapor, carbon dioxide, methane, and nitrous oxide. There are also various complex molecules called halocarbons that are of human origin and did not exist in nature before industrialization, which contribute to the total greenhouse effect.
Note that the most abundant molecules in the atmosphere, oxygen and nitrogen, are totally transparent to infrared radiation. The molecules able to absorb and emit long-wave radiation constitute a very small fraction of the atmosphere.
The next lecture explains how much each gas contributes to the greenhouse effect and how these gases have been changed by human activities.