The greenhouse effect occurs when a planet's atmosphere is transparent to short-wave radiation arriving from its sun, but opaque to long-wave radiation emitted by the planet itself. Gases that bestow these properties upon an atmosphere are called greenhouse gases. So far, we have explored the greenhouse effect in the context of hypothetical examples such as our Extreme Greenhouse and Planetary Greenhouse.
In a planetary greenhouse, the lower atmosphere absorbs long-wave radiation. But the ability of any substance to absorb radiation is exactly matched by its ability to emit radiation. Thus the lower atmosphere emits long-wave radiation with the same efficiency that it absorbs long-wave radiation. Furthermore, although the lower atmosphere is opaque to long-wave radiation, the atmosphere gets progressively thinner as we ascend, so we are sure to come to an altitude where the atmosphere is so thin that it is transparent. Thus the middle atmosphere radiates heat out into space through the thin, upper atmosphere. The amount of heat the middle atmosphere radiates into space is proportional to the fourth power of its absolute temperature. The middle atmosphere will cool down until it radiates into space no more heat than it gains from the planet surface.
Greenhouse gases give rise to atmospheric convection. Gas in the middle atmosphere radiates heat into space and cools down. It contracts as it cools down, and becomes more dense. It sinks towards the planet surface. As it sinks, the surrounding atmosphere compresses it, causing it to warm up. But the sinking gas remains marginally cooler than the surrounding atmosphere, and so it continues to sink. When it reaches the planet surface, it stops sinking. The planet surface warms it up. Once it is warm enough, it becomes less dense and rises. As it rises, it compresses the surrounding atmosphere, and so cools down. But the rising gas remains marginally warmer than the surrounding atmosphere, and so it continues to rise. When it reaches the middle atmosphere, it starts to radiate heat into space. It cools down. The top of this convection cycle is the altitude at which the atmosphere stops cooling. We call it this altitude the tropopause.
In Work by Convection we showed that atmospheric convection generates power. This power causes the weather. Powerful weather is a feature of the greenhouse effect. The powerful weather extends only up to the tropopause, which is the upper extent of the convection cycle.
Atmospheric convection transports heat from the planet surface to the tropopause. But the amount of heat the convection cycle transports has very little effect upon the temperature drop from the bottom to the top of the cycle. Instead, it is the ratio of the pressure at the bottom to the pressure at the top that determines the temperature drop. An understanding of convection allows us to use the equations for adiabatic compression to calculate the warming caused by the greenhouse effect. If, for example, the pressure in the tropopause 30 kPa and the temperature is 210 K (−63°C), while the pressure at the surface is 100 kPa, then the temperature at the surface must be close to 300 K (27°C).
The temperature of the tropopause itself is, however, determined by the amount of heat it must radiate into space. The amount of heat it must radiate is determined by the manner in which the atmosphere absorbs long-wave radiation. We have considered a transparent atmosphere, an atmosphere that is opaque to all long-wave radiation, and an atmosphere that is transparent to some long-wave radiation but opaque otherwise. In more recent posts, we have considered the long-wave absorption spectra of water vapor and carbon dioxide. We found that doubling the concentration of a trace greenhouse gas causes progressively smaller increases in surface temperature.
So far, we have ignored the effect of clouds. Clouds have a strong effect upon the Earth's climate. They are made up of tiny water droplets, and are opaque to all long-wave radiation. At the same time, they reflect all short-wave radiation from the sun. Roughly a third of the Earth's surface is covered by clouds at any one time, and they reflect roughly a third of the Sun's radiation back into space, which tends to lower the temperature of the surface. But clouds block some surface radiation from reaching outer space, which tends to raise the surface temperature.
We are going to ignore clouds for now, but we will consider them in detail eventually. We are building up our understanding of the Earth's greenhouse effect one step at a time. In our next post, we will consider the absorption spectra of carbon dioxide and water vapor as they occur together in various layers of the Earth's atmosphere. We will obtain our spectra with the help of astronomy's Spectral Calculator.