1.1 The Earth’s mean temperature

The two main factors that influence the mean temperature of the Earth and its neighbouring planets are distance from the Sun and the nature and extent of their atmospheres. Consider first the situation if the Earth had no atmosphere.

The ultimate source of almost all the energy reaching the Earth’s surface is the Sun. (Some heat energy flows from the interior of the Earth, but is tiny compared to the solar contribution.) The incoming energy is transmitted across space by solar radiation (the Sun’s rays), and when it reaches the Earth it is transferred to its surface, or to your skin which, as you will know from experience, warms.

All bodies give off heat in the form of infra-red radiation that depends on their temperature. Temperature also affects the regions of the spectrum in which the radiation is emitted (such as the visible spectrum, but also infra-red and ultraviolet and beyond). So the Earth, in turn, radiates energy back into space, but because it is at a much lower temperature than the Sun, this energy is in a more restricted part of the spectrum – namely the infra-red (it doesn’t glow visibly hot!). Its surface temperature would have risen in the past until it reached a value where the rate at which energy is transferred to it is balanced by the rate at which energy is radiated out to space from the surface of the Earth, as illustrated in Figure 2. Over time, if the output from the Sun is constant, the mean surface temperature of the Earth will stabilise at a particular value.

At what temperature would the Earth stabilise? If it had no atmosphere, its surface temperature can be calculated to be approximately −18 °C (minus 18 degrees Celsius). A similar calculation can be applied for our neighbouring planets: Venus, which is nearer to the Sun, should have a mean surface temperature of +50 °C, while for the more distant Mars the figure is −57 °C.

In fact the mean surface temperature of the Earth is usually quoted as plus 15 °C, and it has been getting warmer in recent years. The surface temperature is defined as the air temperature measured by standard instruments close to the surface, when averaged over the whole planet, over both sea and land. For this reason it is generally referred to as the global mean surface temperature (GMST).

The difference of +33 °C between the figure of −18 °C and +15 °C represents a significant warming. This warming is the difference the atmosphere makes, and is known as the greenhouse effect. This is an entirely natural effect, which has existed as long as the Earth has had an atmosphere, although the amount of warming will have changed with the composition of the atmosphere. Note that we are not describing human impact here.