We turn now to the third of the three branches of science that underpin our changing climate: radiant energy – the energy carried by light and other electromagnetic waves – and the ways it interacts with the gases and particles in our atmosphere. In this chapter we will outline the necessary science, and especially the laws of thermal emission. It turns out that we are bathed in electromagnetic radiation that we can’t see. We will then show how these laws lead to the counter-intuitive outcome that the ‘temperature’ of our planet is actually well below freezing! Fortunately, however, our Earth has a ‘good’ atmosphere, containing some very important gases – greenhouse gases – which act as a blanket, keeping the surface at a suitable temperature for liquid water, and hence for life. A simple model will help you gain a basic understanding of the greenhouse effect.

In our galaxy, the existence of dust is revealed by the fact that dust grains absorb, scatter, polarize, and emit light. The interaction of dust grains with light depends on the size and shape of the grains, as well as on the index of refraction of the material making up the grains. Observations indicate that the mass of dust in our galaxy is about 1% the mass of interstellar gas. Most grains are either graphite or silicate, with a typical grain radius of ∼0.1 micron. The equilibrium temperature of dust grains is set by the balance between absorbing starlight and emitting thermal radiation; for interstellar grains, the equilibrium is at T ∼ 20 K. Cool stellar winds, like those of Mira variable stars, give rise to circumstellar dust grains. As these grains are spread through interstellar space, they can grow by accretion of atoms or be destroyed by sputtering or be vaporized by shock-heating.