1.2 What determines the Earth's GMST?

The Sun is the ultimate source of energy for the Earth's climate. A planet such as the Earth will have a stable temperature as long as there is a balance between the rate at which energy comes in from the Sun and the rate at which it is returned to space by the planet. If the two rates fail to match, the planet will either warm up or cool down until a balance is restored. Thus, it is appropriate to begin with a review of this global balancing act. The heart of the matter is that the energy flows to and from space are in the form of radiation - or to be more precise, electromagnetic radiation. You should consult Box 1 (Section 1.1) if you need to refresh your memory about this form of energy transfer.