From the point of view of determining our climate, the Earth is basically a large rock floating in cold, empty space with an enormously bright searchlight shining on one side of it. If the Earth were not rotating and did not have an atmosphere, then the side being illuminated would be hotter than boiling water and the dark side would be solidly frozen. A rotating Earth would be more evenly heated, like a rotisserie chicken being cooked, but without an atmosphere the average temperature around the globe would still be below the freezing point of water. The warming due to the atmosphere, known as the greenhouse effect, is necessary to keep the Earth habitable.

What we call “climate” is a complex interaction between the heating of the Sun and the processes that distribute this heat over the planet. This chapter provides a perspective on the forces determining the overall climate of the Earth. The goal is to provide the “big picture,” the global climate and its relation to the Solar input as an organic whole. We will then be in a position to discuss the major causes of climate variability, and the role played by the Sun in climate change. As we will see, in the 20th century the natural changes due to the Sun's variable radiation were overwhelmed by the human influences on global climate.