The atmosphere: its circulation, the heat (terrestrial radiation) and light (solar radiation) which pass through it, and the processes which go on in it, such as the formation of clouds and the atmospheric chemical reactions that determine the concentrations of some of its important constituents, such as methane and ozone.

The ocean: There is a constant exchange of heat, momentum and water between ocean and the atmosphere. The ocean acts as a heat sink to delay climate change. In addition, ocean currents transport large amounts of heat and water around the world.

The land surface, including its vegetation and seasonal snow cover, has an important influence on the flow of air over it, the absorption of solar energy, and the water cycle.

The cryosphere: those parts of the world whose surface is affected by ice, principally sea-ice in the Arctic and Southern Oceans and the land-based ice-sheets of Greenland and Antarctica.

The biosphere: Life on land (the terrestrial biosphere) and in the ocean (the marine biosphere) play a major role in the carbon cycle and hence in determining the atmospheric concentration of carbon dioxide.