We briefly review the dominant role of clouds in Earth’s climate. The earliest observational studies of heat transfer through Earth’s atmosphere, for example, those of John Leslie around 1800, showed that clouds have a large effect on radiative heat transfer from Earth’s surface to space. Greenhouse gases also affect heat transfer, but much less than clouds. For example, “instantaneously doubling” CO2 concentrations, a 100% increase, only decreases radiation to space by about 1%. To increase solar heating of the Earth by a few percent, low cloud cover only needs to decrease by a few percent. The first half of this paper reviews observational facts about how clouds affect heat transfer. The second half gives a brief summary of the new 2n-stream radiation transfer theory for quantitatively analysing how clouds scatter radiation incident from outside the cloud, and how they emit thermal radiation generated by their particulates.
