Physics of the Earth's radiative energy balance

The purpose of this paper is to review the status of research into one of the most challenging and important problems facing physics today: how does the atmosphere moderate the radiative energy balance that determines the Earth's climate? Behind this question lies the whole basis for understanding the greenhouse effect, including coupled feedback processes: this understanding has consequences for major issues of public concern, climate change and global warming. Following an introduction to the basic physics of the energy balance and the greenhouse effect, a discussion is given of how the spectrum of outgoing thermal radiation (by which the planet cools to space) depends on internal parameters such as surface temperature and atmospheric humidity. This includes a discussion of the sign and magnitude of the water vapour-climate feedback, and the 'super greenhouse effect'. It is shown that the role of cloud in the energy balance is extremely important, although poorly understood. Recent work to exploit the information contained in the resolved spectrum of outgoing long-wave radiation (OLR) is described, including a new technique to search for the 'signal' of climate change within the 'noise' of natural climate fluctuations. Finally, some comments are offered on the problem of the predictability of future climate changes.