The solar radiation hitting the Earth’s surface. Global variations in insolation are critical in the general circulation of the atmosphere. ‘Climatic changes result from variables in planetary orbits which modulate solar energy emission and change seasonal and latitudinal distribution of heat received by the Earth. Small insolation changes are multiplied by the albedo effect of the winter snow fields of the Northern Hemisphere, by ocean-atmosphere feedbacks, and, probably, by the stratospheric ozone layer’ (Kukla (1972) Boreas 1, 1). Insolation forcing functions include the eccentricity of the Earth’s orbit, the angle from the vernal equinox to perihelion, obliquity, and time (Huybers (2011) Nature 480, 229).