A causal change or perturbation on the climate system. Radiative forcing is the net flux of radiation in or out of a system. The five major greenhouse gases account for about 97% of the direct radiative forcing by long-lived gases. Interannual variations in the growth rate of radiative forcing due to CO₂ are large, and probably related to natural phenomena such as volcanic eruptions and ENSO events, as well as to anthropogenic activity (Hoffman et al. (2006) Tellus B 58, 5). Precisely the same forcing can produce hugely divergent evolutions of the climate system given infinitesimally small differences in initial conditions (Washington (2000) PPG 24, 4; E. N. Lorenz 1963). Orbital forcing (also known as astronomical forcing) is the climatic effect of slow changes in the tilt of the Earth’s axis and shape of the orbit.

A forcing factor would therefore be a factor that affects forcing (for example, Cuffy and Vimeux (2001) Nature 412), but Tooth (2008) PPG 32, 1 lists climate, tectonics, and human activity as forcing factors in an arid geomorphology, and Burningham (2008) Geomorph. 97, 3–4 writes of sea-level rise as a forcing factor on estuaries. (The literature is littered with references to forcing factors—clearly the buzzwords—but the writers seem to mean major/dominating factors.) Coco and Murray (2007) Geomorph. 91, 3–4 go one further, and refer to a forcing template of spatial structures and geological constraints in the evolution of sand patterns.