Slide 22: Climatic forcing
The effect of various factors on the climate are expressed as a forcing value (ie: to what extent they force global warming)
The forcing value is measured in Wm-2 (the increase in effective energy caused per square metre).
There are many natural forcing factors operating over different time periods, from desertification increasing albedo of a planet to the passage of the solar system through the galaxy (Williams, 1975).
Even the movements and placement of continents over time has some forcing activity.
However, these natural (also called external, or non-radiative) forcing factors are independent of those which directly affect the energy balance of the Earth-atmosphere (called radiative forcing factors) (Shine et al, 1990).
Shine, K., Derwent, R.G., Wuebbles, D.J. & Morcette, J.J., 1990. Radiative forcing of climate. In: Climate Change: The IPCC Scientific Assessment, Intergovernmental Panel on Climate Change (IPCC), Houghton, J.T., Jenkins, G.J. & Ephraums, J.J. (eds.). Cambridge University Press, Cambridge, pp. 41-68.
Williams, G.E., 1975. Possible relation between periodic glaciation and the flexure of the Galaxy. Earth Planet. Sci. Lett., 26, pp. 361-369.