Slide 22: Climate change forcings
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).
Forcings are discussed in more detail in the next chapter, on GCMs, but for now it is important to understand that there are many sources of climate forcing.
However, human activity has introduced significant radiative forcing to the atmosphere through various factors, and these will be touched upon in the next couple of slides.