4. Radiative forcing is the change in the balance between radiation coming into the atmosphere and radiation going out. A positive radiative forcing tends on average to warm the surface of the Earth, and negative forcing tends on average to cool the surface. The figure shows estimates of the globally and annually averaged anthropogenic radiative forcing (in Wm-2) due to changes in concentrations of greenhouse gases and aerosols from pre-industrial times to present day and to natural changes in solar output from 1850 to present. The height of the rectangular bar indicates a mid-range estimate of the forcing and the error bars show the uncertainty range. Confidence level shows the author's confidence that the actual forcing lies within the given error range.

Note: forcing associated with stratospheric aerosols resulting from volcanic eruptions is not shown because it is very variable over this time period.