Adding it all up and Radiative Forcing

As we have seen, a wide range of GHG and aerosols influence the global climate, or more precisely change the natural radiation balance of the Earth. In order to add up and compare these different factors scientists use the concept of "radiative forcing" to quantify how much each of these is moving the earth away from its natural radiation balance. This is measured in Watts per square metre and is a very important calculation.
 
The figure below has been prepared by the IPCC to demonstrate the issue of radiative forcing. Radiative forcing results from one or more factors that affect climate and are associated with human activities or natural processes. The values represent the forcing since 1750. Human activities cause significant changes in long-lived gases, ozone, water vapour, surface albedo, aerosols and contrails. The only increase in natural forcing of any significance between 1750 and 2013 occurred in solar irradiance. Positive forcing leads to warming of the climate and negative forcing leads to a cooling.

Radiative Forcing relative to 1750 (IPCC, 2013)
IPCC, Working Group 1, AR5 (2013)

This figure sums up the various impacts of GHG aerosols, as well as solar radiation. It shows clearly that most of the radiative forcing is caused by carbon dioxide, with methane being the next most important individual GHG.
  
The figure also shows that part of the warming is being masked by some short lived aerosol types, as well as aerosol impacts on clouds. Also, variations in solar radiation have had a small warming effect. More details on this chart are available from the IPCC.