Global average radiative forcing estimates and ranges in 2005 for anthropogenic greenhouse gases and other important agents and mechanisms.
Understanding global warming requires understanding the changes in climate forcings (radiative forcing) that have occurred since the industrial revolution.  These include positive forcing from increased greenhouse gases, negative forcing from increased sulphate aerosols and poorly constrained forcings from indirect aerosol feedbacks as well as minor contributions from solar variability and other factors.  The poorly constrained aerosol effects results from both limited physical understanding of how aerosols interact with the atmosphere and limited knowledge of aerosol concentrations during the pre-industrial period.  This is a significant source of uncertainty in comparing modern climate forcings to past states.

Contrary to the impression given by this figure, it is not possible to simply sum the radiative forcing contributions from all sources and obtain a total forcing.  This is because different forcing terms can interact to either amplify or interfere with each other.  For example, in the case of greenhouse gases, two different gases may share the same absorption bands thus partially limiting their effectiveness when taken in combination.
The figures used to generate this plot were obtained from the IPCC Working Group I Fourth Assessment Report Summary for Policymakers, page 16. [1]
This figure was produced by Leland McInnes (Radiative-forcings.svg) using gnuplot and Inkscape and is licensed under the GFDL. All data is from publicly available sources.