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.