The further step of attributing some part of observed temperature changes to human influences
makes use of climate models, which have been employed to estimate the climatic effects of a
range of human-induced and natural factors. The human factors include recent changes in the
atmospheric concentrations of both greenhouse gases and sulfate particles (called "aerosols").
The natural factors considered include solar variability, the effects of volcanic eruptions, and
internal variability of the climate system resulting from interactions among its individual