What are positive forcings?

Positive forcings tend to cause warming. CO₂ has the largest forcing, but CH₄, when its indirect effect on other gases is included, causes a forcing half as large as that of CO₂. CO₂ is likely to be increasingly dominant in the future, but the other forcings are not negligible

Figure 1. Estimated change of climate forcings between 1850 and 2000, based on with five principal aerosols delineated.

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Source: http://www.giss.nasa.gov/research/features/200105_senate/page5.html