O - adds up to as much. Methane, including its indirect effects, causes a forcing half as large as CO 2 . Then there are all these aerosols. These are fine particles in the air - solid or liquid particles. Sulfates and organic aerosols, from fossil fuel burning, cause cooling.
Aerosols also affect the properties of clouds, with a cooling effect, but a large uncertainty.
Their growth needs to be stopped anyhow, for reasons of human health. Second , CO 2 emissions can continue but the emissions rate should be no larger than it is today - preferably declining slowly. The resulting forcing of 1 watt would be expected to cause some climate change, but less than 1 degree Celsius warming in 50 years. Black carbon is a product of incomplete combustion - you can see it in the exhaust of diesel trucks. Microscopic soot particles are like tiny sponges, they soak up toxic organics and other aerosols.