A. Trap heat
The suns radiation is absorbed by the earth's surface. It is then re-emitted as Infrared radiation and travels to the atmosphere. Here, it is absorbed by the bonds in GHGs, causing the warming of the atmosphere, and overall, global warming. This effect, in the past, was usually incorporated into cycles such as the water and carbon cycles, and didn't really have a big effect on the atmosphere but now, due to the increased concentration of these GHGs from human emissions, global warming is a very real problem.
