In the context of the Enhanced Greenhouse Effect, what forces the effective altitude of radiation emission to rise higher?

Answer

The atmosphere becoming more opaque at lower levels

When GHG concentrations increase, the atmosphere becomes more opaque nearer the surface, forcing the effective altitude from which radiation can escape to space to rise to colder heights, thus reducing outgoing energy.

In the context of the Enhanced Greenhouse Effect, what forces the effective altitude of radiation emission to rise higher?
Earthclimateatmospheregreenhouse effectwarming