Since the original definition of ‘Radiative Forcing’ in IPCC Second Assessment the concept has evolved include the cooling effects of aerosols.
IPCC Third Assessment Report (2001) “Since the Second Assessment Report, significant progress has been achieved in better characterizing the direct radiative roles of different types of aerosols. Direct radiative forcing is estimated to be –0.4Wm–2, for sulfate –0.2Wm, –2 for biomass burning aerosols, –0.1 W m–2 for fossil fuel organic carbon and +0.2 W m–2 for fossil fuel black carbon aerosols. There is much less confidence in the ability to quantify the total aerosol direct effect and its evolution over time, than that for the gases listed above. Aerosols also vary considerably by region and respond quickly to changes in emissions.”
IPCC Fourth Assessment Report (2007) “Anthropogenic contributions to aerosols (primarily sulphate, organic carbon, black carbon, nitrate, and dust) together produce a cooling effect, with a total direct radiative forcing of –0.5 [–0.9 to –0.1] W m–2 and an indirect cloud albedo forcing of –0.7 [–1.8 to –0.3] W m–2. These forcings are now better understood than at the time of the TAR due to improved in situ, satellite and ground-based measurements and more comprehensive modeling, but remain the dominant uncertainty in radiative forcing. Aerosols also influence cloud lifetime and precipitation.”
Dry cloudless deserts with little greenhouse effect get hotter than humid, cloudy places that which precipitate more often. There is more incoming solar radiation heating surfaces in the desert and high-pressure system warm fronts adiabatically heating the air. In humid conditions, however, increased cloud cover and precipitation are associated with low-pressure systems that adiabatic cools air temperatures. Cloud cover and precipitation have negative forcing on surfaces.
The hydrological cycle shows solar radiation heats surfaces creating evaporation and humidity that becomes cloud cover and latent heat is released on precipitation.
The NASA Earth Energy Budget Flowchart below confers with the Atmospheric Transmission Chart. The surfaces are heated in the first instant by INCOMING SOLAR RADIATION that heats the air near the surfaces and adiabatically heating the air further through high-pressure system warm fronts raising air temperatures. There is no apparent ‘forcing’ by the greenhouse gases on the surfaces in the flowchart below. CO2 is transparent, there are adiabatic lapse rates, and even if it did “mirror” or “reflect” up-going IR heat back to the surfaces, whether it be light or heat, it is less the original solar radiation heating the surfaces. For example, if you touch sand during the day it is hotter than the air near the surface.
Due to Specific Heat Capacities between ocean and air, warm water creates warm wind high-pressure systems that raise global air temperature and similarly cool water creates low-pressure systems and cold fronts that lower global air temperature. The incoming solar radiation heats surfaces in the first instant. There is also atmospheric adiabatic heating from the rise and expansion of heated air mass near surfaces and from Foehn winds down hills.
The reason I suspect GHG’s are a net thermal sink is by comparing air temperatures in a dry cloudless desert near the Equatorial where the sun is most direct and intense to air temperatures in a Tropical rainforest or rice farming agricultural land near the Equatorial where most ‘greenhouse effect’ exists and the GHG’s and cloud cover definitely keep it from getting too hot in the day and too cold at night, precipitation then cools and cleans the air and surfaces. In Cambodia, for example, the driest time of the year between Jun-Aug is the hottest and the cloud cover still builds up every day blocking the solar radiation heating surfaces and precipitates every several days cooling the surfaces again. Compare this to a dry cloudless desert region such as Death Valley that is in a rain shadow, the incoming solar radiation heating surfaces and heat island effects heating air near surfaces plus adiabatic heating high-pressure systems makes air temperatures hotter in the desert than if there were cloud cover. Foehn winds downhill and convection superheat the air in the desert valley as it has steep hillsides either side. In Australia, the hottest places are in arid regions in the interior characteristic of dry weather and clear skies.
“The IPCC 2007 report  shows that this cooling effect may be large enough to offset 50% of the radiative heating due to the build up of greenhouse gases. This indirect effect is acknowledged to be the largest source of uncertainty in understanding the human impact on the global climate.”
“Cloud radiative forcing (CRF) is defined as the difference between the radiation budget (net incoming solar radiation minus the out- going long wave) over a cloudy (mix of clear and clouds) sky and that over a clear sky. If this difference is negative clouds exert a cooling effect, while if it is positive, it denotes a heating effect. The five-year average of the cloud radiative forcing  is shown in Fig. 2. The global average forcing is about –15 to –20 W m-2 and thus clouds have a major cooling effect on the planet. Two major puzzles posed by this data are germane to the topic of this paper.”
I prepared this diagram to help demonstrate the Greenhouse Cooling Effect.