A simulation experiment is conducted to inquire into the mean climate state and likely trends in atmo-

 spheric infrared radiation spectra. Up- and down-welling spectra at five vertical levels from surface to the top

 of the atmosphere (TOA) are rigorously calculated from climate model simulated atmosphere for a 25-year

 period. Tracing the longwave radiation flux vertically and spectrally renders a dissection of the greenhouse

 effect of Earth atmosphere and its change due to climate forcings and feedbacks. The results show that

the total outgoing longwave radiation (OLR) at the TOA may be conserved due to: 1) compensating tem-

 perature and opacity effects; and 2) contrasting temperature changes in troposphere and stratosphere. The

tightly coupled tropospheric temperature and opacity effects reduce the overall tropospheric contribution to

 OLR change to be comparable to the overall stratospheric contribution, which suggests that transient OLR

 change is constrained by the relative strengths of stratospheric and tropospheric temperature changes.

 The total OLR energy, however, is redistributed across its spectrum. The earliest detectable global

 climate change signal lies in the CO2 absorption bands, which results from stratospheric cooling and CO2

opacity eect. This signal can be detected much sooner than surface temperature change and is little affected

 by achievable instrument accuracy.