A simulation of the possible consequences of a doubling of the CO2 con
tent of the atmosphere has been performed with a low resolution global
climatic model. The model included the diurnal and seasonal cycles, c
omputed sea ice amount and cloud cover, and used implied oceanic heat
fluxes to represent transport processes in the oceans. A highly respon
sive 2-layer soil moisture formulation was also incorporated. Twenty y
ear equilibrated simulations for control (1 x CO2) and greenhouse (2 x
CO2) conditions were generated. The major emphasis of the analysis pr
esent ed here is on the intra-annual and interannual variability of th
e greenhouse run with respect to the control run. This revealed consid
erable differences from the time-averaged results with occasions of ma
rked positive and negative temperature deviations. Of particular inter
est were the periods of negative temperature departures compared to th
e control run which were identified, especially over the Northern Hemi
sphere continents. Temporal and spatial precipitation and soil moistur
e anomalies also occurred, some of which were related to the surface t
emperature changes. Substantial sea surface temperature anomalies were
apparent in the greenhouse run, indicating that a source of climatic
forcing existed in addition to that due to doubling of the CO2. Compar
ison of the intra-annual and interannual variability of the control ru
n with that of the greenhouse run suggests that, in many situations, i
t will be difficult to identify a greenhouse signal against the intrin
sic natural variability of the climatic system.