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We have developed a
radiative-convective model for the purpose of investigating
the climate sensitivity to cloud radiative properties and the
related feedback processes.
Using this model, we find that
the surface temperature increases with cloud optical thickness when
the clouds are very thin and decreases with cloud optical depth when
the shortwave cloud radiative
forcing dominates the longwave cloud radiative forcing.
A few percent variation in the cloud droplet size has a
larger
impact on the equilibrium state lower atmosphere temperature than the
warming caused by doubling of the 
concentration.
When clouds are included in the model,
the magnitude of the greenhouse effect due to doubling of
the concentration
varies with the cloud optical thickness.
The thicker the clouds, the weaker
the greenhouse warming.
A 2% increase of cloud equivalent radius per degree increase in
temperature is sufficient to double the warming caused by the doubling of
concentration. This finding suggests that a physically reliable
relation between cloud equivalent radius and the macrophysical
meteorological properties on a global climate scale is needed to assess the
climate impact of increases in greenhouse gases.
