Surprisingly to most, there is no greenhouse effect at all, and you can prove it for yourself.
From the temperature and pressure profiles for the Venusian atmosphere, you can confirm that, at the altitude where the pressure = 1000 millibars, which is the sea level pressure of Earth, the temperature of the Venusian atmosphere is 66ºC = 339K.
This is much warmer than the temperature at the surface of the Earth (at pressure = 1000 millibars), which is about 15ºC = 288K. HOWEVER
Venus is closer to the Sun, and gets proportionally more power from it. Earth is 93 million miles from the Sun, on average, while Venus is only 67.25 million. Since the intensity of the Sun's radiation decreases with distance from it as 1 over r-squared, Venus receives (93/67.25) squared, or 1.91 times the power per unit area that Earth receives, on average.
Since the radiating temperature of an isolated body in space varies as the fourth-root of the power incident upon it, by the Stefan-Boltzmann law, the radiating temperature of Venus should be the fourth-root of 1.91 (or the square-root of 93/67.25) = 1.176 times that of the Earth. Furthermore, since the atmospheric pressure varies as the temperature, the temperature at any given pressure level in the Venusian atmosphere should be 1.176 times the temperature at that same pressure level in the Earth atmosphere, INDEPENDENT OF THE DIFFERENT LEVELS OF INFRARED ABSORPTION in the two atmospheres. In particular, the averaged temperature at 1000 millibars on Earth is about 15ºC = 288K, so the corresponding temperature on Venus, WITHOUT ANY GREENHOUSE EFFECT, should be 1.176 times that, or 339K. But this is just 66ºC, the temperature we actually find there from the temperature and pressure profiles for Venus.
[Note: The derivation of the radiating temperature above is for absolute temperature, in degrees Kelvin (K), so the 1.176 factor relates the Kelvin temperatures, not the Celsius temperatures.]
According to the calculations here the proportion of CO2 in the atmosphere, massive on Venus and low on Earth, has NO "greenhouse" effect on planetary temperatures - rather it is the distance from the Sun that is the main factor.
So far there have been no serious challenges to the observations made at the link. Presently it appears the Green House Effect, with regards to CO2, is totally BOGUS.
[Posted at the SpookyWeather blog, July 29th, 2011.]