Why is it hot on Venus?
Hansen started out his career with a dumb idea about Venus. His doctoral dissertation blamed the high temperatures on aerosols, which is pretty funny because now he blames low temperatures on aerosols. (Science was never his strong point.)
Later he admitted he was wrong, and instead latched on to Marijuana chain smoker Carl Sagan’s theory – the heat was caused by 95% CO2. Wrong again.
Venus is hot because it has a very thick troposphere. Atmospheric pressure on Venus is almost 100X higher than Earth. It is hot on Venus for the same reason it is hot in Death Valley and cold on Mount Everest.
If you traveled to Venus and descended into the atmosphere to an altitude of 50km, the atmospheric pressure and the temperature would be similar to that on Earth. This is in spite of the fact that the greenhouse effect at 1 bar on Earth is much stronger – because we have water vapor.
Compare the LW absorption of Earth’s atmosphere (Total Absorption and Scattering) to Venus atmosphere (Carbon Dioxide) at 1 bar, in the chart below. As you can see, the greenhouse effect on Earth is much stronger.
If you descended down to the surface, temperatures would rise about 8C per km – similar to Earth. The lapse rate is fixed, regardless of pressure broadening towards the surface, or whatever other dumb arguments people will bring up here.
Venus is hot because it has a thick convective atmosphere. Falling air heats, and rising air cools. If the atmosphere on Earth were as thick as Venus, temperatures would be very hot here too.