Because there's very little water on Venus?

Hi,

Basically the 'venus effect' is what happens when the feedback system breaks down.

Once CO2 is practically zero but the sun is so hot that temperatures are higher than today, you reach the point that water vapour feedback dominates. Essentially, once the oceans reach an average temperature of about 28 Degrees C (about 13 degrees higher than today), you get the situation that evaporation exceeds precipitation in an ongoing cycle (Higher evaporation = higher temperatures = more evaporation, where it cannot rain out fast enough to stop). Very, very quickly (perhaps a century), surface temperatures exceed the boiling point of water; the oceans boil into the atmosphere, increasing the greenhouse effect even more.

Now, of course, the temperature gets so extreme that carbonates start breaking down, so all exposed Carbonates release CO2 back into the atmsophere. Volcanic CO2 will also build up. Even more greenhouse effect.

The huge increase in atmospheric water means that more will br broken down by UV light into oxygen and hydrogen, with the hydrogen escaping. Several hundred million years hence, the water will mostly be gone and the oxygen gradually removed by reaction with lava. We will now have a massive CO2 atmosphere and a searing desert of a surface, much like Venus today.

The implication is, of course, that Venus was once like Earth, although this is uncertain - it may never have been cold enough to keep the ocean/CO2 equlibrium. It is also interesting that sea temperatures in the late creatceous super-greenhouse may have averaged 25 degrees C or higher.. placing us a couple of big volcanic eruptions away from the runaway greenhouse.

Something to think about next time you start your car up..