Venus

As the Astronomer Patrick Moore once said, "If you were to stand on the surface of Venus, you would be simultaneously fried, poisoned, squashed and corroded."

The surface temperature on Venus is about 462 degrees Celsius (hot enough to melt lead), the atmosphere is composed of 97% carbon dioxide, the pressure is about 100 times the atmospheric pressure on Earth, and the clouds are composed of corrosive sulphuric acid.

Venus is a particularly interesting case study because planetary scientists believe that it originally possessed water, much like the Earth, but then underwent a runaway greenhouse effect, and all the water boiled away. The initial cause of this may simply have been Venus's closer proximity to the Sun. The higher temperatures increased the water vapour content in the atmosphere, and because water vapour is a greenhouse gas, this, in turn, increased the temperature further. As temperatures increased, greater and greater quantities of carbon dioxide would have been liberated into the atmosphere, which, of course, would also have amplified the temperature increase. As a consequence of the higher temperatures, the hydrogen atoms in water molecules were separated from the oxygen atoms, and the hydrogen atoms then escaped into space.

Whilst CO2 levels on Earth have never reached the current levels on Venus, the presence of life on Earth has acted to regulate the atmospheric content of CO2 on the Earth, as explained in this nice passage by James Lovelock:

Living organisms act like a giant pump. They continuously remove carbon dioxide from the air and conduct it deep into the soil where it can react with the rock particles and be removed. Conside a tree. In its lifetime it deposits tons of carbon gathered from the air into its roots, some carbon dioxide escapes by root respiration during its lifetime, and when the tree dies the carbon of the roots is oxidized by consumers, releasing carbon dioxide deep into the soil...There it comes into contact with, and reacts with, the calcium silicate of the rocks to form calcium carbonate and silicic acid. These move with the groundwater until it enters the streams and rivers, on their way to the sea. In the sea, the marine organisms continue the burial process by sequestering the silicic acid and calcium bicarbonate to form their shells. In the continuous rain of microscopic sea shells, the products of rock weathering - sedimented limestone and silica - and buried on the sea floor and eventually subducted by the movements of plate tectonics. (The Ages of Gaia, p127).

Whilst the atmosphere of Venus contains 300,000 as much carbon dioxide as the current atmosphere of the Earth, the Earth's crust contains almost as much CO2 chemically bound in the form of such limestone.