Hi Mark

Inorganic processes remove carbon dioxide as precipitable carbonates long before they hit ~ 70 bars in the presence of an ocean. James Kasting has modelled carbon dioxide and its inorganic control for years - Earth's pre-Industrial level (~ 300 ppm) was about right for its current 288 K surface temperature at current insolation levels.

For years Archean geologists have argued over the initial carbon dioxide levels of Earth's atmosphere with some arguing it was several bar to off-set the 70-80% solar luminosity during the Archean, while others have pointed to paleosols (fossil soils) that indicated much lower levels. Recently the paleosol data has been explained in a rich CO2 context and other paleosol data indicates a level x 1000 the present day - that's 30% CO2. With some methane chucked in, the Archean's average temperature due to the greenhouse was ~ 55 degrees C (131 F), with evidence of ~ 100 C temperatures in the early Archean. In the Hadean Earth probably had a 400 bar atmosphere of steam and CO2.

An interesting look forward in time was published by Kasting in 1992. As insolation rises the carbonate formation rate will rise and CO2 levels will steadily drop until it's unsuitable for C3 photosynthesis - most trees will then die out or adapt. That's in about 500 million years. In about ~ 900 million the CO2 will be too low for even C4 plants ( grasses, some shrubs ) and so plant-life on land will become extinct or need a new carbon source. The sea will retain a low background dissolved carbonate level so oceanic plants will last a lot longer. Land-plants might evolve into floating forests? However in about ~ 1.5 billion years the Earth will have dried out from hydrogen loss to space once the stratospheric water-trap is overwhelmed by rising surface temperatures.

Clouds are expected to drag out the timespan, as David Grinspoon and Mark Bullock have suggested for Venus. But the Sun will eventually hit (in 4 billion years) ~ 150% of current insolation, past which point the surface is sure to be too hot for stable water oceans.

An interesting look back is the origins of Larry Niven's "Known Space" idea that the Moon's formation saved Earth from the fate of Venus, and Jinx and Lookithat's lowlands. Prior to Soviet Venera landers the chief component of Venus' atmosphere in the early 1960s was thought to be N2. Mariner 2 had confirmed the high surface temperature and Carl Sagan had then modelled Venus as having ~ 80 bar of N2, causing so much Rayleigh scattering that it made the planet yellow even without clouds. A.G.Cameron had modelled Earth's formation with such data in mind and proposed that Darwin's fission-hypothesis would strip Earth of its primary N2 atmosphere as the Moon was spat forth.Thus Larry Niven's "Known Space" Venus, Jinx and Lookithat were born.

His explosive-on-water-contact Martians (from his first published story in 1963) were from another nitrogen-based theory from 1960 proposed by Kiess (et al) - that Mars' red colours and white polar caps came from NOX compounds... the white caps were supposed to be solid nitrogen tetroxide!!! Would've made self-refuelling easy with all that oxidiser lying around. The Mariners of the later 1960s killed that idea too, but Niven stuck with (now inexplicably) exploding Martians.

Adam