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Re: Mystery of Mass Extinctions Is No Longer Murky

Hi Jeff

That's all true - c. wet greenhouse 1.1 Gyr from now with 50% cloud cover - perhaps twice as long if we do a full GCM of the situation. If Earth desertifies from mantle subduction of the oceans then parts will remain "habitable" but dry for somewhat longer - so the water could disappear, but not via the greenhouse effect. And solar UV is declining through time, so the efficiency of water-loss will be somewhat lower.

During the Cryogenian the insolation was lower by about ~ 6% and that probably helped keep the Earth from tumbling into a "wet greenhouse" state, but the real reason was that a water-based greenhouse is too damned unstable and needs another IR-active gas, like a lot of CO2, to remain in a stable situation. And of course CO2 and open oceans don't mix well. When the greenhouse gas can condense or precipitate things get too complex...


Jeff Hecht wrote:

A moist greenhouse (or wet greenhouse) is inevitable because the sun is gradually warming as it burns hydrogen, but calculations I saw predicted that it would take 1.1 billion years, at which point the sun would be 11% brighter. The theory is that the oceans will gradually evaporate, and water vapor will rise into the stratosphere, where solar ultraviolet will split the water molecules into hydrogen and oxygen and the oxygen will escape.

Any proper model of climate in deep geologic time has to consider the lower solar flux earlier in the Earth's history. It's only a few percent back in the Cretaecous, but becomes more significant earlier.

At 6:22 PM +1000 7/6/08, Adam wrote:

Hi Guys

Having studied atmospheres a bit I concur - Earth can't get so hot that it 
wouldn't rain anymore. I'm not entirely sure the Earth would even be dehydrated 
the same way Venus is supposed to have lost its ocean. Hydrodynamic outflow of 
gases from Venus' exosphere in the early days of the Solar System was driven 
partly by the much higher UV put out by the early Sun.

However a "wet Greenhouse" is hotter than a high-pressure autoclave, even if 
the oceans are still around - with about 10 bars worth in the atmosphere. Could Earth be 
tipped towards that end-state? We really don't know, but the end of the Cryogenian 
glaciations via a massive greenhouse was the worst seen since the hot times of the 
Archean - and the Earth didn't tip then. Would our little perturbation really be enough 
to push the Earth's systems past that point? Maybe not, but it could end up with 50 C 
temperatures in the Caribbean like during the Cretaceous...