Re: Dirty Snowball Worlds

Logan Kearsley wrote:
> On Feb 18, 6:12 pm, Bryan Derksen <bryan.derk...@[EMAIL PROTECTED]
> wrote:
>> If it is cold enough for CO2 to start freezing out on the antistellar
>> pole, then the atmosphere is already demonstrably not capable of
>> sufficient heat transport thaw frozen CO2 back there.
> 
> It's demonstrably not capable of sufficient heat transport to thaw
> frozen CO2 given not-necessarily-permanent current conditions. That
> doesn't mean it can't warm up again.

No, but with CO2 absent from the atmosphere you'll have to warm it up 
again via some mechanism other than greenhouse effect.

>> The point the paper was making is that once the antistellar pole's
>> temperature drops that far for whatever reason it's a runaway process.
>> All of the CO2 freezes out, the planet loses its greenhouse gases, and
>> barring something weird like a big increase in the remaining
>> atmosphere's thickness the temperature stays down permanently. In order
> 
> Or something not so weird, like sunspot activity decreasing, thus
> increasing insolation.

As long as the insolation increases again before too much CO2 has 
condensed, sure. The paper discussed some timeframes for this sort of
thing.

> Or dust condensing out of the atmosphere- a
> particularly large volcanic eruption might temporarily freeze the
> planet. And I don't know how stable it would be, but I can imagine a
> cyclic situation where an evaporating ice cap could induce planet-wide
> dust storms such as are seen on Mars, which then cool the planet and
> cause the cap to re-condense, after which the dust settles and it
> starts to warm up again.

But with the CO2 gone you don't get any of the warming that it might 
have been contributing before the freeze. According to 
<http://web.uccs.edu/geogenvs/ges100-online/Chapt4.doc>
without the 
greenhouse effect Earth's average temperature would be -15 C whereas the 
current average is 15 C. If Earth was a tide-locked planet balanced 
close to the point of having its CO2 freeze out, and something dropped 
the temperature enough to trigger it, to recover you'd need to increase 
the heating of the planet enough to overcome that 30 degrees C 
temperature deficit that you're no longer getting from the greenhouse 
effect. If you've got snowball conditions to deal with the albedo will 
be boosted, too, making it even harder.

Personally, I think the "CO2 collapse" scenario their models predicted 
sounds quite plausible. Reversing it could be an interesting 
terraforming scenario, you could perhaps set up orbital mirrors and try 
to raise the antistellar point's temperature enough to get the 
greenhouse effect going again.