On 19/11/2011 14:28, Nelson wrote:
> Please consider what would happen on Earth starting with an atmosphere
> of Carbon dioxide mixed 50:50 with water vapour. My estimate is that
> the sun would visually dim very little but the whole IR band of
> radiation would fall by some 38%. Therefore there would be 38% less
> heat
> reaching the ground.
You have fundamentally failed to understand that the bulk of the
radiation coming from the sun is characteristic of a black body at 5500K
strongly peaked in the visible with a tail into uv and ir. The incoming
radiation in a clear sky would be barely affected and it will warm the
ground. The tricky bit is predicting the effect of clouds.
The higher proportion of CO2 and H2O would block *outgoing* thermal
infra red radiation characteristic of temperatures of around 300K.
Reasonably nice graphic presentation of the main components at:
http://www.globalwarmingart.com/images/7/7c/Atmospheric_Transmission.png
>
> That atmosphere would warm and convection currents would start to
> operate. Due to expansion of gases the rising vapour would cool at the
> saturated lapse rate some 5 °C/km). Soon a distillation process would
> begin and Earth would regain something like the current temperatures
> but actually cooler due to a decrease in thermal energy reaching the
> ground.
Almost certainly clouds would form and rain would fall until the
atmosphere contained water vapour at a concentration that was roughly
consistent with the prevailing ambient temperature at each latitude.
After that you would have weather characteristic of those temperatures.
Snow and ice would be considerably rarer.
It only remains to estimate the temperature of an atmosphere of CO2 at
0.5bar (ie 50% instead of 400ppm). 50/0.4 = 125 which is close enough to 2^7
Doubling CO2 concentration gives a optimistic 2K increase and possibly
as much as 5K if we are unlucky increase in global mean temperature. So
the ambient temperature would be in the ball park of 14K to 35K higher
than at present (best guess at present about 20C). This could in a worst
case scenario allow lakes in the tropics to boil. It is also getting a
bit borderline for having permanent ice at the poles in the worst case
scenario. There are a few online simulators that will let you play with
model planets - this is pushing them to their limits.
>
> At night the heat retention of the carbon dioxide and water vapour
> atmosphere would save the planet from excessive temperature
> fluctuations as observed on the Moon of 123°C to -238 °C.
The main effect would be from clouds. And they are a double edged sword.
In daytime they do reflect away a fair proportion of the incident
sunlight. But equally they also stop long wave radiation from escaping
and reflect back a proportion of sunlight that has penetrated. If you
get mostly high cirrus or status clouds then they can enhance the
greenhouse effect by trapping heat. Clear skies allow the ground to cool
rapidly at night as astronomers are well aware.
You should also be aware that Venus has a high very CO2 concentration
and a surface temperature sufficient to melt lead. And this is despite
having the highest albedo of any planet in the solar system of 0.9
--
Regards,
Martin Brown