B Why would an atmosphere without greenhouse gasses be colder than with them?

[Graeme M]

TL;DR

How would a hypothetical atmosphere without greenhouse gasses cool?

Quick question that I haven't been able to find the answer to. Greenhouse gasses both warm and cool the atmosphere by slowing heat loss to space. But what would happen without GHGs?

I read that the earth would be colder (though still relatively warm), but why? Without GHGs the atmosphere would still be a similar mass and still warmed by conduction from the surface, yet without a means to radiate that heat to space.

Why wouldn't the atmosphere accumulate heat over time, becoming warmer? How is this hypothetical GHG-less atmosphere reaching thermal equilibrium colder than today?

 

[Drakkith]

Without GHGs the atmosphere would still be a similar mass and still warmed by conduction from the surface, yet without a means to radiate that heat to space.

I'm not sure what this last part means. Greenhouse gases absorb IR radiation that would otherwise be radiated away from the planet and into space. This heats the GHG's which then pass the heat to the rest of the surrounding air or radiate it away. Of the energy radiated away, about half of it is radiated back towards the surface where it is absorbed. The net effect is to heat the surface and air to a higher temperature than they would otherwise be without the GHG's.

Why wouldn't the atmosphere accumulate heat over time, becoming warmer? How is this hypothetical GHG-less atmosphere reaching thermal equilibrium colder than today?

Without GHG's much of the thermal radiation that is currently absorbed and redirected back to Earth's surface or atmosphere is lost to space. Sort of like removing all the glass from a greenhouse. Without the ability to trap all that energy, the surface and atmosphere are cooler.

 

[Graeme M]

Without GHG's much of the thermal radiation that is currently absorbed and redirected back to Earth's surface or atmosphere is lost to space.

That's what I'm struggling with. I thought the point of the GHGs is that they can emit IR, so non-GHG gasses transfer heat via conduction (molecular collisions) and the GHGs then radiate that out to space. But if non-GHGs cannot radiate IR AND there are no GHGs, how does the atmosphere lose its heat to space?

The process without GHGs would be: incoming insolation heats the earth's surface, the surface warms oxygen and nitrogen etc via conduction, convection transfers heat through the atmospheric column, and with no GHGs to radiate that heat away, the atmosphere seems like it should simply grow ever hotter until some other process is involved. I can't see the mechanism for thermal equilibrium at -18C.

 

[Dale]

With GHG in the atmosphere the surface of the earth is in radiative “contact” with a reservoir that is at the temperature of the atmosphere. Without GHG in the atmosphere the surface of the earth is in radiative “contact” with a reservoir that is a few degrees warmer than absolute zero.

 

[sophiecentaur]

with no GHGs to radiate that heat away,

Sorry if I'm stating the obvious here
The GHGs are in a long vertical column ('cylinder' of air) of the atmosphere. This is a diurnal effect so it's not simple; you can't really have an equilibrium state. BUT the GHGS near the ground will be in temporary equilibrium with the ground. They do not absorb the higher frequencies wavelengths so that (most of) solar energy gets to the ground and the GHGs will also absorb some incident energy. The ground radiates IR (solids will end up at a black body temperature of about 300K). This IR is absorbed by the nearby GHGs which then warm up and radiate IR upwards and downwards. A (brief) equilibrium will occur. The upward radiation is absorbed by the higher GHG layers etc. etc. and the column is then in equilibrium with a temperature gradient. The ground heats up a lot more than if there were no GHGs in order to radiate incoming and outgoing energy. The presence of clouds is also a similar effect. On clear nights the ground cools very much faster than with clouds.

With no GHGs, most of the ground's radiant IR will exit to space (even more than on a clear cloudless night) so the average ground temperature would end up at about 10C with huge diurnal / annual variations.

From https://space.stackexchange.com/que...temperature-of-a-satellite-orbiting-the-earth

 

[Bandersnatch]

The OP thinks that only GHGs can radiate heat. - is the issue here.

 

[sophiecentaur]

Why wouldn't the atmosphere accumulate heat over time, becoming warmer?

Hmm. That's something he needs to sort out.
Nothing 'only' absorbs or loses heat. Emissivity is the same value as absorptivity. Take a bubble of GHG and leave it somewhere and it will end up at an equilibrium temperature; loss and gain will eventually be the same.

The whole process of heat gain and loss from the Earth takes place throughout the atmosphere and at the surface. Take away all the GHGs and the process will be simpler and the diagram above gives a clue about what a final temperature would be at 1AU. Compare the other planets. Mars is in the news a lot and it has very little atmosphere. Its average surface temperature is around -45C, corresponding to its distance from the Sun.

 

[russ_watters]

That's what I'm struggling with. I thought the point of the GHGs is that they can emit IR, so non-GHG gasses transfer heat via conduction (molecular collisions) and the GHGs then radiate that out to space. But if non-GHGs cannot radiate IR AND there are no GHGs, how does the atmosphere lose its heat to space?

The surface radiates and not all of that energy is absorbed by the atmosphere:

https://scied.ucar.edu/learning-zone/how-climate-works/energy-budget

Without greenhouse gasses, the 6% that is directly radiated to space from the surface goes up and the 15% radiated by the surface that is absorbed by the atmosphere goes down.