What's wrong with a bit of global warming?

[Andre]

It should also be considered that a greenhouse gas emits infrared radiation, losing energy in the process, as much as it gains energy by absorpting infrared radiation.

So, since we have two more agents for transfer of 'heat'-eneregy, conduction, convection; it would be interesting to see what kind of role they play in the temperature of the atmosphere. The importance of convexion is generally estimated to be low, compared to radiation. However, radiation balances heating and cooling of the atmosphere but convection does not.

Parcels of surface air are heated by conduction (and radiation) decreasing their density so they will rise up (convection). A similar effect have horizontally moving air masses of different temperatures, where the warmer, less dense, air is forced over the cooler air (advection). Also this transport of energy higher into the atmosphere is enhanced considerably by the http://daphne.palomar.edu/jthorngren/latent.htm due to evaporation and condensation. Meanwhile, cooler air descends balancing the pressure. But there is no such thing as back convection or back advection, transferring the energy back down to the Earth surface.

So, without radiative greenhouse gasses, this convected heat would have accumulated in the atmosphere and it could be argued that this one way traffic of convection contributes significantly towards accumulating heating of the atmosphere, while the radiative greenhouse gasses moderate its heating effect, cooling it down again. See also Chilingar et al 2008 which is discussed in this thread.

these processes may also put the 'certainty' of heating or cooling effects in perspective, due to the variation in concentration of radiative gasses in the atmosphere.

*G. V. CHILINGAR, L. F. KHILYUK, and O. G. SOROKHTIN, 2008, Cooling of Atmosphere Due to CO2 Emission, Energy Sources, Part A, 30:1–9, 2008 ISSN: 1556-7036 print/1556-7230 online DOI: 10.1080/15567030701568727

 

[sylas]

It should also be considered that a greenhouse gas emits infrared radiation, losing energy in the process, as much as it gains energy by absorpting infrared radiation.

Good point. This is also known as Kirchoff's law; the emissivity of a radiator is equal to its absorptivity. This is frequency dependent. That is, a gas (or any other material) will interact with some wavelengths better than others... and that interaction applies for both emission and absorption. This is crucial for the basic physics of the atmospheric greenhouse effect.

We can see this on Earth... and in fact this is one of the ways we can be confident that temperature changes in the present are being driven by these atmospheric greenhouse interactions of certain gases with thermal radiation!

At any given altitude, at equilibrium the energy flowing up is equal to the energy flowing down. Any excess or deficit will go to heating up or cooling down the gas, driving it towards an equilibrium. Since the atmosphere interacts very weakly with visible light, the solar energy flowing down is mostly unimpeded. The Earth's surface radiates back up in thermal wavelengths; and the atmosphere is opaque to most of this radiation... mostly because of interactions with water and with carbon dioxide.

There is also energy flowing up from the surface by convection, and also by latent heat of evaporation of water. The atmosphere is thus mainly heated from below; and then it radiates also in turn, with the same wavelengths that it absorbs. The energy radiated from the atmosphere goes both upwards, and back down. This is the guts of the greenhouse effect. The surface receives most of the solar input, plus an additional flux of thermal radiation from the atmosphere.

If the atmosphere was dry nitrogen and oxygen only, then it would still be heated by convection; but the thermal radiation would escape straight out to space. The atmosphere would still be heated; but it would radiate very inefficiently; there would be minimal backradiation coming back down to the surface. Earth's temperature would be an equilibrium with solar radiation, substantially below the freezing point of water.

There's one very illuminating feature of how the atmospheric profile has been changing over recent decades as the planet has heated up somewhat. The stratosphere actually cools... very strongly... and this is driven mostly by greenhouse gases and their capacity to radiate more efficiently. In the atmosphere, temperatures fall with altitude up to the tropopause, which is extremely cold. Above this level, the radiation escaping to space is strongly depleted in infrared radiation, and this is directly measured. You can see great chunks cut out of the spectrum, particularly with the effects of water and carbon dioxide.

You DO still get radiation coming up at the wavelengths that interact with CO2... but that radiation is not coming from the surface. It comes mainly from the upper atmosphere, because the mean path length of thermal radiation is much reduced. Hence the radiation coming into the stratosphere which is able to interact with CO2 or H2O is weak and cold... colder indeed that the stratosphere itself. Hence the net effect of more effecient interaction with thermal radiation is that the stratosphere SHEDS heat more effectively than otherwise, and is becomes cooler as greenhouse gas concentrations increase.

I've given some descriptions of this, with references, in other threads. See, for example [post=2170740]msg #23[/post] of "Estimating the impact of CO2 on global mean temperature"; the first page of that thread also shows some spectra and energy flow diagrams ([post=2165483]msg #3[/post]).

So, since we have two more agents for transfer of 'heat'-eneregy, conduction, convection; it would be interesting to see what kind of role they play in the temperature of the atmosphere. The importance of convexion is generally estimated to be low, compared to radiation. However, radiation balances heating and cooling of the atmosphere but convection does not.

The thread I mention above gives some numbers, with references, that give a good idea of the magnitude of energy flows, as global averages. At the surface, the Earth absorbs about 161 W/m² of incoming solar radiation. By virtue of its own temperature, the surface radiates about 396 W/m² of radiation; and there is also a flux of about 17 W/m² as convection and 80 W/m² as latent heat of evaporation. So you are quite right; in terms of actual energy flux convection and latent heat is about 20% of the total. The thermal backradiation from the atmosphere is about 333 W/m²; less than the upwards radiation because of course it is cooler than the surface; but still actually the largest energy flux to the surface, on average.

Often, people just consider the difference between the up and down flux, in which case there is about 63 W/m² net radiation into the atmosphere from the surface; this is less than convention, but it is certainly positive; which means that the net effect of interaction with radiation is to heat the atmosphere. Another approach is to remove the radiation that escapes direct to space, in which case you have just 23 W/m² into the atmosphere from the surface.

Of course, this is a net positive; the net effect of interactions with radiation is to heat up the atmosphere, not to cool it.

As for "balance"... ALL these energy flows balance together; energy conservation does not distinguish. Also, the flux of thermal radiation reduces with altitude, because temperatures fall with altitude. So I don't think there's any real difference between radiant and other energy fluxes in terms of balance. Energy balance calculations have to consider special and radiant energy on the same footing.

Cheers -- sylas