6
$\begingroup$

I have never heard an explanation as to what traps GHGs. What holds them in place? Has research been conducted to assess the efficacy of say triggering a change in their chemical composition into a compound that would render them inert or less impactful or 'setting them free' so to speak?

$\endgroup$
7
26
$\begingroup$

The reason the atmosphere (including GHGs) stays attached to the earth is gravity. This is called the Hydrostatic Equilibrium. There is one GHG that this does not fully apply to: water vapor. While water vapor can be considered to be in hydrostatic balance, the fact that it undergoes a phase change (and becomes dissociated if it reaches too far up) prevents it from being more evenly distributed vertically like $\ce{CO_2}$ or $\ce{CH_4}$.

$\endgroup$
3
  • 5
    $\begingroup$ And while there's some (very slow) escape of gas, that depends on the molecular weight of the gas. So while hydrogen & helium escape relatively easy, the principle GHG, carbon dioxide, is heavier than either atmospheric oxygen & nitrogen. So if anything, escape into space would result in a HIGHER GHG concentration. $\endgroup$ – jamesqf Apr 13 at 17:04
  • $\begingroup$ So, the best way to turn GHG into something that wants to escape is by turning it into water, at which time we no longer want it to escape? $\endgroup$ – Mast Apr 15 at 9:49
  • $\begingroup$ @Mast, Water doesn't 'want to escape'. It falls back to the surface as precipitation. Once air gets cold, it can hold very little moisture. This is why the majority of the water in the atmosphere is in the troposphere. $\endgroup$ – Mehmet Apr 17 at 7:18
14
$\begingroup$

Trapping compounds and changing composition are two very different things.

The composition of an atmosphere is set by equilibrium chemistry. Equilibrium chemistry can be understood as mapping of a set of input atoms into molecules and remaining atoms. For example, at the given numbers of N, O, C, H... and given temperature and pressure, one will always find the same amount of GHG, e.g. $\rm CO_2$, $\rm CH_4$, etc. Hence, changing the amount of GHG would require to remove atoms first, i.e. split molecules in the first place. The latter requires some kind of large-scale chemical or photoionization processes.

The actual trapping in Earth's gravitational well, just as Venus', is due to the high molecular weight of GHG's, keeping most of the GHG gas in the troposphere. There is some upwards loss to space however. This process is strongest in a planets early history when the light hydrogen carries the heavier GHGs along. Nowadays the upwards loss is controlled via upwards diffusion, limited by 'cold traps' in atmospheres.

$\endgroup$
6
  • $\begingroup$ Thank you. The life expectancy of a GHG is different for CO2 than it is for CH4 but in both cases there must be a factor or factors that cause them to change from their current composition into their individual molecules or they combine to form other compounds but there must be a 'mass-balance' equation to describe this and there must be a loss of the compounds and their molecules or the concentration of them would be much larger than it is. When we say that CO2 lasts for ~ 100 years and CH4 17 years, what happens to them? What do they turn into and or where do they go? $\endgroup$ – Blair Irwin Apr 12 at 23:24
  • 3
    $\begingroup$ @BlairIrwin I wrote a (hopefully accessible) paper on where CO2 goes, which you can find here pubs.acs.org/doi/pdf/10.1021/ef200914u (pre-print here theoval.cmp.uea.ac.uk/~gcc/publications/pdf/ef2011a.pdf). An individual molecule lasts only about 4 years in the atmosphere before it is exchanged with carbon from the oceans or terrestrial biosphere (this is known as the "residence time"). By "CO2 lasts ~100 years" we mean that it takes natural carbon sinks about 100 years to take up pulse of additional CO2 injected into the atmosphere (called the "adjustment time") ... $\endgroup$ – Dikran Marsupial Apr 13 at 11:12
  • 1
    $\begingroup$ However, that 100 years is just the initial response of the "fast" carbon cycle, mostly uptake into the surface ocean down to the thermocline. When this has equilibriated there will still be about 20% of the pulse left in the atmosphere, which can only be removed by being taken into the deep ocean and into the lithosphere, which will take much longer. $\endgroup$ – Dikran Marsupial Apr 13 at 11:14
  • 2
    $\begingroup$ CH4 is different in that it just gets oxidised (?) to form CO2, which then gets gradually taken up with the regular CO2. $\endgroup$ – Dikran Marsupial Apr 13 at 11:14
  • 1
    $\begingroup$ @Dikran Marsupial: Yes, CH4 + 2 O2 -> CO2 + 2H2O is an exothermic reaction, so it can happen easily when the two gasses are in contact. (That's why we use natural gas as a fuel, after all.) Decomposing CO2 is endothermic, thus doesn't happen unless energy is applied - usually solar energy via photosynthesis. $\endgroup$ – jamesqf Apr 14 at 3:24
3
$\begingroup$

Carbon dioxide has the same components as an oxygen molecule, plus a carbon atom, so the forces that keep oxygen from escaping the atmosphere apply even more to CO$_2$.

The escape velocity of the Earth is about 11 km/s, and the speed of sound is 300 m/s. So a molecule would have to be traveling at more than Mach 30 to have escape velocity. That's not a complete answer (that would require discussing Boltzmann distributions and such), but it should be enough to give an intuitive idea why the Earth doesn't lose its atmosphere.

$\endgroup$
1
$\begingroup$

Simplest explanation I can think of (hopefully no logical mistakes in it): gases, just like liquids, are fluids; bound to the same physics laws. Lighter gases "rise up", above heavier ones which "sink down".

Between all these, there's gravity, which attracts them all.

$\endgroup$
1
  • 2
    $\begingroup$ What you say is only true above the turbopause, at ~100 km altitude. Below that, gases are well mixed and no weight-dependend separation of species occurs. $\endgroup$ – AtmosphericPrisonEscape Apr 13 at 16:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.