CO2 levels are increasing, they have crossed 400 ppm, which means that of every million gas molecules in the air, 400 are of CO2.

It has been increasing. Does that mean the total volume of air in the atmosphere is increasing or that some other gas is decreasing as CO2 increases?

Thank you.

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    $\begingroup$ The other option, which you have not considered, is the bulk density of the air might be increasing. $\endgroup$ – Fred Jun 25 '18 at 17:32
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    $\begingroup$ 400/1 million is a tiny part, keep in mind. Even if it started at 0, and though it's 44 vs 29 g/mol, that's only a change 0.006 g/mol (or 0.02%) $\endgroup$ – JeopardyTempest Jun 25 '18 at 23:01
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    $\begingroup$ Meanwhile water vapor varies much more wildly over much shorter time periods. Common summer water vapor values in subtropical climates can be over 10 g H2O per kg of air air (so in other words, more like 10,000 ppm, if my math were all solid. And 18 vs 29 is as impactful as 44 vs 29. So in other words... the atmosphere varies more on a regular basis (not even considering the impact of pressure systems!), so it's a very tiny reality probably fed back in a mix of factors. $\endgroup$ – JeopardyTempest Jun 25 '18 at 23:08
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    $\begingroup$ when burning carbon you combine two oxygen atoms(from the air)with one atom of carbon so the mass of atmosphere changes very little. $\endgroup$ – trond hansen Jun 27 '18 at 12:20
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    $\begingroup$ If you mean "are there more atoms in the air in the form of gas?, I think the comment above answers that (more carbon atoms in the air). It would be a small percentage increase though as also mentioned above. $\endgroup$ – Jack R. Woods Jul 1 '18 at 1:59

[Major edits below]

In short the answer is NO.

Before we get into volume changes, I have to say that volume is a tricky measure to apply to the atmosphere as a whole, because there is no clear limit between the atmosphere and outer space. Also, the thickness of the atmosphere is quite insensitive to changes in atmospheric mass. For example, the atmosphere of Venus is much more massive than Earth's (93 times more massive). Leading to surface pressures 92 times those of Earth. However, the thickness of the atmosphere is quite similar, with the mesosphere extending to 120 km, only 41% higher than on the Earth, despite having 9300% the mass.

This planetary comparison suggests that more CO$_2$ will lead to a more massive atmosphere and higher atmospheric pressures, but little change in its actual volume (if you somehow manage to define how to measure the volume of the atmosphere).

As an analogy, imagine you have one meter of snow cover, and there is one meter of snowfall. You won't end up with two meters of snow cover. Thise is because the new snow will compact the snow below, leading to something like 1.7 meters of snow cover.

Therefore, more gases in the atmosphere will lead to a denser atmosphere, but have little impact on its thickness.

Now we have to make a very important distinction: It is not the same to add CO$_2$ from a CO$_2$ source (like a volcanic eruption) than add it due to combustion of fossil fuels. This is because the latter do not add molecules to the atmosphere, just turn O$_2$ molecules into CO$_2$. Now, if you look into what this transformation means in terms of volume, you will get a surprise (at least it was a surprise to me):

Given standard conditions (20°C and 1 atm). One cubic meter of O$_2$ weights 1.331 kg. When you use that oxygen to burn fossil fuels all the molecules in it will go from a molecular weight of 32 (O$_2$) to 44.01 (CO$_2$), meaning you end up with 1.831 kg of CO$_2$. Now comes the interesting part: under the same conditions, one cubic meter of CO$_2$ weights 1.842 kg, consequently, our initial cubic meter of O$_2$ turned into 0.994 m$^3$ of CO$_2$, equivalent to a volume reduction of 0.6%. Therefore, burning fossil fuels reduces the volume of the atmosphere.

I was surprised at first by this result, but it nicely explain the classic experiment where a boiled egg get sucked into a bottle: enter image description here This is an screenshot of this video where you can see the trick in action (skip to minute 1:10)

Now, despite that burning stuff decrease the volume of the atmosphere. It is important to note that if you add greenhouse gases like CO$_2$ or methane, you are not only increasing the amount of those gases in the atmosphere, because the increase in temperature will lead to an increase in water vapor too. So the atmospheric mass increase will be larger than the mass of CO$_2$ or methane added. And that extra mass is related to additional molecules, so it will lead to extra volume (contrary to the combustion case where the amount of molecules is constant).

Thus, as we burn fossil fuels the mass of the atmosphere increases (as well as the atmospheric pressure), but the volume decreases, and due to the fact that CO$_2$ is the product of combustion, that increase will be at the expense of reducing the amount of molecular oxygen (O$_2$).

Although, this decrease is very small: the atmosphere have ~21% of Oxygen and 0.04% of CO$_2$, so an increase in CO$_2$ of a 30% (similar to going from 300 to 400 ppm), means a reduction of O$_2$ of 0.01%, a relatively small change (using Wikipedia numbers it would go from 20.946% Oxygen to 20.934%). Also, that's strictly true only if you make an instantaneous combustion. Because over time there could be negative feedback working against this imbalance. For instance, plants will increase the rate of photosynthesis (CO$_2$ fertilization effect)

That said, the mass increase (and volume decrease) associated to the CO$_2$ jumping from 300 ppm (pre-industrial levels) to 400 ppm is negligible, and probably dwarfed by day to day changes in particulate material or water vapor.

As a side note, the book "Red Mars", makes a fairly scientifically sound description of how to "terraform" Mars, and how adding CO$_2$, or water vapor increase the atmospheric pressure and surface temperature.

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    $\begingroup$ This is a great answer but now I'm curious about "reducing the amount of molecular oxygen" and it's relative magnitude. $\endgroup$ – f.thorpe Sep 13 '18 at 1:12
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    $\begingroup$ I am unable to find reference but I have read if all of the organic carbon on Earth's surface is burned down, the reduction in O2 amount will be less than 2%. $\endgroup$ – Harish Sep 13 '18 at 9:28
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    $\begingroup$ @farrenthorpe Well, combustion will take one molecule of oxygen for each one of CO2. However, the atmosphere have ~21% of Oxygen and 0.04% of CO2, so an increase in CO2 of a 30% (similar to going from 300 to 400 ppm), means a reduction of O2 of 0.01%, a relatively tiny change (using wikipedia numbers it would go from 20.946% Oxygen to 20.934%). That's strictly true only if you make an instantaneous combustion. Because over time there are CO2 sinks and negative feedbacks working against this imbalance. For instance, plants will increase the rate of photosynthesis (CO2 fertilization effect). $\endgroup$ – Camilo Rada Sep 13 '18 at 15:50
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    $\begingroup$ @farrenthorpe Have a look at the answer now, I did some mayor edits. $\endgroup$ – Camilo Rada Sep 13 '18 at 17:59
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    $\begingroup$ @Harish Makes sense. Maybe up to 9% as long as you don't consider the carbon on sedimentary rocks. I just did a quick calculation and you would need 446891 Gt (giga tons) of carbon to turn all O2 into CO2. And all carbon reservoirs beside rocks add up to 41163 Gt (according to this d32ogoqmya1dw8.cloudfront.net/images/integrate/…), so that would take 9% but if you consider only land carbon (that you can easily burn) is would be 0.1% $\endgroup$ – Camilo Rada Sep 14 '18 at 2:03

Let me answer this question indirectly. When a pressure cooker partially filled with liquid water is heated up, there will be more water vapour molecules in the air inside pressure cooker but volume of pressure cooker still remains same. Only thing that changes inside the pressure cooker is pressure and temperature of air.

Volume of gas is determined by ideal-gas law which states that PV = nRT where P is pressure, V is volume, n is amount of gas, R is a constant and T is temperature. If P and T remain same then adding more gas (increasing n) would result in change in V (volume). However, the factors that affect pressure and temperature of air are so highly variable and large that minuscule volume increment expected from amount of CO2 does not matter. However, CO2 by absorbing infrared radiation increases temperature and that results in increase in volume which is a topic of active research.

  • $\begingroup$ But, with all other factors held constant, and considering that the atmosphere is not an ideal gas, what would be the effect of replacing O2 with CO2? $\endgroup$ – Gimelist Sep 13 '18 at 2:45
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    $\begingroup$ Equation for non ideal gas looks like (P- a)(V + b) = nRT where a is coefficient dependent on inter-molecular forces and b is coefficient that depends on volume of individual molecules of gases. Overall argument present in answer holds for non-ideal gas. $\endgroup$ – Harish Sep 13 '18 at 9:22
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    $\begingroup$ While answering I did not consider the fact that CO2 molecule is added at the expense of O2 molecule in the atmosphere. In other words total number of molecules remains same but are getting replaced by heavier molecules but effect will be negligible as pointed out by @camilo-rada $\endgroup$ – Harish Sep 13 '18 at 9:25

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