Even today, CO2 represents just 1/2,375th of the atmosphere.

Before the industrial revolution it was 1/3,571th

That's a change of less than 1/7,000th of the atmosphere.

Oxygen represents 1/5th of the atmosphere.

Some of that oxygen will have been consumed to make CO2, but not enough to matter, and more will have been emitted because CO2 is a limiting factor for plants.

The atmosphere has gotten very slightly thicker.

EDIT: as /u/mfb- notes below, these numbers are off by a factor of 10 - all these percentages should be one decimal point to the right (e.g. 0.2% -> 0.02%).

Yes. Most of the CO2 was made by burning carbon-containing molecules using oxygen from the atmosphere, so each CO2 molecule roughly corresponds to one less O2 molecule.

But since CO2 is a small portion of the atmosphere's total, this doesn't make a big difference. Today, CO2 is about 420 ppm, or about 0.42%, of the atmosphere; prior to humans it was about 280 ppm (~0.28%). That's a huge difference in terms of how much CO2 there is; there's almost 50% more today than there was a couple centuries ago. But it implies a change of only about 0.14 percentage points in the oxygen amount.

Since oxygen is about 21% of the atmosphere, that's a relative change of only about 1 part in 150 of the oxygen content, which isn't a big deal. Air pressure already varies by more than that (it's the equivalent of about 7 mb of pressure, roughly the difference between a mild storm and a clear day) as weather systems pass by, so your body is already well-adapted to handling such small changes in oxygen content.

(Actually, I wonder if typical sea-level pressure is a bit higher today than it used to be. CO2 is heavier than oxygen, so the atmosphere should "weigh" slightly more than it used to - by a factor of, give or take, about 0.07 ppt. That's not nothing! It should correspond to a global increase in surface atmospheric pressure of about a millibar, which should be detectable. [EDIT: okay, a tenth of a millibar is less.])

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We have slightly more atmosphere than we used to. That's really the short answer to it. We added "stuff" without taking away "stuff". So we have more "stuff". A little bit more. Not a lot.

Which doesn't sound like much but little changes can have major impact to life that's evolved to a very narrow band of conditions.

Most of the earth’s atmosphere is nitrogen and oxygen, making up 99% of the atmosphere (approximately). So the small changes in oxygen are insignificant to the total—changes in trace gases though, especially greenhouses gases, are significant at much smaller changes in concentrations.

No, it doesn't necessarily mean that other gases like O2 are decreasing. The increase in CO2 is due to human activities like burning fossil fuels, and it's causing a shift in the composition of the atmosphere.

well that's not how combustion works

...what? were you under the impression clouds are just big fluffy white pieces of coal?

That is impossible lol. We didn't "add stuff without taking away stuff".

Where are you supposing we source most of that O2 in CO2?

....so oxygen bonds to carbon and releases energy and co2. We took oxygen, added carbon that was stored in physical matter, and released it as co2. So oxygen was removed, bonded with carbon, and released back into the atmosphere this time with added carbon.

So the net amount of oxygen atoms remains the same but we've now added carbon.

The carbon is "stuff". We've taken away no net oxygen atoms and we've added carbon.

You understand that 1-1+1+2 is the same as 1+2 right? Am I assuming correctly that this is at least something you get? Do I need to slow down?

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I see your logic, and at first glance I was about to defend your statement, however it's not correct because:

1. If you burn pure carbon, you get O2 + C = CO2. That's the same number of gas molecules after burning. 1 gas molecule in the input, 1 gas molecule on the output. A gas's volume is determined by temperature and number of molecules, not the number of atoms(technically an approximation, but a very good one). Yes there are more atoms making up the gas, but once the combustion gas has cooled to ambient, the volume of CO2 is basically the same as the volume of O2 you started with. While the CO2 is heavier, this doesn't equate to more gas, it's just heavier gas. (If I give you a litre of petrol and a litre of water, you've got the same amount, although the water weighs more.)

However this is kind of moot because:

2. All this combustion can actually make he atmosphere weigh less and have a lower volume! Most of the stuff we burn is a hydrocarbon. Hydrocarbons burn to produce CO2 and H2O. The H2O precipitates out of the atmosphere, so there's actually fewer gas molecules in the air because we've removed oxygen and turned it in to water. In addition, the mass of the oxygen precipitated, is greater than the mass of carbon added, so the air is lighter too. Now it really depends what hydrocarbons we've burned. Natural gas (methane) produces twice as many water molecules as CO2 so would have the strongest depleting effect. Liquid fuels like petrol, diesel and oil produce about equal numbers of CO2 and water molecules so more slightly deplete the atmosphere. Coal produces fewer water molecules than CO2 I haven't done the math on whether this is a net mass increase or decrease (I expect it's pretty mass neutral) but it definitely still decreases the air's volume.

We turned solid stuff (Hydrocarbons) into atmosphere. CO2 and H20

The volume of both of those things in the atmosphere has increased.

There are literally tons of Carbon released into the atmosphere and while it will come down eventually we do have more atmosphere than before.

H20 joins the precipitation cycle but due to increasing temperatures there is more H2O in the atmosphere than before on average.

So yes. We do have more atmosphere than before.

Both by mass and by number of molecules.

The energy we've put into the system will eventually go back to the previous equilibrium after hundreds of years. So it's temporary on the earth's timescale at the moment.

Unless we put too much energy in then it releases more energy, methane released, ice caps melted, less heat reflected, and the change becomes more of an earth timescale one than a human timescale one.

The CO2 weighs more but has the same volume as the O2 it replaced. However you've missed the face we've also burned a bunch of hydrogen since almost everything we burn is a hydrocarbon. This produces water (H2O) which precipitates out of the atmosphere with a net result of depleting the amount of oxygen atoms in the air. (See my longer answer).

The amount of O2 in the atmosphere has decreased, but it's only a fraction of a percent.

420 ppm is 0.042%, so the change in oxygen is even smaller.

1000 ppm = 1/1000 is 0.1%.

Added a note at the top of my post. Thanks for the correction.

Regarding your last point, some of the mass gain is going to be mitigated from water as a byproduct of hydrocarbon burning, but then also to consider is that a higher average temp is going to correlate with a lower average pressure.

Not sure which factor would win out, I'd tend to think the temp increase would be a larger factor and therefore lower pressure