[Luminary (OP)]

What happens if everybody, or say most of us, on the earth stop to breathe at the same time for say 15-20 seconds? How does it effect the climate/atmosphere? What will happen to the gas (oxygen, co2 etc) levels?

[Bored chemist]

What happens if everybody, or say most of us, on the earth stop to breathe at the same time for say 15-20 seconds? How does it effect the climate/atmosphere? What will happen to the gas (oxygen, co2 etc) levels?

Practically nothing.
People breath about 10 litres in a minute. There are about 6 billion of us so we would alter about 60 billion litres of air if we stopped breathing for a minute
That's of the order of 60 billion grams or about 60 million tonnes.
The earth's atmosphere weighs about 5 quadrillion tonnes. Roughly ten million times more.
We really are rather small.

[litespeed]

Bored

I the body's metabolism continues during the breath holding, and CO2 probably continues to accumulate in the lungs. However, this accumulation is near the inside surface and blocks oxygen uptake. After holding the breath we would breath harder to get everything back into balance.

Incidentally, a scuba diver I know told me only a small proportion of the oxygen in a breath of air is actually absorbed. Theoretically, breathing out and then in from a collapsed paper bag could settle the issue. Specifically, how long can you hold your breath versus how long you can breath in and out of the bag [the air movement mixes the gases as you do so] before gasping for more air.

I'm not going to try it though!

[Bored chemist]

I agree that it wouldn't affect the atmosphere in the long run because, after holding your breath you breathe more rapidly to "catch up" again.
The point is that, even for those few seconds the effect would be tiny.

[litespeed]

BC

Yes, I agree. Very tiny indeed! Still, I would like to know how much oxygen remains in an exhaled breath that might be reused. I have actually researched this [just a little bit] and have not come up with anything informative so far.

Submariners have traditionally used CO2 absorption devices to prolong the breathability of enclosed air. That certainly supports the idea the oxygen was not used up. But does not answer the question of how much the enclosed air could be re-breathed without CO2 scrubbing.

[Bored chemist]

Exhaled air is about 5% CO2 so roughly 5% of the O2 gets used up by breathing. The exhaled air contains about 16% O2.
Wiki gives this data
 "The permanent gases in gas we exhale are roughly 4% to 5% carbon dioxide and 4% to 5% less oxygen than was inhaled. Additionally vapors and trace gases are present: 5% water vapor, several parts per million (ppm) of hydrogen and carbon monoxide, 1 part per million (ppm) of ammonia and less than 1 ppm of acetone, methanol, ethanol (unless ethanol has been ingested, in which case much higher concentrations would occur in the breath, cf. Breathalyzer) and other volatile organic compounds. The exact amount of exhaled oxygen and carbon dioxide varies according to the fitness, energy expenditure and diet of that particular person."
Here
http://en.wikipedia.org/wiki/Breathing