a hard question to answer unless you're a scientist

what have you got planned?

story idea.

step 1: go to a scuba web site and see how much air a person uses per hour
step 2: figure the volume of air on the earth at any given time. (i googled and didn't find an answer, but you could multiply surface area of the earth times the number of vertical miles of air, which goes something like 50 km)
step 3: multiply the amount of air used by one person in an hour times the earth's population (6,736,518,644.00 as of this evening) That will tell you how much air is used every hour.
step 4: divide the total air amount from step two by the air used from step 3. That would give you an answer
HOWEVER
I'm not certain that air isn't created by other chemical processes. Air the oxygen molecules released when water changes state? I'm not sure.

I'm going to take a stab at this since it's an interesting question and I love math. Obviously, absurd estimations, approxomations and assumptions follow:

Assuming the average radius of the earth is 6,370 km and that over 90% of the atmosphere's mass is below a 20km altitude, I figure the rough volume of the atmosphere is 10230139898282839965 liters.

I had a chemistry class where the professor told us that the average human breath was around half a liter. There are about 6.6 billion people on the planet, so that's 3.3 billion liters gone per global breath. Wikipedia tells me that the adult breathing rate is 12 to 20 breaths per minute. Kids breath more, but have smaller lungs of course, so I'll go with 16 as the average and say that's 52,800,000,000 liters of breathing per minute.

Through the power of division, that's... 196,733,460 minutes, or 368.6 years.

For purposes of a story, you could shrink that further of course, since I didn't take into account the breathing all the animals are doing. I don't even know where to start to try to estimate that. You could tell me they cut it down to 10% of that time, or 80% and I wouldn't be surprised at either.

EDIT: It also just occurred to me that in the process of using that air, the oxygen in would be coming out carbon dioxide. Since all the plants (and presumably microbes) that normally sink that carbon dioxide are dead, it seems to me that the air would also be unbreathable through lack of oxygen or too much CO2 well before then. Figuring that out would probably involve some fancy calculus based statistical analysis and research that I just don't feel like doing right now. Also, the climate change would be ridiculous well before that 368 years was up. That might kill us all first.

Anyway, I'd say that 368.6 years is a good deal longer than the absolute actual ceiling for life support.

There are at least 2 more significant facts that you have to take into account, but both have the effect to "shorten" the amount of time that would actually be left.

#1 at levels above 50,000 ppm carbon dioxide is in fact toxic--no I don't mean carbon monoxide, which is even more toxic, but carbon dioxide ([URL=http://en.wikipedia.org/wiki/Carbon_dioxide]see Wikipedia article[/URL]). This is not just due to asphyxiation by lack of oxygen. Case in point a release of carbon dioxide from Lake Nyos in Cameroon killed about 2,000 people in 1986. What this means is that the levels of carbon dioxide would become toxic long before all of the oxygen got used up.

#2 burning shit also uses up a lot of oxygen, and if the shit that you're burning contains carbon, it's also going to produce carbon dioxide. If I had to wager, I would bet that all the stuff that we burn to make energy, drive cars, and whatnot converts a heck of a lot more oxygen to carbon dioxide than the total of all people breathing.

If I had to come up with a sci-fi solution to "fix" the problem, it would be to use non-combustive energy sources, like nuclear power, solar power, wind energy, hydro power to generate electricity to generate oxygen from water by hydrolysis. But I would have to check out how "carbon neutral" the various energy sources really are, because it still takes energy to make solar panels and build power plants and so on, the energy you get back might not be enough to offset the energy that you used up.

I want the result to be as close to the truth as possible. In fact, I want it to be the truth.

Get a hollow sphere exactly 1/6,750,000 the volume of the earth's atmosphere, pack your car with food, drive inside the sphere and seal it shut, then live inside the sphere and go about your normal daily routine until you die, then you will have the answer.

Come to think of it, research related to Biosphere 2 might have some relevant information. See also space travel / terraforming / colonies on the moon / Mars.

If all of the plants stopped producing oxygen, and therefore are dead, people would use up all the food stores, the food chain would collapse and people would starve to death. Herbivores would die out, but maybe life could evolve to become all carnivorous and chemosynthetic. Bacteria and fungii would still remain and probably some sea life and insects. Are we counting algea as plants? Most likely some mutant plants would survive and produce offspring regardless of whatever happened, might even thrive with very little competition. Life on earth has almost been wiped out before in the distant past.

I like this thread.
I'm going with Polumna's idea, that whilst still breathable, as soon as the shift in Carbon Dioxide to Oxygen levels hits around 30%, the climate would just go insane. We're talking 60 degrees Celcius in Canada, polar ice-caps melting. Not to mention that if the trees themselves are dying, in that heat most of the world is going to go to desert in a matter of months. Also dead trees + heat = fires. Fires = more oxygen being burned. A whole lot more. If we're taking into account those animals and fires, that 368.6 years would become 3.68 years, seing as a candle flame can would burn about the same as an adult's lung per minute.

So in the first few months the climate's gonna change.
Three laters there's no air to breathe.

I'm guessing the sentence between would make for an awesome story Justin. How are you going to have the trees stop producing oxygen? A disease? Trees get diseases too.

I can't believe I forgot burning! Thanks a lot, meatthinker. I just can't bring myself to calculate the increased levels of CO2 in ppm over time. I'd have to go get at least one chemistry book, at least two math books and a proper calculator. However, sticking to the end-of-all-usable-oxygen model, I'll revise as best I can.

I found a source saying that global CO2 emissions for 2004 were 7,376 million tons. Wikipedia says that there were, in 2002, 590 million passenger cars. Wikipedia also says that transportation is responsible for 19.2% of CO2 emissions.

I'll pull a number out of nowhere for an average automobile engine displacement... 3 liters. I'm sure this is quite high for the whole world, if not for the US. I'm partially trying to reduce error since I can't find any numbers on the population of non-passenger vehicles, and surely trucks and airplanes are covered in the previously mentioned 19.2%. Adding in airplanes alone, I'm sure the 3 liters will produce a number that is significantly lower than reality.

So with a four stroke engine burning its full displacement every two revolutions, I'll make another ashamedly inaccurate assumption and say that those engines are all running at 2000RPM at an average. Feel free to criticize my assumptions.

Then that gives us 590 million cars burning 3 liters 1000 times a minute. That's 1770000000000 more liters/minute right there.

I'll also assume that this combustion is complete and use it to extrapolate the rest of the burning stuff. Round that 19.2% up to 20% and we can say that burning other stuff 4 times as much oxygen used as cars, bringing the total oxygen used by burning stuff to 8850000000000 liters per minute... passing my previous human breathing estimate by two orders of magnitude!

Using the prior estimated volume of useable atmosphere to recompute, I'm getting:
1149092 minutes. 798 days. 2 years and 68 days. That number is much more satisfyingly scary. I don't think (but I don't know what I'm talking about) that that's enough time for climate change to kill everyone before the CO2 would. Maybe some people on coasts would succumb to rising ocean levels. Assuredly, as meatthinker pointed out, the CO2 would kill you before the not-enough-oxygen would.

I will remind: Inaccuracy times inaccuracy = inaccuracy squared. This estimate is inaccurate to the twelfth or thirtheenth power at least. I might use this number for a piece of fiction, but if it were being published, I'd ask the publisher if one of their staff editors has a degree in math/climatology/chemistry/statistics/physics/whatever for a second opinion.

Yes, I believe they are. However, I just realized that I assumed that all those engines were burning 3 liters 1000 times a minute 24 hours a day.

Of course, that only reduces the burning-stuff estimate a percentage of the automotive 20% since those "other burning stuff" figures were constant over a year... and coal power plants don't stop burning, to my knowledge.

90% of the earths oxygen is produced by phytoplankton in the oceans.

also...[URL=http://www.abc.net.au/news/newsitems/200601/s1545977.htm]Scientists question trees' role in global warming[/URL]

for any of you that are interested, this is an idea that came to me whilst I was doing . . . well, something. i dont really rememeber when the idea came to me, just that it came. anyway, I have no plans on writing this story anytime soon. I plan on it being a cerebral-type science fiction story. look that up if you dont know what it means. I have a bunch of story ideas like this that I have no intention of writing until much later when Ive become a more experienced writer.

anyway, the story goes like this: for some reason unknown to me at this point, every plant and everything that produces oxygen ceases production. soon, everyone on earth will suffocate. very [I]children of men [/I]or [I]blindness[/I]. ([I]blindness[/I] is about a city where one day everybody wakes up blind and no I havent read it.)

thats all I really have. what happens next entirely depends on how long it will take until everybody and everything suffocates.what I do plan on is inserting something mystical. a Tree of Life type myth everyone tells in the story about a great tree somewhere that still produces oxygen.

[QUOTE=polumna;1015603]Yes, I believe they are. However, I just realized that I assumed that all those engines were burning 3 liters 1000 times a minute 24 hours a day.

Of course, that only reduces the burning-stuff estimate a percentage of the automotive 20% since those "other burning stuff" figures were constant over a year... and coal power plants don't stop burning, to my knowledge.[/QUOTE]

Jesus, you are a genius.