Hello. That's an interesting question. Have you learned about how plants and animals live together? You see, plants make oxygen and put it into the air. People and animals use that oxygen to live. We make carbon dioxide and put it into the air (when we breathe out, or exhale). Plants use that to live. So you see, animals and plants are in balance.

If you killed all the plants in the world, the only oxygen available for animals to use is that in the air, or atmosphere (we may be able to get some from the water, but I'll only talk about the air). Anyway, the amount of oxygen in our atmosphere (remember that's a big word for air) is 2,000,000,000,000,000,000 pounds. WOW! That's about the same weight as all the houses in America, Canada, Europe and India put together. That's a lot of oxygen. Remember though, there are a lot of animals on this planet, and all of them need that oxygen to live.

If we killed of all the animals in this world so that only we could use the oxygen, it would last 1,014 years. That is not a long time. Remember too that if we don't kill every animal on the planet the oxygen in the air would only last a few hundred years.

In other words, the plants are very important for us to survive, so we should take care of them.

I don't know but can figure out an estimate. First calculate the volume of air by subtracting the volume of the earth plus the atmosphere from the volume of the earth. I'll say that the atmosphere is only 2-kilometers high (it is actually much higher but the amount of oxygen decreases quickly as you go up--I am assuming a constant concentration to make things easier). So the volume of the atmosphere turns out to be 255,300,000 cubic kilometers (V=4/3pi*r³, r1 = 3186, r2 = 3188). Now lets find out how long it would take 6,400,000,000 people to breathe in all of this air and remove all of the oxygen with each breath (vital capacity; amount of air breathed in and exhaled is 4500cc and lets say 15 breathes per minute per person [Toxicology, 5th Ed.]). This turns out to be about 5.9x10¹¹ minutes or about 1200 years. This number is a high estimate. Can you tell why? A: Left out breathing rates for other animals and oxygen-using decomposition rates for the dead plants. The atmospheric concentration for oxygen is 21% but the minimum required for humans is about 17%, so only a little less than 1/4 of the atmosphere would have to be respirated before the concentration could not support humans. A confounding problem would be global warming due to increased CO₂. How would this affect oxygen and other gasses (including more CO₂) dissolved in the oceans? Is this really an easy problem to solve? The true answer is probably 100 to 400 years.

Oxygen has been forming on planet Earth for a very long time (several billion years). The origins of this 'oxygen pool' stems from microorganisms, which at first were very few in number. With the continual expansion of the 'oxygen pool', competition for resources slowly favored anaerobic microorganisms, which further increased the total amount of oxygen available. Similarly, other species (like plants) have also evolved to use this new means of competition (oxygen production) to exclude others from the limited resources necessary for life. Plus, oxygen is formed in other natural chemical processes. Thus, microbes and plants facilitated the separation of an oxygen atom from some other atoms (like CO₂) to form pure oxygen molecules (O₂). This relates to conservation of mass that a lot of oxygen atoms have been here a long time (since the formation of the planet), but not necessarily as an oxygen molecule.

So, to answer your question: How long would we last if all the plants died tomorrow? A very long time because the present day oxygen pool is soooooo large. This would be on a geological time scale of several thousands of years. Plants are, basically, a non-factor for oxygen formation.

That's a great question. About 20% of our atmosphere is made up of oxygen, but some is always being "used up". It actually becomes part of other molecules like CO₂ and H₂O (what are these molecules?) It is also being "created" (it is actually being released from the other molecules). There are also 'reserves' of oxygen in the ocean. Scientists are not sure what the levels of oxygen were in the distant past, and it's hard to say how long it would take to use up our reserves. It wouldn't matter too much because without plants to make usable energy from sunlight, the entire food web would collapse and all animals would starve. Guess we'd better take care of those plants. Thanks for asking.