During the day plants photosynthesize and give out oxygen through stomata. But during the night, when half of the planet is dark, half of the earth's trees/plants consumes oxygen to survive. At that time when an entire hemisphere is producing only $\ce{CO_2}$ and there is no source of $\ce{O_2}$, why don't half of the life forms die? And 12 hours later, the other half when they are in darkness?

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    $\begingroup$ Your assumption that plants are able to consume a significant percentage of atmospheric oxygen during night is flawed. You are ignoring a massive reservoir of oxygen in our atmosphere and the planetary general circulation that moves air around the globe. You might also ask yourself how the respiration rate of plants at night compares to that of humans and animals. How do people, under your assumption, survive the polar night, which lasts far longer than 12 hours? $\endgroup$
    – casey
    Commented Oct 21, 2015 at 18:39

1 Answer 1


The rates at which plants consume CO$_2$ and animals consume O$_2$ is minuscule compared to the vast amounts of CO$_2$ and O$_2$ in the atmosphere. The amount of change overnight is not even measurable, unless the plants are in an enclosed structure.

Interestingly, there is a variation in CO$_2$ concentraion over the course of a year as the Northern Hemisphere (where most of the land vegetation is) goes through its seasons. Here is a plot of CO$_2$ over several decades. It shows an annual cycle superimposed on the general upward trend from the accumulation of man-made CO$_2$.

Full Mauna Loa CO2 record

The red trace is the actual data, and the black trace is the time averaged fit to the data.

  • $\begingroup$ This answer would be better with some (referenced) numbers to support the first paragraph. $\endgroup$
    – Matt Hall
    Commented Oct 29, 2015 at 15:48
  • $\begingroup$ @kwinkunks I agree, but I will not have the time for the next week. Feel free to edit. $\endgroup$
    – Eubie Drew
    Commented Oct 29, 2015 at 17:57

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