Explain how scientists know the oxygen level in the atmosphere has changed because of plants. Could our atmosphere still be changing?

The chemical reaction by which plants produce complex food molecules (sugars) requires an energy input (i.e., from sunlight) to occur. In this reaction, carbon dioxide and water combine to form carbon-based organic molecules and release oxygen.

Explanation:

so basically plants create oxygen and If there were no plants on the planet we wouldn't be able to breathe.

and yes it could be changing since large quantities of trees are cut down every day in rainforests which produce large amounts of oxygen for us. hope this helps!

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