What is the oxygen content in Earth's atmosphere?
Earth's atmosphere contains approximately 21% oxygen (O₂) by volume, making it the second most abundant atmospheric gas after nitrogen (78%). This oxygen concentration is maintained through a delicate balance between oxygen-producing processes (primarily photosynthesis by plants, algae, and cyanobacteria) and oxygen-consuming processes (respiration, combustion, and oxidation reactions). The current oxygen level represents billions of years of biological evolution; early Earth had virtually no free oxygen until photosynthetic organisms began releasing it as a byproduct. This oxygen-rich atmosphere enables aerobic respiration, which is far more energy-efficient than anaerobic processes, allowing complex multicellular life to evolve and thrive.
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