Climate Number: 21 Percent
Without a steady supply of oxygen in the air around us, we would suffocate. Compared to other planets in our solar system, Earth’s concentration of oxygen in the atmosphere is exceptional. Of the air that goes into your lungs, 78 percent is nitrogen and 21 percent is oxygen. The remaining one percent is a combination of trace gases such as carbon dioxide, water vapor and stable “noble” gases like argon and helium. While lots of oxygen is critical to the survival of complex animal life, including human beings, Earth’s ancient atmosphere was by most estimates almost completely void of oxygen. Instead, the atmosphere was composed almost entirely of carbon dioxide and methane until about 2.4 billion years ago. At this time, bacteria that convert atmospheric carbon into sugars, emitting oxygen as a waste product (called photosynthetic organisms), became a dominant life form. Back then, photosynthetic organisms were the exception and the oxygen they emitted would have been considered a pollutant to other life forms living on Earth. But the photosynthetic organisms reproduced successfully and quickly (in geologic terms) built up the atmosphere we know today. By about 450 million years ago, oxygen concentrations were high enough – about 12.6 percent of the atmosphere – to allow fire to burn through Earth’s early forests. Fire has been part of Earth’s ecosystems ever since.
For Comparison: Venus’ atmosphere has almost no elemental oxygen and is about 96.5 percent carbon dioxide and 3.5 percent nitrogen. The Martian atmosphere is about 95.3 percent carbon dioxide, 2.7 percent nitrogen, 1.6 percent argon and has a trace amount of oxygen (0.13 percent).
Seasons: Winter, Spring, Summer, Fall
Source: Kump, LR. “The rise of atmospheric oxygen.” Nature 451 (2008): 277-278.
