Approximately 20%.
The earths atmosphere is made up of 78% nitrogen and 21% oxygen
An oxygen deficient atmosphere has an oxygen concentration less than the normal level of around 21%.
97%
no
Nitrogen is about 79% of the Earth's atmosphere.
Nitrogen is about 79% of the Earth's atmosphere.
Free oxygen
Oxygen gas
Plants
oxygen
The increased oxygen levels in Earth's atmosphere today compared to earlier in its history are due to the evolution and proliferation of photosynthetic organisms like plants and cyanobacteria. These organisms release oxygen as a byproduct of photosynthesis, gradually increasing its concentration in the atmosphere over millions of years.
There is no indication at present that the concentration of oxygen in the atmosphere is declining.