Today's dry air is about 78% nitrogen, about 21% oxygen, and small amounts of argon, carbon dioxide (about 0.04%) and other gases. The proportions have been roughly stable for about 200 million years.
The early atmosphere was very different. Scientists think volcanoes released mostly carbon dioxide, with water vapour, nitrogen and small amounts of methane and ammonia, and there was little or no oxygen. As the Earth cooled, water vapour condensed to form the oceans, and some carbon dioxide dissolved in them. Algae and later plants used photosynthesis to remove carbon dioxide and release oxygen. Carbon was also locked away in sedimentary rocks such as limestone and in fossil fuels. Evidence is limited because the time scale is billions of years.
Worked example
Give two ways carbon dioxide in the early atmosphere decreased.
- It dissolved in the newly formed oceans.
- Algae and plants absorbed it by photosynthesis.
Answer:It was locked into limestone and fossil fuels.
Key words
- atmosphere
- the layer of gases around the Earth
- nitrogen
- about 78% of today's air
- photosynthesis
- removes CO₂ and releases O₂
- sedimentary rock
- rock such as limestone that stores carbon
New questions every time.
NGSS-referenced: HS-ESS2-6