We know how the atmosphere has changed over the years because evidence has been found in rocks and other sources. But nobody was there millions of years ago to record changes as the happened, so all that we know about how today's atmosphere happened, are only theorys.
The atmosphere is still changing today because of...
- Human activity - burning fossil fuels (releases CO2), deforistation (contributes to increase in CO2 levels) & livestock farming (releases huge ammount of methane)
- Volcanic activity - sulphur dioxide (thrown high into the atmosphere when volcanoes errupt) & carbon dioxide (also put into the atmosphere during erruptions)
Scientists learnt alot about the past atmosphere from Antarctic ice cores. Every year a layer of ice forms and bubbles of air get trapped inside, then are burried by the next layer. The deeper the ice, the older the air. It is very dificult to be precise about how the atmosphere has changed and some of it is down to guess work.
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