Cascades to Coast

How long has air been on the Earth?

Air, similar to what we know today, has probably been on Earth for only about 1.5 billion years. Oxygen did not appear in the Earth's atmosphere till about 2 billion years ago, and it took at least another 500 million years for the amount of oxygen in the air to reach the amount we see in the atmosphere today. Scientists think that the atmosphere we see today is probably the third atmosphere the Earth has had. The first atmosphere (about 4.5 billion years ago) was likely mostly hydrogen, and Earth lost it all in the heavy solar winds of that time. The Earth's second atmosphere was formed about 4.4 billion years ago by the release of gases by volcanoes and meteor strikes. It was mainly made up of CO2, N2, water vapor, ammonia and methane, and no oxygen. The second atmosphere was transformed into our current atmosphere by the action of cyano-bacteria. Our current atmosphere still has a lot of nitrogen (78%), very little carbon dioxide (0.04%) and quite a bit of oxygen (21%).