How Has The Atmosphere Changed

Learn the basics about the changes to our atmosphere, as a part of environmental chemistry. The Earth’s early atmosphere had no oxygen. By examining rocks laid down over earth history we can piece together the story of our atmosphere, to the present day where mankind has polluted the atmosphere. 4.5 billion years ago the atmosphere probably was made mostly of nitrogen, with some carbon dioxide, water vapour and hydrogen compounds such as methane and ammonia. Within 800 million years life evolved in the oceans. 3.2 billion years ago photosynthesis had evolved with the first oxygen producing cyanobacteria, similar to today’s blue-green algae. At first the oxygen given off simply reacted with the rocks, but gradually it built up in the oceans and started to bubble out into the atmosphere. To start with it was a toxic gas harmful to life, but soon life evolved to make use of the oxygen through aerobic respiration. By 2 billion years ago, about half-way through earth’s history, there was oxygen in the atmosphere. Without oxygen there was no ozone in the upper atmosphere, so UV light from the sun rained down on the land making it impossible for life to exist except under water where the UV could not reach. Moving to the land was difficult for plants which relied on the water to support them, and it was not until about 400 million years ago, that life learned to live on land. The atmosphere remained much the same until the present day: mostly nitrogen, with 21% oxygen and just enough carbon dioxide to give plants their carbon during photosynthesis. About 100 years ago mankind started to change the atmosphere. We found seams of coal and pockets of oil and natural gas under ground and began extracting them and reacting them with the oxygen in the atmosphere, increasing the carbon dioxide concentration.