5.9 Chemistry of the atmosphere The atmosphere today The atmosphere has remained the same for about 200 million years or so. Around 80% nitrogen Around 20% oxygen Small amounts of other gases including water vapour. Theories There are many theories about how our atmosphere evolved. The information on this page is just one of them. Theories rely on evidence. The evidence for these theories can be got from: Volcanoes Other planets Rocks Living things Photosynthesis Plants and algae absorb carbon dioxide and lock the carbon away in organic compounds. They also produce oxygen. As oxygen levels built up in the atmosphere, more complex life could evolve. Eventually (200 million years ago) the composition of the atmosphere reached a similar composition to what it is today. A lot of carbon is locked away in fossil fuels and limestone (calcium carbonate). As we burn fossil fuels, the concentration of carbon dioxide in the atmosphere increases. Air pollution Combustion of fuels releases three major pollutants into the air: Carbon particulates: These lead to respiratory problems in people, especially in cites. These are also linked to global dimming (less light reaching earth). Carbon monoxide: The product of incomplete combustion. This binds to haemoglobin in the blood stream and prevents the blood from carrying as much oxygen. Sulphur dioxide and oxides of nitrogen: These can react with cloud water to form sulphuric acid or nitric acid. This causes acid rain. Burning trees and fossil fuels adds carbon dioxide to the atmosphere. This increases the greenhouse effect. The early atmosphere The earths surface was molten. During the first billion years on earth there were frequent volcanic eruptions. Scientists think that the gases released went on to form the early atmosphere and the oceans. This early atmosphere was probably mostly carbon dioxide (CO2) with very little oxygen (O2). The volume of carbon dioxide gradually decreased over time via absorption by the oceans and absorption by plants and algae (see photosynthesis). The carbon in the carbon dioxide was locked up in plants and algae. When they died they sank to the bottom of oceans, became buried and compressed. Over millions of years these formed fossil fuels. As we burn these fossil fuels, we release the carbon that was locked up in the fuel. 5.9 Chemistry of the atmosphere Carbon footprint The volume of greenhouse gases that are released over the life cycle of something. Greenhouse gases Carbon dioxide Methane Water vapour These act like an insulating layer around the planet. This layer lets in short wave radiation from the sun but does not allow the long wavelength radiation (emitted by the planet) to leave the atmosphere. This layer keeps the planet warm enough to support life. This is called the greenhouse effect. Adding to these greenhouse gases is thought to cause global warming. Global warming can lead to climate change. Increase in agriculture means more cattle are farmed. Cattle produce methane. This increases the greenhouse effect.