The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today. We are, however, causing our atmosphere to change by human activity. Burning fossil fuels and deforestation are two examples of human effect on the environment.
Composition of the Earth's atmosphere
Air is made up of nitrogen (78%), oxygen (21%) and other gases (1%)
The composition of air
You need to know the proportions of the main gases in the atmosphere.
The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.
It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to respire, and fuels to burn - is the next most abundant gas. These two gases are both elements and account for about 99% of the gases in the atmosphere.
The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.
Evolution of the atmosphere
The early atmosphere:
Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.
The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.
It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.
Scientists can’t be sure about the early atmosphere and can only draw evidence from other sources: for example, volcanoes on other planets release high quantities of carbon dioxide or nitrogen and iron-based compounds which are present in very old rocks that could have formed only if there was little or no oxygen.
Changes in the atmosphere
So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?
The proportion of oxygen went up because of photosynthesis by plants.
The proportion of carbon dioxide went down because: it was locked up in sedimentary rocks such as limestone, and in fossil fuels it was absorbed by plants for photosynthesis it dissolved in the oceans.
The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing.
Changes to the atmosphere
The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis, respiration and combustion.
Green plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms - including all plants and animals - release energy from their food using respiration. Respiration and combustion both release carbon dioxide into the atmosphere.
Carbon dioxide is produced by burning fossil fuels. Increased energy consumption is leading to a rise in the use of fossil fuels, which in turn increases the release of carbon dioxide into the atmosphere.
The rising human population is adding to atmospheric carbon dioxide in other ways too. When land is cleared for timber and farms (deforestation), there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. This is particularly important when forests are cleared to make way for farms: not only are there then fewer trees to absorb carbon dioxide, but the burning of the trees releases carbon dioxide.
Rocks are classified (organised) into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. This classification is based on how they were