Almost all of the Earth's atmosphere (99%) is made up of nitrogen (about 78%) and oxygen (about 21%). While both of these gases play important roles in the vast number of processes that support life on Earth, they play almost no direct role in regulating the climate.

This is carried out by some of the trace gases (the solcalled 'greenhouse gases') in the remaining 1% of the atmosphere which occur in relatively small amounts:an excess of greenhouse gases can raise the temperature of a planet to lethal levels, as on Venus where the 96.5% carbon dioxide (CO2) atmosphere results in surface temperatures of about 467 °C (872 °F). 

Apart from carbon dioxide, the other important greenhouse gases are methane, nitrous ocide, ozone, water vapour, and halocarbons. Although the proportion of the trace gases in the atmosphere appears relatively small, they can still have a big impact on climate change - and they are mainly caused by human activities.