Asked by: Jonathan Lodge, Doncaster
In a word, life. The two most abundant gases in the atmosphere, nitrogen and oxygen, are maintained by natural cycles driven by living organisms. For example, oxygen is created by plants via photosynthesis, as they convert carbon dioxide and water into energy using sunlight. Within the last 10,000 years or so, humans have also begun to affect the mix of atmospheric gases, notably boosting levels of the greenhouse gases carbon dioxide and methane.