proportion of atmospheric gases

• What percentage of the Earth's atmosphere is oxygen?

  21%
• What percentage of the Earth's atmosphere is nitrogen?

  78%
• What types of gases are present in small quantities in the Earth's atmosphere?

  Noble gases, carbon dioxide, and water vapour
• What has been the trend in the proportions of gases in the Earth's atmosphere over the past 200 million years?

  • The proportions have stayed more or less the same.
  • No significant changes in gas composition.