The atmosphere and climate change

Air today is about four fifths nitrogen and one fifth oxygen, but the early atmosphere was mostly carbon dioxide from volcanoes. Living things changed it over billions of years. Now human activities are adding carbon dioxide and methane, greenhouse gases that trap heat, and burning fuels also releases pollutants such as carbon monoxide and sulfur dioxide.

Key facts

  • Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide, with small amounts of other gases.
  • The early atmosphere came from volcanoes: mostly carbon dioxide, with water vapour and nitrogen. The water vapour condensed to form the oceans.
  • Algae and plants made oxygen by photosynthesis. Carbon dioxide went down as it dissolved in the oceans and was locked up in rocks and fossil fuels.
  • Greenhouse gases (carbon dioxide, methane, water vapour) absorb infrared radiation from the Earth and keep it warm. More of them causes global warming.
  • Human activities add carbon dioxide (burning fossil fuels, cutting down forests) and methane (cattle, rice fields, landfill). Effects include rising sea levels and more extreme weather.
  • Complete combustion of a fuel gives carbon dioxide and water. Incomplete combustion also gives carbon monoxide (toxic) and soot (global dimming, lung damage).
  • Sulfur dioxide (from sulfur in fuels) and oxides of nitrogen (from hot engines) cause acid rain and breathing problems.

Worked examples

Example 1

What is in dry air? How much oxygen is there in 500 cm³ of air?

  1. The tallest bar is nitrogen (78%), then oxygen (21%). Argon and carbon dioxide are tiny.
  2. Oxygen is 21% of the volume. 21 ÷ 100 × 500 = 105 cm³

Answer: about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide; about 105 cm³ of oxygen

Example 2

The carbon dioxide in the air rose from 317 ppm in 1960 to 414 ppm in 2020. Find the rise and the percentage increase.

  1. Subtract. 414 − 317 = 97 ppm
  2. Divide by the starting value and multiply by 100. 97 ÷ 317 × 100 = 30.6%
  3. The rise is getting faster. Most scientists agree it is caused by burning fossil fuels.

Answer: 97 ppm; 30.6%

Example 3

Explain how greenhouse gases keep the Earth warm.

  1. Sunlight (short wavelengths) passes through the atmosphere and warms the Earth's surface.
  2. The warm Earth gives off infrared radiation (longer wavelengths).
  3. Greenhouse gases absorb some of it and re-emit it in all directions, some back to the Earth, so the Earth stays warmer than it would otherwise.

Answer: they absorb infrared radiation given off by the warm Earth and re-emit it, some of it back towards the Earth