Carbon Dioxide Levels & the Greenhouse Effect (HL IB Chemistry)

Carbon dioxide levels 

• The amount of carbon dioxide (and carbon) in the atmosphere is increasing, mainly due to human activities such as:
  • Combustion, including 
    • Electricity generation - estimated at around 85% of all fossil fuel consumption
    • Transportation
  • Construction
  • Deforestation / conversion of land from forestry to farming
  • Increased livestock farming 
    • Livestock, such as cattle, release methane into the atmosphere
• Both carbon dioxide and methane gases contribute to atmospheric carbon levels and influence global temperatures
• The main reason for increasing carbon dioxide levels is the combustion of fossil fuels which releases carbon that has been stored for millions of years
  • Increased use of fossil fuels is contributing to an increase in the carbon dioxide content of the atmosphere

• Scientists from research stations, such as the Mauna Loa Observatory, have been taking quantitative measurements of atmospheric carbon dioxide (and methane) concentrations for many years
  • Scientists have records for carbon dioxide levels dating back to 1958 and for methane levels from 1984

Graph of carbon dioxide levels over time

There has been a general increase in the volume of carbon dioxide over the years (blue line) with annual fluctuations (red line) due to seasonal changes in photosynthesis rates

• The amount of carbon dioxide is constantly changing due to seasonal fluctuations in rates of photosynthesis 
  • Photosynthesis removes carbon dioxide from the atmosphere

What are greenhouse gases?

• A greenhouse gas is a gas that absorbs radiation emitted from the Earth's surface, trapping it in the atmosphere so that it is not lost to space
  • Greenhouse gases in the atmosphere have a similar effect to the glass in a greenhouse, hence the term greenhouse gas and their effect is known as the greenhouse effect
• The greenhouse effect is important to ensure that Earth is warm enough for life
  • Without the insulating effects of greenhouse gases, Earth would see similar temperature fluctuations to planets such as Mars, where temperatures range from 20°C to −153°C
• There are many greenhouse gases, including:
  • Carbon dioxide - produced when living organisms respire and when fuels are burned
  • Methane - produced by livestock and landfill sites as well as being released during mining 
  • Nitrous oxides - commonly released during the combustion of impurities in fossil fuels
  • Water vapour - produced during combustion as well as by evaporation of the oceans and lakes as part of the water cycle

• The major focus of greenhouse gases and the greenhouse effect is the action of atmospheric carbon dioxide
  • Sources of atmospheric carbon dioxide include:
    • Combustion of wood and fossil fuels
    • Respiration of plants and animals
    • Thermal decomposition of carbonate rocks
    • The effect of acids on carbonates

What is the greenhouse effect?

• When shortwave radiation from the sun strikes the Earth’s surface it is absorbed and re-emitted from the surface of the Earth as infrared radiation
• The infrared radiation passes through the atmosphere where some thermal energy passes straight through and is emitted into space
• But some infrared radiation is absorbed by greenhouse gases and re-emitted in all directions
• This reduces the thermal energy lost into space and traps it within the Earth’s atmosphere, keeping the Earth warm
• This process is known as the greenhouse effect
• As the concentration of greenhouse gases in the atmosphere increases due to human activity, more thermal energy is trapped within the Earth's atmosphere causing the Earth’s average temperature to rise (global warming)
• This process is called the enhanced greenhouse effect

Greenhouse effect diagram

Greenhouse gases absorb the infrared radiation emitted by Earth, warming the atmosphere