Carbon Dioxide

Carbon dioxide (CO₂) is the most abundant of the greenhouse gases. It is thought to have been in the atmosphere for over 4 billion years and in much larger proportions (up to 80% more) than today. Most of the carbon dioxide was removed from the atmosphere as early organisms began the process of photosynthesis. This locked away carbon dioxide as carbonate minerals, oil shale, coal and petroleum in the Earth's crust when the organisms died. This left approximately 0.04% in the atmosphere today.

The natural carbon dioxide cycle

Atmospheric carbon dioxide comes from a number of natural sources, mainly animal and plant respiration, decay of plants and volcanic eruptions. It is removed from the atmosphere by photosynthesis in plants and by dissolving in water, especially on the surface of oceans.

The amount of carbon dioxide taken out of the atmosphere by plants is almost perfectly balanced with the amount put back into the atmosphere by respiration and decay. Changes to this cycle as a result of human activities can have a large impact on this delicate balance.

Learn more about the Carbon Cycle.

Deforestation

Deforestation

Courtesy of the EPA

Deforestation
Courtesy of the EPA

Deforestation

Deforestation

Courtesy of the EPA

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The impact of human activities

Burning fossil fuels releases the carbon dioxide stored millions of years ago. We use fossil fuels to run vehicles (petrol, diesel and kerosene), heat homes, businesses and power factories. Deforestation releases the carbon stored in trees and also results in less carbon dioxide being removed from the atmosphere, because there are fewer trees to absorb it. This process also degrades soils and releases large amounts of carbon previously locked away in the earth.

Since 1750 the atmospheric concentration of carbon dioxide has increased from 280 ppm (parts per million) to 414.7 ppm in December 2020. The annual growth rate of carbon dioxide during the last ten years alone has been almost 2.2 ppm per year. This is the highest rate recorded since direct measurements began in 1958.

It is estimated that about 70% of the increased atmospheric carbon is from fossil fuel use and that 30% comes from land use changes. Annual emissions are over 36 billion tonnes (Gigatonnes) per year.