Carbon dioxide (CO2) is the most important greenhouse gas produced by human activities, primarily through the combustion of fossil fuels. It accounts for the largest proportion of the 'trace gases' and is currently responsible for 60% of the 'enhanced greenhouse effect'. It is also thought that it's been in the atmosphere for over 4 billion of the Earth's 4.6 billion year geological history and in much larger proportions (up to 80%) than today.
Most of the carbon dioxide was removed from the atmosphere as early organisms evolved photosynthesis. This locked away carbon dioxide as carbonate minerals, oil shale and coal, and petroleum in the Earth's crust when the organisms died. This left 0.03% in the atmosphere today.
 The natural carbon dioxide cycle
- Atmospheric carbon dioxide comes from a number of natural sources, mainly the decay of plants, volcanic eruptions and as a waste product of animal respiration.
- It is removed from the atmosphere by photosynthesis in plants and by dissolving in water, especially on the surface of oceans.
- Carbon dioxide stays in the atmosphere for approximately 100 years.
- 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. Small changes as a result of human activities can have a large impact on this delicate balance.
 The impact of human activities
Burning fossil fuels releases the carbon dioxide stored millions of years ago. Fossil fuels used 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.
Since the Industrial Revolution in the 1700’s, human activities, such as the burning of oil, coal and gas, and deforestation, have increased CO2 concentrations in the atmosphere. In 2005, global atmospheric concentrations of CO2 were 35% higher than they were before the Industrial Revolution.
The best case scenario for the increase in carbon dioxide emissions predicts that the concentration of carbon dioxide in the atmosphere will reach double the level of before the Industrial Revolution, in 2100. The worst case scenario brings this forward to 2045.
 Reducing carbon emission
Promote the use carbon-free or reduced-carbon sources of energy. Carbon-free sources of energy have their own associated impacts, but in general, these technologies generate energy without producing and emitting carbon dioxide to the atmosphere. Carbon-free energy sources include solar power, wind power, geothermal energy, low-head hydropower, hydrokinetics and nuclear power. Alternatively, switching from high-carbon fuels like coal and oil, to reduced-carbon fuels such as natural gas, will also result in reduced carbon dioxide emissions.
Another method of reducing emission is carbon sequestration which involves the capture and storage of carbon dioxide that would otherwise be present in the atmosphere, contributing to the greenhouse effect.