Carbon Emissions

A greenhouse gas (GHG) is any gas in the atmosphere which absorbs and re-emits heat, and thereby keeps the planet’s atmosphere warmer than it otherwise would be.  Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C , rather than present average of 15 °C. The main GHGs in the Earth’s atmosphere are water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N20) and ozone. 

There are both natural and human sources of carbon dioxide emissions but since the Industrial Revolution humans have produced a 40% increase in the atmospheric concentration of carbon dioxide.

Carbon dioxide is naturally present in the atmosphere as part of the Earth’s carbon cycle and natural sources include decomposition, ocean release and respiration.  Human sources come from activities like manufacturing processes, deforestation and the burning of fossil fuels like coal, oil and natural gas.  Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. Human activities are altering the carbon cycle by adding more CO2 to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO2 from the atmosphere.

Changes in CO2 emissions from fossil fuel combustion are influenced by many long term and short term factors including seasonal temperatures, population growth and economic growth.  The most effective way to reduce CO2 emissions is to reduce fossil fuel consumption. Regulating this carbon pollution through carbon taxes or a cap and trade scheme are ways that governments seek to reduce carbon emissions. 

CO2e, or carbon dioxide equivalent, is a standard unit designed to express the impact of each different greenhouse gas in terms of the amount of CO2 that would create the same amount of warming.  Different greenhouse gases last in the atmosphere for different lengths of time and they absorb different amounts of heat. Standard ratios are used to convert the various gases into equivalent amounts of CO2. These ratios are based on the ‘global warming potential’ (GWP) of a GHG which indicates the amount of warming a gas causes over a given period of time, normally 100 years.  GWP is an index, with CO2, have the index value of 1, and the GWP for all other GHGs is the number of times more warming they cause compared to CO2.  For example, 1kg of methane causes 25 times more warming over a 100 year period compared to 1kg of CO2.  In this way methane has a GWP of 25.  CO2e is a useful term as it allows GHGs to be expressed as a single number and it allows bundles of GHGs to be easily compared in terms of their total global warming impact.