Most greenhouse gases occur naturally and are what made life on Earth possible thanks to their heat-trapping properties. However, human activity is now increasing greenhouse gas concentrations and tipping the Earth’s climate off balance through rising temperatures and species and habitat loss.
None of us would exist if it wasn’t for the Earth’s natural greenhouse effect. As radiation from the sun reaches the Earth’s atmosphere, some of it is reflected back into space, while some passes through and is absorbed by the Earth’s surface, thereby heating it up. Heat from the Earth bounces back into the atmosphere where it gets trapped by a canopy of gases, which allows the warmth to stay and sustains life as we know it.
However, too much trapped warmth spells trouble. Human activity, primarily the burning of fossil fuels and deforestation, have greatly intensified the natural greenhouse effect, causing global warming and throwing the Earth’s climate off balance. The major enhanced (human-produced) greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6), each having their own specific heat-trapping abilities. Water vapour is also a greenhouse gas, but its presence in the atmosphere is not directly affected by human activity.
In order to compare greenhouse gases, it is helpful to use their Global Warming Potential (GWP), which refers to their warming effect over a set time compared to the same amount (by weight) of carbon dioxide. This takes into account both the warming potential of each molecule of gas and its atmospheric lifetime (how long it survives in the atmosphere). The commonly accepted point of reference is carbon dioxide (with a GWP of 1), because it is the most significant greenhouse gas contributed by human activity.
Despite being a relatively weak greenhouse gas, carbon dioxide has the largest impact on the enhanced greenhouse effect because it has the highest concentration. Primarily produced by the burning of fossil fuels in cars, power plants, and industry, humans have been spewing CO2 into the atmosphere at an accelerated rate since the Industrial Revolution. Emissions increase every year and remain in the atmosphere for at least 100 years. Since plants absorb CO2, their loss through deforestation amplifies the concentration of this greenhouse gas.
Methane is responsible for roughly 20 per cent of the enhanced greenhouse effect. It is emitted during the production and transportation of coal, natural gas, and oil. Grazing livestock (especially the digestive system of cows) release methane, as does decomposing waste and agriculture. As global temperatures increase, permafrost in the northern hemisphere begins to melt, releasing huge quantities of methane. Although methane remains in the atmosphere for only about 10 years and its concentration is lower than that of CO2, it traps 20 to 25 times more energy per molecule in one decade than CO2 does in one century. Its atmospheric concentration has increased globally by about 150 per cent since the 1750s and is currently at a level unsurpassed in the last 400,000 years.
Nitrous oxide is released naturally from oceans and soils. Man-made sources include crop fertilisation and other agricultural procedures, the burning of fossil fuels, and sewage treatment. On a molecular level, it is 300 times more powerful a greenhouse gas than CO2 and survives in the atmosphere for about 100 years.
Other gases adding to the greenhouse effect stem from man-made chemicals used in industry. Found in refrigeration, air conditioning, aerosol sprays, foam blowing agents, solvents, fire fighting agents and aerosol propellants, chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) are not as abundant as the above mentioned gases, but thousands of times more powerful than CO2. As CFCs were phased out after they were found to be ozone depleting, HFC use and production surged after they were actively promoted as replacement refrigerants.
Perfluorocarbons are emitted through aluminum smelting and are used to make semi-conductors and substitute ozone depleting chemicals. Although concentrations are very small, PFCs are 5,000 to 10,000 times more powerful than CO2, with an atmospheric lifetime of up to 50,000 years.
According to the Intergovernmental Panel on Climate Change, the most potent greenhouse gas is sulphur hexafluoride. 23,900 times more powerful than CO2 and with an atmospheric lifetime of 3,200 years, it doesn’t take much to have a potent effect. SF6 is found in car tires, electrical insulation, sports shoes, semiconductor manufacture, and in the magnesium industry.
The largest international effort to slow global warming by curtailing emissions of greenhouse gases, the Kyoto Protocol, was signed in 1997 and came into effect in 2005. It is an international agreement that sets binding targets for 37 industrialised countries and the European community for reducing greenhouse gas emissions. These targets amount to an average of 5 per cent against 1990 levels over the five-year period 2008-2012.
Although many greenhouse gases occur naturally, human activities are clearly increasing the rate at which they are released. One of the major concerns about the future is that climate changes may happen too fast to allow many animal and plant species to respond in time, leading to species extinction.
While strategies for curbing greenhouse gas emissions are dealt with at a government level, we can all help out by reducing our own carbon footprint. Calculate your own carbon footprint or go to the Eco Tips section of our Green Guide to learn about practical, easy-to-embrace steps that you can take at home, work or on the road to cut down your greenhouse gas emissions.
Photo credit: glennia, flickr/Creative Commons