The primary human source of carbon dioxide (CO2) in the atmosphere is from the burning of fossil fuels for energy production and transport. Changes in land use and deforestation also contribute significantly. Trees, for example, are natural 'carbon sinks' - they absorb carbon dioxide while alive and when they are destroyed, carbon dioxide is released into the atmosphere. Once in the atmosphere, most of the carbon dioxide stays there for 50 to 200 years, and some of it stays there indefinitely.
What are fossil fuels?
Oil, coal and natural gas are called fossil fuels because it is believed they are formed from the remains of plants and animals living millions of years ago. All fossil fuels are made up of hydrocarbons, and release carbon dioxide when burned.
Currently, fossil fuels are the primary source for almost 80 percent of the industrial world's energy. They are a non-renewable resource, so we'll eventually run out of them. However, if we want to avoid dangerous climate change we can only afford to burn less than one-fourth of the known oil, coal and gas reserves – burning any more will almost certainly release enough carbon dioxide to change the climate dramatically.
Who does the most burning?
The simple answer is that because industrialised nations have bigger economies and have been burning fossil fuels for a hundred years or more, they are responsible for most of the cumulative carbon dioxide emissions in the atmosphere. However, all nations are responsible to one degree or another.
This can, and should, change in the future. In some countries, it is changing today. Thanks to renewable energy technology and energy efficiency, economic success and fossil fuel use are no longer synonymous.
However, among the world's top economies, the US still stands out as the number one polluter. With less than 5 percent of the world's population, the US is the world's largest producer of greenhouse gases and is responsible for almost a quarter of global emissions of carbon dioxide.
But to look at carbon dioxide emissions only by country is perhaps too narrow. The same question applies per business or even individual. Someone driving a gas-guzzler of a car is burning more fossil fuels then someone with a more efficient car, for example. Of course nations and businesses must be held accountable, but as individuals we each also make decisions the affect the climate.
Although carbon dioxide is the most significant greenhouse gas in terms of human emissions, we are also adding others to the atmosphere that are even better at trapping heat. The Kyoto Protocol covers emissions of five gases beside carbon dioxide: methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6). In addition, water vapour is also a greenhouse gas, but its presence in the atmosphere is not directly affected by human activity.
Gasses with natural and significant human sources:
Methane (CH4)
Methane is the second biggest contributing greenhouse gas, and is responsible for 20 percent of the enhanced (human caused) greenhouse effect. It is about 23 times more powerful a greenhouse gas than carbon dioxide, and has an atmospheric lifetime of roughly 12 years.
Sources of methane include decomposing organic waste (in nature and in garbage dumps), and the raising of livestock. It's also emitted during the production and transport of coal and natural gas. Although natural sources exist, human activities are significantly contributing to the amount of methane in the atmosphere. Globally, atmospheric concentrations of methane have increased by about 150 percent since 1750, and are now at higher levels than in the last 400,000 years. Once in the atmosphere, methane decays into carbon dioxide over a period of a few years.
Nitrous oxide (N2O) Nitrous oxide is 296 times more powerful a greenhouse gas than carbon dioxide, and remains in the atmosphere for 114 years. It is naturally emitted from oceans and soils, but human driven sources are increasing its atmospheric concentrations. Uses include some agricultural (mostly nitrogen fertilization) and industrial activities, and it is created during combustion of fossil fuels and other organic matter. Nitrous oxide also has a variety of direct uses - including as an aerosol propellant and as an anaesthetic (i.e. "laughing gas").
Artificial gasses with very high global warming potential:
Hydrofluorocarbons (HFCs)
HFCs make up only a small portion of greenhouse gas emissions, but they are extremely potent greenhouse gases. Depending on the exact type of HFC, they are up to 20,000 times more powerful greenhouse gasses than carbon dioxide, and have atmospheric lifetimes of up to 260 years.
Some uses of HFCs are in refrigeration (both commercial and domestic), in air-conditioning (homes, cars, offices etc), and they are also used as foam blowing agents, solvents, fire fighting agents and aerosol propellants. HFC use and production surged after they were actively promoted as replacement refrigerants when a phase out of the ozone depleting chlorofluorocarbons (CFCs) was mandated by the Montreal Protocol. This is despite Greenpeace's successful Greenfreeze project, which proved that more natural and benign alternatives are commercially viable for refrigeration. In fact, safer alternatives exist for almost every use of HFCs - making them a good target for emission reductions.
Perfluorocarbons (PFCs)
PFCs are from 5,700 to 10,000 times more powerful greenhouse gasses (depending on the exact type) than carbon dioxide, and have an atmospheric lifetime of up to 50,000 years. PFCs are by-products of aluminium smelting. They are also used in semi-conductor manufacture, and as substitutes for ozone depleting chemicals. Emissions of PFCs are small even compared to HFCs. However, given their potency, long lifetimes and availability of alternatives already on the market, PFCs should be urgently phased out.
Sulphur Hexafluoride (SF6)
Sulphur Hexafluoride is the most potent greenhouse gas evaluated by the Intergovernmental Panel on Climate Change. It is 23,900 times more powerful a greenhouse gas than carbon dioxide, and has an atmospheric lifetime of 3,200 years. It has a number of uses including in Nike Air shoes, car tyres, for electrical insulation, semiconductor manufacture, and in the magnesium industry.
Like PFCs, the effects of Sulphur Hexafluoride to date are fairly small. However, since it is a very persistent and potent greenhouse gas, there is concern about its continuing build up in the atmosphere. Given its potency, long lifetime and availability of alternatives already on the market, Sulphur Hexafluoride should be urgently phased out.
The European Union is currently designing legislation to control emissions of these gases. For more information see the Greenpeace's EU unit website.
Water and ozone:
Ozone (O3)
Ozone occurs both naturally, and from human activities. It is present both in the upper atmosphere, where it forms the ozone layer shielding us from harmful levels of ultraviolet solar radiation, and in the lower atmosphere, where it is the main component of smog.
Some people confuse the issue of ozone depletion with climate change. In reality, they are separate but related. The man made chemicals that destroy the ozone layer are greenhouse gases, as are some of the chemicals that are replacing them. Also, as the Earth's lower atmosphere warms and traps more heat, the upper atmosphere (where the ozone layer is) becomes colder, which facilitates the chemical reactions that damage the ozone layer.
Water vapour (H2O)
Water vapour is the most abundant greenhouse gas. The direct effect of human activity on global water vapour concentrations is thought to be negligible. However, water vapour is important for climate change because of an important feedback effect. Warmer air can hold more moisture, enhancing climate change. The exact size of this important feedback remains to be determined by scientists.
Editor's note: Gases are commonly compared to one another according to 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. Comparing GWPs is useful because it takes into account both the warming potential of each molecule of a gas, and its atmospheric lifetime (how long it stays in the air). Carbon dioxide is the commonly accepted point of reference (with a GWP of 1) because it is the most significant greenhouse gas from human activities.
For simplicity, this page refers to the warming potential of each gas relative to carbon dioxide over a 100-year period. This is the same as its GWP with a hundred year benchmark. Thus, a kilogram of carbon dioxide emission has a GWP of one, while a kilogram of nitrous oxide has a GWP of 310 - which we have expressed here as "nitrous oxide is 310 times more powerful a greenhouse gas than carbon dioxide".
However, it is worth noting that since some gases will stay in the atmosphere much longer than 100 years, their total greenhouse effect over time is actually greater than expressed here.
Atmospheric lifetime = How long the gas stays in the atmosphere.
What are fossil fuels?
Oil, coal and natural gas are called fossil fuels because it is believed they are formed from the remains of plants and animals living millions of years ago. All fossil fuels are made up of hydrocarbons, and release carbon dioxide when burned.
Currently, fossil fuels are the primary source for almost 80 percent of the industrial world's energy. They are a non-renewable resource, so we'll eventually run out of them. However, if we want to avoid dangerous climate change we can only afford to burn less than one-fourth of the known oil, coal and gas reserves – burning any more will almost certainly release enough carbon dioxide to change the climate dramatically.
Who does the most burning?
The simple answer is that because industrialised nations have bigger economies and have been burning fossil fuels for a hundred years or more, they are responsible for most of the cumulative carbon dioxide emissions in the atmosphere. However, all nations are responsible to one degree or another.
This can, and should, change in the future. In some countries, it is changing today. Thanks to renewable energy technology and energy efficiency, economic success and fossil fuel use are no longer synonymous.
However, among the world's top economies, the US still stands out as the number one polluter. With less than 5 percent of the world's population, the US is the world's largest producer of greenhouse gases and is responsible for almost a quarter of global emissions of carbon dioxide.
But to look at carbon dioxide emissions only by country is perhaps too narrow. The same question applies per business or even individual. Someone driving a gas-guzzler of a car is burning more fossil fuels then someone with a more efficient car, for example. Of course nations and businesses must be held accountable, but as individuals we each also make decisions the affect the climate.
Although carbon dioxide is the most significant greenhouse gas in terms of human emissions, we are also adding others to the atmosphere that are even better at trapping heat. The Kyoto Protocol covers emissions of five gases beside carbon dioxide: methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6). In addition, water vapour is also a greenhouse gas, but its presence in the atmosphere is not directly affected by human activity.
Gasses with natural and significant human sources:
Methane (CH4)
Methane is the second biggest contributing greenhouse gas, and is responsible for 20 percent of the enhanced (human caused) greenhouse effect. It is about 23 times more powerful a greenhouse gas than carbon dioxide, and has an atmospheric lifetime of roughly 12 years.
Sources of methane include decomposing organic waste (in nature and in garbage dumps), and the raising of livestock. It's also emitted during the production and transport of coal and natural gas. Although natural sources exist, human activities are significantly contributing to the amount of methane in the atmosphere. Globally, atmospheric concentrations of methane have increased by about 150 percent since 1750, and are now at higher levels than in the last 400,000 years. Once in the atmosphere, methane decays into carbon dioxide over a period of a few years.
Nitrous oxide (N2O) Nitrous oxide is 296 times more powerful a greenhouse gas than carbon dioxide, and remains in the atmosphere for 114 years. It is naturally emitted from oceans and soils, but human driven sources are increasing its atmospheric concentrations. Uses include some agricultural (mostly nitrogen fertilization) and industrial activities, and it is created during combustion of fossil fuels and other organic matter. Nitrous oxide also has a variety of direct uses - including as an aerosol propellant and as an anaesthetic (i.e. "laughing gas").
Artificial gasses with very high global warming potential:
Hydrofluorocarbons (HFCs)
HFCs make up only a small portion of greenhouse gas emissions, but they are extremely potent greenhouse gases. Depending on the exact type of HFC, they are up to 20,000 times more powerful greenhouse gasses than carbon dioxide, and have atmospheric lifetimes of up to 260 years.
Some uses of HFCs are in refrigeration (both commercial and domestic), in air-conditioning (homes, cars, offices etc), and they are also used as foam blowing agents, solvents, fire fighting agents and aerosol propellants. HFC use and production surged after they were actively promoted as replacement refrigerants when a phase out of the ozone depleting chlorofluorocarbons (CFCs) was mandated by the Montreal Protocol. This is despite Greenpeace's successful Greenfreeze project, which proved that more natural and benign alternatives are commercially viable for refrigeration. In fact, safer alternatives exist for almost every use of HFCs - making them a good target for emission reductions.
Perfluorocarbons (PFCs)
PFCs are from 5,700 to 10,000 times more powerful greenhouse gasses (depending on the exact type) than carbon dioxide, and have an atmospheric lifetime of up to 50,000 years. PFCs are by-products of aluminium smelting. They are also used in semi-conductor manufacture, and as substitutes for ozone depleting chemicals. Emissions of PFCs are small even compared to HFCs. However, given their potency, long lifetimes and availability of alternatives already on the market, PFCs should be urgently phased out.
Sulphur Hexafluoride (SF6)
Sulphur Hexafluoride is the most potent greenhouse gas evaluated by the Intergovernmental Panel on Climate Change. It is 23,900 times more powerful a greenhouse gas than carbon dioxide, and has an atmospheric lifetime of 3,200 years. It has a number of uses including in Nike Air shoes, car tyres, for electrical insulation, semiconductor manufacture, and in the magnesium industry.
Like PFCs, the effects of Sulphur Hexafluoride to date are fairly small. However, since it is a very persistent and potent greenhouse gas, there is concern about its continuing build up in the atmosphere. Given its potency, long lifetime and availability of alternatives already on the market, Sulphur Hexafluoride should be urgently phased out.
The European Union is currently designing legislation to control emissions of these gases. For more information see the Greenpeace's EU unit website.
Water and ozone:
Ozone (O3)
Ozone occurs both naturally, and from human activities. It is present both in the upper atmosphere, where it forms the ozone layer shielding us from harmful levels of ultraviolet solar radiation, and in the lower atmosphere, where it is the main component of smog.
Some people confuse the issue of ozone depletion with climate change. In reality, they are separate but related. The man made chemicals that destroy the ozone layer are greenhouse gases, as are some of the chemicals that are replacing them. Also, as the Earth's lower atmosphere warms and traps more heat, the upper atmosphere (where the ozone layer is) becomes colder, which facilitates the chemical reactions that damage the ozone layer.
Water vapour (H2O)
Water vapour is the most abundant greenhouse gas. The direct effect of human activity on global water vapour concentrations is thought to be negligible. However, water vapour is important for climate change because of an important feedback effect. Warmer air can hold more moisture, enhancing climate change. The exact size of this important feedback remains to be determined by scientists.
Editor's note: Gases are commonly compared to one another according to 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. Comparing GWPs is useful because it takes into account both the warming potential of each molecule of a gas, and its atmospheric lifetime (how long it stays in the air). Carbon dioxide is the commonly accepted point of reference (with a GWP of 1) because it is the most significant greenhouse gas from human activities.
For simplicity, this page refers to the warming potential of each gas relative to carbon dioxide over a 100-year period. This is the same as its GWP with a hundred year benchmark. Thus, a kilogram of carbon dioxide emission has a GWP of one, while a kilogram of nitrous oxide has a GWP of 310 - which we have expressed here as "nitrous oxide is 310 times more powerful a greenhouse gas than carbon dioxide".
However, it is worth noting that since some gases will stay in the atmosphere much longer than 100 years, their total greenhouse effect over time is actually greater than expressed here.
Atmospheric lifetime = How long the gas stays in the atmosphere.
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