One of the characteristics of the gases that contribute to climate warming is that they are transparent to short-wave radiation from the sun that reaches the Earth, but they are able to absorb some of the heat that is radiated from the surface of the Earth.
Of the current climate-influencing emissions, carbon dioxide accounts for around 76 per cent of the effect, followed by methane, around 16 per cent, then nitrous oxide (6 per cent) and fluorinated gases (2 per cent) (ignoring the climatic influence of ozone).
Total annual anthropogenic greenhouse gas emissions by groups of gases 1970–2010. Source: IPCC AR5 WGIII
Carbon dioxide (CO2)
Carbon dioxide is by far the most important greenhouse gas. Analysis of ice cores from Greenland and Antarctica shows that pre-industrial levels of carbon dioxide in the atmosphere were about 280 ppmv (parts per million by volume). The level in 2016 was about 44 percent higher, 403 ppm. The rate of increase amounts to 1–2 ppm per year.
Global carbon dioxide emissions from fossil fuel combustion, cement production and other industrial processes are the major source of global greenhouse gas emissions. Currently, they account for about 68 per cent of total global greenhouse gas emissions, and were estimated to be 36.2 GtCO2 in 2015. Once released from fossil storage, carbon dioxide remains in the atmosphere for a very long time and can affect the climate long into the future.
Emissions per capita in developed countries are more than double the global average (emissions from deforestation excluded).
Per capita emissions of carbon dioxide per sector in 2008. Source: IPCC AR5 WGIII
Changes in land use (mainly deforestation) also contribute to carbon dioxide emissions. They represent slightly less than a fifth of the total emissions of carbon dioxide. In recent years, there has been a decline in carbon dioxide emissions from land use, largely due to a lower pace of deforestation and increased afforestation.
Methane is formed naturally by the bacterial decomposition of organic matter under oxygen-free conditions. Because of various types of human activity, emissions of methane have roughly doubled. Rice cultivation, cattle breeding, emissions from coal mines and the leakage of fossil gas represent significant anthropogenic sources around the world, as do the treatment of wastewater and organic waste.
The pre-industrial concentration of methane is estimated to have been 0.7 ppm. Today’s level (2011) is more than twice as high, about 1.8 ppm. The life of methane in the atmosphere is relatively short, on the average 10–15 years.
Nitrous oxide (N2O)
Our knowledge of the extent of emissions and the factors that control them is incomplete, but denitrification is the main source of nitrous oxide in the atmosphere. This process, which is carried out by micro-organisms, occurs naturally in the soil. However, the more nitrogen is made available to plants by adding it in the form of fertilizer or through the deposition of airborne nitrogen, the more nitrous oxide is formed.
Another source of nitrous oxide emissions is all sorts of combustion. During the combustion process, small amounts of N2O are formed in addition to the “ordinary” nitrogen oxides (NO and NO2). This amount depends largely on the combustion conditions.
Nitrous oxide is a greenhouse gas whose pre-industrial level is estimated to have been 270 ppb (parts per billion). The level in 2011 was 324 parts per billion, an increase of 20 percent. About a third of the nitrous oxide emitted today are caused by humans.
Nitrous oxide has a long residence time in the atmosphere, an average of about 120 years.
The greenhouse gases described so far occur naturally in the atmosphere. This does not apply to the group of synthetic fluorine compounds, which in many cases are very long-lived and potent greenhouse gases. Their large heating effect, per molecular weight, is due to their ability to absorb radiation in a previous fully transmissive part of the infrared spectrum.
The most familiar substances in this group are the chlorofluorocarbons (CFC gases, known as CFCs), which have mainly attracted attention because of their ability to break down the stratospheric ozone. CFC gases are also powerful greenhouse gases. Measured per molecule, some of them are tens of thousands of times more effective than carbon dioxide. CFC gases are however being phased out globally.
Other substances in this group are so-called f-gases, which have significant greenhouse effects and include:
- HFCs, which are similar to CFCs but do not contain chlorine and therefore do not affect the ozone layer. Used as a replacement for CFCs in many applications. They are not as long-lived in the atmosphere as CFCs and not as powerful in their greenhouse effect.
- Sulphur hexafluoride (SF6), used in the electronics industry, for example.
- PFCs (also called fluorocarbons, FCs) emitted during aluminium production, but also used in the electronics industry.
Since the released amounts of these substances are small, their contribution to the greenhouse effect is so far only a few percent, calculated over a hundred-year period.
However, global emissions are increasing rather sharply, particularly of HFCs, and several of them have effects that last for a very long time – the mean residence time of SF6 in the atmosphere is estimated at 3,200 years.
Among the substances that could have significant impact as greenhouse gases, ozone is the most short-lived. Its retention time in the troposphere is weeks or a month. Ozone in the lower troposphere acts as a greenhouse gas and the level has risen an average of 1–2 percent per year in recent decades. The increase occurred primarily across North America and Europe, so the climate effect in this case is regional.
One special effect in the context of climate change is due to emissions of nitrogen oxides from aviation in the upper troposphere, where most commercial aviation travel takes place. At this height nitrogen oxides cause extensive formation of ozone.
Particles in the atmosphere also affect the radiation balance. Sulphate particles reflect incoming sunlight and hence reduce the amount of solar energy that reaches the Earth’s surface. Sulphate particles originate from sulphur dioxide emissions.
There are also carbon particles (“black carbon”) in the air. These can both absorb heat and reflect incoming light. Their net effect on climate is therefore difficult to assess. Particles of black carbon can both absorb heat and reflect incident light.
Particles also have an effect on the environment by forming condensation nuclei for water vapour in the atmosphere, which can affect cloud formation and precipitation. Unlike greenhouse gases, the residence time of particles in the air is short, about two weeks.
The net effect of particles is difficult to assess and contributes to a high degree of uncertainty, but has been estimated by the IPCC to be somewhere between -0.1°C and -1.9°C.
The relative contributions of different gases
To evaluate the effect of different greenhouse emissions on the climate you need to know the volume of the emission, its ability to absorb heat radiation in different wavelength bands, its lifetime in the atmosphere and possible secondary effects.
As an example, methane has a residence time in the atmosphere of about 10 years, compared with nitrous oxide, which stays for about 150 years.
An example of secondary effect is CFCs, which although they are powerful greenhouse gases are only expected to give a small net contribution to the greenhouse effect. That’s because they break down into another greenhouse gas, namely the ozone found in the stratosphere. These two effects cancel each other out.
In order to compare the contributions of the various greenhouse gases, it is necessary to calculate how much carbon dioxide would be required to achieve the same effect on the earth’s radiation balance; the quantity is GWP (global warming potential), and the unit is a carbon dioxide equivalent.
Since the lifetime of gases in the atmosphere varies, it is also of importance what time span is used for the comparison. Usually a hundred-year time span is assumed, and then the following applies:
With a shorter perspective, gases with short residence time, such as methane, will have a greater relative importance, while the importance of very long-lived gases increases if you take a long-term perspective.
>> Further reading
The greenhouse effect. Chapter 4 in the secretariat's book Air and the Environment, 2004.
Working Group I Report: The Physical Science Basis. Intergovernmental Panel on Climate Change, fourth assessment report, 2007