Greenhouse gases are substances that contribute to the greenhouse effect. The most significant gases contributing to climate change are water vapour (H2O), carbon dioxide (CO2), methane (CH4), ozone and dinitrogen oxide (N2O), which are also naturally present in nature. In addition to these, many human-made chemicals are powerful greenhouse gases, most importantly chlorofluorocarbons (CFSs and HCFSs), hydrofluorocarbons (HFCs, PFCs and SF6), and bromine compounds (halons, such as CF3Br). The harmfulness of the gases is assessed in terms of their lifecycle and global warming potential. Some gases contribute to the greenhouse effect directly, while others have an indirect effect.
The lifecycle of greenhouse gases in the atmosphere and their global warming potential
Water vapour is a greenhouse gas and the most important natural cause of the greenhouse effect. It increases the Earth’s surface temperature by 21 degrees centigrade. At its natural concentration, carbon dioxide has an effect of seven degrees, while other greenhouse gases account for a total of five degrees.
Global warming potential (GWP) is used for gas comparisons. It is a measure of each gas's warming effect per mass unit over a period of 20 or 100 years, relative to carbon dioxide.
Life cycle (y.)
GWP 20 y.
GWP 100 y.
|Carbon dioxide||50 - 200||1||1|
|HFCs||1,4 -270||437 - 12 000||124 - 14 800|
|PFCs||2 600 - 50 000||5 210 - 8 630||7 390 - 12 200|
|SF6||3 200||16 300||22 800|
|CFCs||45 - 1700||5 310 - 11 000||4 750 - 14 400|
|HCFCs||1,3 - 17,9||273 - 5 490||77 - 2 310|
|Halons||16 - 65||3 680 - 8 480||1 640 - 7 140|
Source: IPCC 2007
Indirect greenhouse gases
Greenhouse gases hold some of the sun's heat in the atmosphere and contribute to greenhouse effect this way. Some gases have an indirect effect:
- nitrogen oxides (NOx), volatile organic compounds (VOCs) and carbon monoxide (CO) react in the atmosphere, forming tropospheric ozone
- carbon monoxide (CO) reacts in the atmosphere, forming carbon dioxide
- volatile organic compounds (VOC) form methane and water vapour
- sulphur dioxide (SO2) reacts in the atmosphere, forming cooling sulphate aerosols
- nitrogen oxides and sulphur dioxide form aerosols that increase cloud formation. This possibly cools down the climate more than it warms it.
Climate warming gases are carbon monoxide, nitrogen oxides (NOx) and volatile organic compounds (VOCs). Sulphur oxide (SO2), on the other hand, cools down the climate.
The global warming potential (GWP) of indirect greenhouse gases relative to carbon dioxide
Life cycle (y.)
GWP 100 y.
|Carbon monoxide (CO)||0,08 - 0,25||40 603|
|Nitrogen oxides(NOx)||0,01 - 0,03||40|
*Non-methane Volatile Organic Compounds.
Source: IPCC 2001, Ministry of the Environment 1997
Methane and other gaseous organic compounds are collectively known as Volatile Organic Compounds, or VOCs. In greenhouse gas assessments, methane is addressed separately, as it is the most significant VOC, and VOCs refer to the other compounds. The abbreviation NMVOC (Non-Methane Volatile Organic Compounds) is also commonly used.
VOCs have a direct effect by absorbing heat radiation from the ground, but their indirect contribution is more significant. They react with hydroxyl radicals (OH-) in the air, forming tropospheric ozone, which is a greenhouse gas. When they reach the stratosphere, they react with OH-, forming water vapour, which is a greenhouse gas. VOCs are also converted to methane.