How important are greenhouse gases

The age of industry

Greenhouse gases and other emissions released by humans are the most important cause of current >> climate change. On this page you will find more information about what greenhouse gases actually are, their importance for climate change and other emissions that affect the climate.

What are greenhouse gases actually?

Greenhouse gases are gases in the earth's atmosphere that change the earth's >> radiation budget. Like every warm body, the earth gives off heat radiation to its surroundings. The heat radiation of an ideal "black body" (that's what physicists call bodies that can radiate their heat unhindered) depends on its temperature and is described by Planck's law of radiation. The heat radiation of most real bodies differs more or less clearly from this ideal, since their heat radiation is hindered. This is also the case with the earth, as the following figure shows:

Satellite measurements of the earth's thermal radiation
compared to the theoretical radiation of an ideal
"black body". The measurement was made in 1970 over the Sahara
with the IRIS (Infrared Interferometer Spectrometer) NASA
carried out. Illustration © Makiko Sato for James Hansen,
Storms of my Grandchildren, with permission.
Own translation. Original >>

In the case of Earth, it is gases in the atmosphere that hinder heat radiation (how this was discovered is >> here). Gases absorb radiation in very specific wavelengths by which they can be clearly recognized - "absorption spectra" are therefore also used in environmental analysis to identify gases or to study the atmosphere of distant planets. The gases that hinder the earth's heat radiation must therefore also be clearly identified: the absorption at the wavelength of 15 μm is caused, for example, by carbon dioxide. When heat radiation is absorbed, the gases themselves heat up and radiate heat - but in all directions, not just into space, but also back to earth. As a result, the earth warms up more than would actually be expected based on the solar radiation (>> more).

Since the gases in question do not hinder the short-wave solar radiation, but only the heat radiation, they act in the atmosphere like the panes of a greenhouse: hence the name greenhouse gases.

The contribution of greenhouse gases to climate change

The warming of the earth by greenhouse gases is a natural process, without which the earth would be around 33 degrees Celsius colder (>> climate). But when the concentration of greenhouse gases increases due to human activities, the temperature of the earth also increases - and then we speak of human-made climate change. It has now been proven that human activities increase the concentration of greenhouse gases (see also the >> Example of carbon dioxide); the following figure shows the contribution of the individual greenhouse gases:

Share of the individual greenhouse gases in the total man-made
Greenhouse gas emissions in 2004 in carbon dioxide equivalent - that is,
the picture shows the Contribution of greenhouse gases to climate change. Illustration
from the >> 4th UN Climate Report 2007, synthesis volume, own translation.

Carbon dioxide (CO2)

Carbon dioxide was recognized as a greenhouse gas by the Irish physicist John Tyndall as early as the middle of the 19th century (>> more); it is the cause of more than three quarters of man-made warming, making it by far the most important “artificial” greenhouse gas today. A doubling of the content of carbon dioxide in the atmosphere could increase the temperature of the earth between 2 and 4.5 ° C (>> more). By energy-related emissions - The combustion of fossil fuels (coal, oil, gas) to generate electricity, in industry, in domestic heating systems and in road traffic is well over 30 billion tons worldwide (2011: 34.7 billion tons [50]) this greenhouse gas is released.

This was added in 2011 3.3 billion tons from the Burning down forests, especially the tropical rainforests of the Amazon and Southeast Asia. Almost half of the total emissions of 38 billion tons are absorbed by natural carbon sinks, especially the oceans, but also by terrestrial ecosystems. On average between 2002 and 2011, the world's oceans absorbed 26 percent (that would be the same in 2011 almost 10 billion tons) and terrestrial ecosystems (growing forests and peat bogs) 28 percent (this would correspond to 10.6 billion tons in 2011; or net - after deducting emissions from burning forests - 7.3 billion tons). However, this value can fluctuate significantly from year to year, depending on the condition of the vegetation.

Contribution of the relevant sources to the increase in
Carbon dioxide concentration in the atmosphere (51).
(One ton of carbon is equivalent to 3.667 tons
Carbon dioxide; Figure from IPCC report 2007
[Working group 1, p. 513], own translation).

This leaves an average of 17.5 billion tons in the atmosphere each year, causing its carbon dioxide content to rise by 2 ppm per year. The concentration of carbon dioxide in the atmosphere rose from pre-industrial 280 ppm to the current 392 ppm. (The pre-industrial values ​​are known from ice cores (>> more); there have been continuous measurements on the Mauna Loa in Hawaii since 1958, which document the increase since then; they are shown in the figure >> here.) Once it got into the atmosphere Carbon dioxide stays there for a long time: after 1,000 years, half of it will still be in the air. Because of this length of stay, the importance of carbon dioxide for climate change will increase in the long term.

In Germany, carbon dioxide already accounts for 85 percent of all greenhouse gases; the energy-related emissions amount to approx. 865 million tons / year.

>> moreon the carbon cycle & climate change

Where does the carbon dioxide come from?

The sources of the increasing carbon dioxide in the earth's atmosphere could be determined with different methods, the isotope analysis is particularly meaningful: Carbon occurs in two common isotopes: 12C (about 99 percent) and 13C (about 1 percent). However, fossil fuels have a lower ratio 13C /12C as carbon dioxide in the atmosphere, and also differ from each other: this “signature” shows carbon dioxide from fossil fuels and the decreasing ratio 13C /12C shows the share of fossil fuels in the increasing carbon dioxide content overall (there are also other causes for a decreasing 13C /12C-ratio, which can be differentiated; For details and references, see IPCC Report 2007, Working Group 1, page 139).

Representation of global carbon emissions (black) and the
Change in the 13C / 12C isotope ratio (red). The scale
of the isotope ratio is shown reversed, so that
the rising curve shows a falling 13C / 12C isotope ratio
means. The curve comes from the Mauna Loa measuring station. Fig .:
Climate Change 2007: The Physical Science Basis. Contribution of
Working Group 1, page 138.

Does breathing have to be forbidden?

There are people who deny climate change, and one of the arguments they use to ridicule concerns about global warming and, above all, the role of carbon dioxide in it, is that there are more than seven billion people on earth every day Exhale two billion tonnes of carbon dioxide - so before you concern yourself with industrial emissions, you should first forbid breathing.

The mistake of reasoning (at least for those who do not use the argument against their better knowledge) lies in the fact that the exhaled carbon dioxide comes from the breakdown of organic carbon built up by means of photosynthesis, which is part of the short-term >> carbon cycle of the earth. The exhaled carbon dioxide does not change the concentration in the earth's atmosphere. When fossil fuels are burned, however, carbon is released that has been withdrawn from this cycle since geological times and is therefore reintroduced into the cycle - and therefore leads to an increase in the concentration in the atmosphere.

Comparative measure of global warming potential

In order to be able to compare the other greenhouse gases with carbon dioxide, you will Global warming potential (after the English Global warming potential often abbreviated as GWP) used for conversion: The global warming potential indicates how strong the effect of a substance is compared to carbon dioxide. The global warming potential of methane, for example, is 21, so one tonne of methane has a greenhouse effect like 21 tons of carbon dioxide. After carbon dioxide, the two most important greenhouse gases are methane and nitrous oxide (laughing gas).

This global warming potential can also be used to indicate the concentrations of all greenhouse gases in the atmosphere; this value becomes Carbon dioxide equivalent concentration and Co2abbreviated e. This concentration is currently around 445 ppm (compared to 392 ppm CO2).

Other greenhouse gases


Methane (CH4) contributes worldwide with a global warming potential of 21 almost 15 percent contributes to the greenhouse effect (the proportion of German emissions is a good 7 percent). Methane is always produced when organic material is broken down under the discharge of oxygen; this mainly happens in the stomachs of ruminants (cattle and sheep), when growing wet rice and in landfills. In some Latin American countries, methane is the most important greenhouse gas from agriculture due to cattle farming; the methane concentration in the atmosphere has increased by 151 percent since the beginning of industrialization.

Increase in methane in the atmosphere since 1979.
Source of the figure: Synthesis Report Climate Change: Global Risks,
Challenges & Decisions. Copenhagen 2009, March 10-12.

Methane reacts in the earth's atmosphere with hydroxyl radicals (“radicals” in chemistry are particularly reactive atoms or molecules with unpaired electrons) and is thereby broken down into carbon dioxide and water vapor over a period of 10 to 12 years. The resulting carbon dioxide has a lower global warming potential than methane. However, the hydroxyl radicals are also involved in the formation of sulfate and other aerosols in the atmosphere, and when the radicals react with methane instead of other air pollutants, the concentration of cooling (>> here) aerosols drops: If this effect is included one, the share of methane in the increase in the greenhouse effect could be higher than previously assumed (60).

Nitrous oxide

Nitrous oxide (laughing gas, N.20) occurs in the soil when mineral nitrogen fertilizers are broken down. It is the most important greenhouse gas released by agriculture worldwide. The global warming potential of nitrous oxide is 310; its share in the greenhouse effect is around eight percent, and its concentration in the atmosphere has increased by 17 percent since the beginning of industrialization.

Increase in the nitrous oxide content in the atmosphere since 1978.
Source of the figure: Synthesis Report Climate Change: Global Risks,
Challenges & Decisions. Copenhagen 2009, March 10-12.


CFC (Chlorofluorocarbons) were mainly used as propellants and refrigerants, as they contribute to the destruction of the ozone layer, their use has been greatly reduced since 1990 (>> here); the fluorocarbons used as substitutes (HFC) do not damage the ozone layer - but are also greenhouse gases.

The Kyoto Protocol also includes those created in the aluminum industry perfused hydrocarbons (PFC) and that used as an insulating gas in high-voltage switches Sulfur hexafluoride (SF6) considered.

And what about the water vapor?

Water vapor is the most important natural greenhouse gas (>> here). Since the water content in the air depends on the temperature, the content of water vapor in the atmosphere increases with rising temperatures and thus intensifies the effects of other greenhouse gases. The greenhouse effect of carbon dioxide in absolutely dry air, for example, would only be about half as great as it actually is, i.e. it is doubled by the water vapor. This effect is already taken into account when considering the global warming potential of carbon dioxide and the other greenhouse gases. Apart from that, the content of water vapor is only changed regionally by human activities - for example by deforestation of rainforests or the introduction of irrigation. However, these interventions do not have any significant global effects on the water balance in the atmosphere (61); and therefore water vapor is not considered separately in the discussion of global warming.

Other emissions that change the climate

In addition to greenhouse gases, other air pollutants can also change the energy balance of the earth. Unlike greenhouse gases, these do not absorb the heat radiated from the earth, but heat the earth in other ways.


Soot particles in the atmosphere cause warming because they absorb solar radiation. Soot particles that have sunk on snow surfaces reduce the albedo (>> here) and thus intensify the warming. In addition, a layer of soot on the glaciers in the Himalayas accelerates their melting; The formation of this soot layer is promoted on the Indian subcontinent by inversion weather conditions (warm air is superimposed on near-earth cold air and prevents air exchange) between the monsoon rains. Soot comes mainly from the combustion of biomass; The largest share is accounted for by slash-and-burn forests, a smaller proportion - around a fifth - by the burning of biomass for cooking and heating purposes. Other sources are coal stoves (especially in China) and diesel vehicles without a soot filter. Overall, soot particles make up 10 percent of the total radiative forcing from greenhouse gases and other emissions.

Nitrogen oxides and hydrocarbons

Nitrogen oxides and hydrocarbons form that when exposed to sunlight tropospheric ozone (>> Summer smog - not to be confused with stratospheric ozone, which forms the ozone layer that protects the earth from the sun's UV radiation, see >> The atmosphere). This is also an important greenhouse gas (but is listed under the other emissions because it is not released directly, but arises from other pollutants). The increase in the tropospheric ozone concentration is highest in industrial conurbations and tropical regions with heavy biomass combustion, in line with the formation of the precursor substances.Century have tripled (80). At higher altitudes, ozone can also be transported far with air currents.

The sources of greenhouse gases

Carbon dioxide from fossil fuels comes primarily from the combustion of coal, oil and gas in power generation, in transport, in buildings and in industry. In industry, additional carbon dioxide is produced in some processes - in the cement industry, for example, roughly the same amount of carbon dioxide escapes from the limestone as comes from fossil fuels. In 2011 the share of fossil fuels was coal 43%, oil 34% and gas 18%; that of the cement industry 5% (50). In addition, there is the carbon dioxide from forestry, mainly from the clearing of tropical forests, and the greenhouse gases that are released in agriculture. The clearing of tropical forests (more >> here) is the largest contribution to greenhouse gas emissions in countries like Brazil and Indonesia (in Indonesia 70 percent of carbon dioxide comes from forest destruction); The most important greenhouse gas from agriculture is nitrous oxide from the soil, closely followed by methane from the stomachs of ruminants such as cattle and sheep. Wet rice cultivation is also an important source. Overall, the following shares in the production of greenhouse gases result for the individual sectors:

The sources of greenhouse gases: share of the individual sectors in the
total greenhouse gas emissions in carbon dioxide equivalent.
Data from 2004 Figure from the >> 4th UN Climate Report 2007,
Synthesis tape, own translation.

The historical responsibility for the greenhouse gases found in the atmosphere today becomes clear when we look at the accumulated emissions of individual countries for the period from 1880 to 2004. In the following figure this is shown as per capita emissions for clarity, the size of the rectangle shows the total emissions of the country:

Cumulative greenhouse gas emissions 1880 to 2004 of the individual countries. Illustration from David JC MacKay: Sustainable Energy - without the hot air, page 14. License: >> cc 2.0. .

Despite all the knowledge about the role that greenhouse gases play in global warming, and despite all political declarations of intent, greenhouse gas emissions have continued to increase so far:

Increase in man-made Greenhouse gas emissions in billions of tons of carbon dioxide equivalent (>> here) per year. For the meaning of the colors see the illustration >> above. Image from the >> 4th UN Climate Report 2007, synthesis volume, own translation.

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>> Main page climate change

© Jürgen Paeger 2006 - 2015

Carbon dioxide. Fig. By Jacek FH, from >> wikipedia, accessed on August 17, 2010. License: >> GNU FDL 1.2

Is carbon dioxide really the most important greenhouse gas?

Some >> “skeptics” claim that carbon dioxide cannot be a (relevant) greenhouse gas, its concentration in the atmosphere is far too low for that. Apart from the fact that we are talking about billions of tons that are released every year: the absorption of heat by carbon dioxide has been proven by measurements. But in fact, this does not explain the effects of carbon dioxide alone (the increase in earth temperature if the carbon dioxide concentration doubled would be "only" one degree Celsius): The warming caused by carbon dioxide also increases the air's ability to absorb water vapor, and this is also a Greenhouse gas (>> here). The increasing content of water vapor increases the effect of carbon dioxide, but is not independent of it and is therefore included in carbon dioxide. But the uncertainties begin with the water vapor: water vapor, for example, can form clouds which, depending on the height and shape, can further increase or decrease the temperature. The effects of complex feedback (>> more) cause uncertainty about the exact extent of the temperature increase that would trigger a doubling of the carbon dioxide concentration (>> here), but do not change the importance of carbon dioxide as the actual central cause of current climate change.