Greenhouse gas

Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about -18 °C, rather than the present average of 15 °C.

The five most abundant greenhouse gases in Earth's atmosphere, listed in decreasing order of average global mole fraction, are: water vapor, carbon dioxide, methane, nitrous oxide, ozone. Other greenhouse gases of concern include chlorofluorocarbons (CFCs and HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride. Water vapor causes about half of the greenhouse effect, acting in response to other gases as a climate change feedback.

Human activities since the beginning of the Industrial Revolution (around 1750) have increased carbon dioxide by over 50%, and methane levels by 150%. Carbon dioxide emissions are causing about three-quarters of global warming, while methane emissions cause most of the rest. The vast majority of carbon dioxide emissions by humans come from the burning of fossil fuels, with remaining contributions from agriculture and industry. Methane emissions originate from agriculture, fossil fuel production, waste, and other sources. The carbon cycle takes thousands of years to fully absorb from the atmosphere, while methane lasts in the atmosphere for an average of only 12 years.

Natural flows of carbon happen between the atmosphere, terrestrial ecosystems, the ocean, and sediments. These flows have been fairly balanced over the past 1 million years, although greenhouse gas levels have varied widely in the more distant past. Carbon dioxide levels are now higher than they have been for 3 million years. If current emission rates continue then global warming will surpass 2.0 C-change sometime between 2040 and 2070. This is a level which the Intergovernmental Panel on Climate Change (IPCC) says is "dangerous".

Properties and mechanisms
Greenhouse gases are infrared active, meaning that they absorb and emit infrared radiation in the same long wavelength range as what is emitted by the Earth's surface, clouds and atmosphere.

99% of the Earth's dry atmosphere (excluding water vapor) is made up of nitrogen (78%) and oxygen  (21%). Because their molecules contain two atoms of the same element, they have no asymmetry in the distribution of their electrical charges, and so are almost totally unaffected by infrared thermal radiation, with only an extremely minor effect from collision-induced absorption. A further 0.9% of the atmosphere is made up by argon (Ar), which is monatomic, and so completely transparent to thermal radiation. On the other hand, carbon dioxide (0.04%), methane, nitrous oxide and even less abundant trace gases account for less than 0.1% of Earth's atmosphere, but because their molecules contain atoms of different elements, there is an asymmetry in electric charge distribution which allows molecular vibrations to interact with electromagnetic radiation. This makes them infrared active, and so their presence causes greenhouse effect.

Radiative forcing


Earth absorbs some of the radiant energy received from the sun, reflects some of it as light and reflects or radiates the rest back to space as heat. A planet's surface temperature depends on this balance between incoming and outgoing energy. When Earth's energy balance is shifted, its surface becomes warmer or cooler, leading to a variety of changes in global climate. Radiative forcing is a metric calculated in watts per square meter, which characterizes the impact of an external change in a factor that influences climate. It is calculated as the difference in top-of-atmosphere (TOA) energy balance immediately caused by such an external change. A positive forcing, such as from increased concentrations of greenhouse gases, means more energy arriving than leaving at the top-of-atmosphere, which causes additional warming, while negative forcing, like from sulfates forming in the atmosphere from sulfur dioxide, leads to cooling.

Within the lower atmosphere, greenhouse gases exchange thermal radiation with the surface and limit radiative heat flow away from it, which reduces the overall rate of upward radiative heat transfer. The increased concentration of greenhouse gases is also cooling the upper atmosphere, as it is much thinner than the lower layers, and any heat re-emitted from greenhouse gases is more likely to travel further to space than to interact with the fewer gas molecules in the upper layers. The upper atmosphere is also shrinking as the result. <!-- Naming these effects contributes to a full understanding of the role of greenhouse gases. However, these effects are of secondary importance when it comes to understanding global warming. It is important to focus on top-of-atmosphere energy balance to correctly reason about global warming. It has been argued that the surface budget fallacy, in which focus on the surface energy budget leads to faulty reasoning, constitutes a common fallacy when thinking about the greenhouse effect and global warming.

==== Chemical process contributions to radiative forcing ====

Some gases contribute indirectly to altering the TOA radiative balance through participation in chemical processes within the atmosphere.

Oxidation of CO to directly produces an unambiguous increase in radiative forcing although the reason is subtle. The peak of the thermal IR emission from Earth's surface is very close to a strong vibrational absorption band of (wavelength 15 microns, or wavenumber 667 cm−1). On the other hand, the single CO vibrational band only absorbs IR at much shorter wavelengths (4.7 microns, or 2145 cm−1), where the emission of radiant energy from Earth's surface is at least a factor of ten lower. Oxidation of methane to, which requires reactions with the OH radical, produces an instantaneous reduction in radiative absorption and emission since is a weaker greenhouse gas than methane. However, the oxidations of CO and are entwined since both consume OH radicals. In any case, the calculation of the total radiative effect includes both direct and indirect forcing.

A second type of indirect effect happens when chemical reactions in the atmosphere involving these gases change the concentrations of greenhouse gases. For example, the destruction of non-methane volatile organic compounds (NMVOCs) in the atmosphere can produce ozone. The size of the indirect effect can depend strongly on where and when the gas is emitted.

NMVOCs include a large variety of chemically different compounds, such as benzene, ethanol, formaldehyde, cyclohexane, 1,1,1-Trichloroethane and acetone. Essentially, NMVOCs are identical to volatile organic compounds (VOCs), but with methane excluded. Methane is excluded in air-pollution contexts because it is not toxic. It is however a very potent greenhouse gas, with low reactivity and thus a long lifetime in the atmosphere. An important subset of NMVOCs are the non-methane hydrocarbons (NMHCs). The same process that converts NMVOCs to carbon dioxide can also lead to the formation of tropospheric ozone. Halocarbons have an indirect effect because they destroy stratospheric ozone. Finally, hydrogen can lead to ozone production and increases as well as producing stratospheric water vapor. -->

Contributions of specific gases to the greenhouse effect
Anthropogenic changes to the natural greenhouse effect are sometimes referred to as the enhanced greenhouse effect.

This table shows the most important contributions to the overall greenhouse effect, without which the average temperature of Earth's surface would be about -18 °C, instead of around 15 °C. This table also specifies tropospheric ozone, because this gas has a cooling effect in the stratosphere, but a warming influence comparable to nitrous oxide and CFCs in the troposphere.

Special role of water vapor


Water vapor is the most important greenhouse gas overall, being responsible for 41–67% of the greenhouse effect, but its global concentrations are not directly affected by human activity. While local water vapor concentrations can be affected by developments such as irrigation, it has little impact on the global scale due to its short residence time of about nine days. Indirectly, an increase in global temperatures cause will also increase water vapor concentrations and thus their warming effect, in a process known as water vapor feedback. It occurs because Clausius–Clapeyron relation establishes that more water vapor will be present per unit volume at elevated temperatures. Thus, local atmospheric concentration of water vapor varies from less than 0.01% in extremely cold regions and up to 3% by mass in saturated air at about 32 °C.

List of all greenhouse gases
The contribution of each gas to the enhanced greenhouse effect is determined by the characteristics of that gas, its abundance, and any indirect effects it may cause. For example, the direct radiative effect of a mass of methane is about 84 times stronger than the same mass of carbon dioxide over a 20-year time frame. Since the 1980s, greenhouse gas forcing contributions (relative to year 1750) are also estimated with high accuracy using IPCC-recommended expressions derived from radiative transfer models.

The concentration of a greenhouse gas is typically measured in parts per million (ppm) or parts per billion (ppb) by volume. A concentration of 420 ppm means that 420 out of every million air molecules is a  molecule. The first 30 ppm increase in concentrations took place in about 200 years, from the start of the Industrial Revolution to 1958; however the next 90 ppm increase took place within 56 years, from 1958 to 2014. Similarly, the average annual increase in the 1960s was only 37% of what it was in 2000 through 2007.

Many observations are available online in a variety of Atmospheric Chemistry Observational Databases. The table below shows the most influential long-lived, well-mixed greenhouse gases, along with their tropospheric concentrations and direct radiative forcings, as identified by the Intergovernmental Panel on Climate Change (IPCC). Abundances of these trace gases are regularly measured by atmospheric scientists from samples collected throughout the world. It excludes water vapor because changes in its concentrations are calculated as a climate change feedback indirectly caused by changes in other greenhouse gases, as well as ozone, whose concentrations are only modified indirectly by various refrigerants that cause ozone depletion. Some short-lived gases (e.g. carbon monoxide, NOx) and aerosols (e.g. mineral dust or black carbon) are also excluded because of limited role and strong variation, alongwith minor refrigerants and other halogenated gases, which have been mass-produced in smaller quantities than those in the table. and Annex III of the 2021 IPCC WG1 Report

a Mole fractions: μmol/mol = ppm = parts per million (106); nmol/mol = ppb = parts per billion (109); pmol/mol = ppt = parts per trillion (1012).

The IPCC states that "no single atmospheric lifetime can be given" for CO2. This is mostly due to the rapid growth and cumulative magnitude of the disturbances to Earth's carbon cycle by the geologic extraction and burning of fossil carbon. As of year 2014, fossil CO2 emitted as a theoretical 10 to 100 GtC pulse on top of the existing atmospheric concentration was expected to be 50% removed by land vegetation and ocean sinks in less than about a century, as based on the projections of coupled models referenced in the AR5 assessment. A substantial fraction (20–35%) was also projected to remain in the atmosphere for centuries to millennia, where fractional persistence increases with pulse size.

Values are relative to year 1750. AR6 reports the effective radiative forcing which includes effects of rapid adjustments in the atmosphere and at the surface.

Factors affecting concentrations
Atmospheric concentrations are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound or absorption by bodies of water).

Airborne fraction


The proportion of an emission remaining in the atmosphere after a specified time is the "airborne fraction" (AF). The annual airborne fraction is the ratio of the atmospheric increase in a given year to that year's total emissions. The annual airborne fraction for had been stable at 0.45 for the past six decades even as the emissions have been increasing. This means that the other 0.55 of emitted is absorbed by the land and atmosphere carbon sinks within the first year of an emission. In the high-emission scenarios, the effectiveness of carbon sinks will be lower, increasing the atmospheric fraction of even though the raw amount of emissions absorbed will be higher than in the present.

Atmospheric lifetime


Major greenhouse gases are well mixed and take many years to leave the atmosphere.

The atmospheric lifetime of a greenhouse gas refers to the time required to restore equilibrium following a sudden increase or decrease in its concentration in the atmosphere. Individual atoms or molecules may be lost or deposited to sinks such as the soil, the oceans and other waters, or vegetation and other biological systems, reducing the excess to background concentrations. The average time taken to achieve this is the mean lifetime. This can be represented through the following formula, where the lifetime $$\tau$$ of an atmospheric species X in a one-box model is the average time that a molecule of X remains in the box.

$$\tau$$ can also be defined as the ratio of the mass $$m$$ (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box ($$F_\text{out}$$), chemical loss of X ($$L$$), and deposition of X ($$D$$) (all in kg/s):
 * $$\tau = \frac{m}{F_\text{out}+L+D}$$.

If input of this gas into the box ceased, then after time $$\tau$$, its concentration would decrease by about 63%.

Changes to any of these variables can alter the atmospheric lifetime of a greenhouse gas. For instance, methane's atmospheric lifetime is estimated to have been lower in the 19th century than now, but to have been higher in the second half of the 20th century than after 2000. Carbon dioxide has an even more variable lifetime, which cannot be specified down to a single number. Scientists instead say that while the first 10% of carbon dioxide's airborne fraction (not counting the ~50% absorbed by land and ocean sinks within the emission's first year) is removed "quickly", the vast majority of the airborne fraction – 80% – lasts for "centuries to millennia". The remaining 10% stays for tens of thousands of years. In some models, this longest-lasting fraction is as large as 30%.

Monitoring


Greenhouse gas monitoring involves the direct measurement of atmospheric concentrations and direct and indirect measurement of greenhouse gas emissions. Indirect methods calculate emissions of greenhouse gases based on related metrics such as fossil fuel extraction.

There are several different methods of measuring carbon dioxide concentrations in the atmosphere, including infrared analyzing and manometry. Methane and nitrous oxide are measured by other instruments, such as the range-resolved infrared differential absorption lidar (DIAL). Greenhouse gases are measured from space such as by the Orbiting Carbon Observatory and through networks of ground stations such as the Integrated Carbon Observation System.

The Annual Greenhouse Gas Index (AGGI) is defined by atmospheric scientists at NOAA as the ratio of total direct radiative forcing due to long-lived and well-mixed greenhouse gases for any year for which adequate global measurements exist, to that present in year 1990. These radiative forcing levels are relative to those present in year 1750 (i.e. prior to the start of the industrial era). 1990 is chosen because it is the baseline year for the Kyoto Protocol, and is the publication year of the first IPCC Scientific Assessment of Climate Change. As such, NOAA states that the AGGI "measures the commitment that (global) society has already made to living in a changing climate. It is based on the highest quality atmospheric observations from sites around the world. Its uncertainty is very low."

Natural sources
The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments are fairly balanced; so carbon levels would be roughly stable without human influence. Carbon dioxide is removed from the atmosphere primarily through photosynthesis and enters the terrestrial and oceanic biospheres. Carbon dioxide also dissolves directly from the atmosphere into bodies of water (ocean, lakes, etc.), as well as dissolving in precipitation as raindrops fall through the atmosphere. When dissolved in water, carbon dioxide reacts with water molecules and forms carbonic acid, which contributes to ocean acidity. It can then be absorbed by rocks through weathering. It also can acidify other surfaces it touches or be washed into the ocean.

Human-made sources


The vast majority of carbon dioxide emissions by humans come from the burning of fossil fuels. Additional contributions come from cement manufacturing, fertilizer production, and changes in land use like deforestation. Methane emissions originate from agriculture, fossil fuel production, waste, and other sources.

If current emission rates continue then temperature rises will surpass 2.0 C-change sometime between 2040 and 2070, which is the level the United Nations' Intergovernmental Panel on Climate Change (IPCC) says is "dangerous".

Most greenhouse gases have both natural and human-caused sources. An exception are purely human-produced synthetic halocarbons which have no natural sources. During the pre-industrial Holocene, concentrations of existing gases were roughly constant, because the large natural sources and sinks roughly balanced. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.

Removal from the atmosphere through negative emissions
A number of technologies remove greenhouse gases emissions from the atmosphere. Most widely analyzed are those that remove carbon dioxide from the atmosphere, either to geologic formations such as bio-energy with carbon capture and storage and carbon dioxide air capture, or to the soil as in the case with biochar. Many long-term climate scenario models require large-scale human-made negative emissions to avoid serious climate change.

Negative emissions approaches are also being studied for atmospheric methane, called atmospheric methane removal.

History of discovery
In the late 19th century, scientists experimentally discovered that and  do not absorb infrared radiation (called, at that time, "dark radiation"), while water (both as true vapor and condensed in the form of microscopic droplets suspended in clouds) and  and other poly-atomic gaseous molecules do absorb infrared radiation. In the early 20th century, researchers realized that greenhouse gases in the atmosphere made Earth's overall temperature higher than it would be without them. The term greenhouse was first applied to this phenomenon by Nils Gustaf Ekholm in 1901.

During the late 20th century, a scientific consensus evolved that increasing concentrations of greenhouse gases in the atmosphere cause a substantial rise in global temperatures and changes to other parts of the climate system, with consequences for the environment and for human health.

Other planets
Greenhouse gases exist in many atmospheres, creating greenhouse effects on Mars, Titan and particularly in the thick atmosphere of Venus. While Venus has been described as the ultimate end state of runaway greenhouse effect, such a process would have virtually no chance of occurring from any increases in greenhouse gas concentrations caused by humans, as the Sun's brightness is too low and it would likely need to increase by some tens of percents, which will take a few billion years.