# Greenhouse gas

Last updated

A greenhouse gas is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect. [1] The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone. Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F), [2] rather than the present average of 15 °C (59 °F). [3] [4] [5] The atmospheres of Venus, Mars and Titan also contain greenhouse gases.

Gas is one of the four fundamental states of matter. A pure gas may be made up of individual atoms, elemental molecules made from one type of atom, or compound molecules made from a variety of atoms. A gas mixture, such as air, contains a variety of pure gases. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible, as indicated by the constant velocity vectors in the image.

In physics, absorption of electromagnetic radiation is how matter takes up a photon's energy — and so transforms electromagnetic energy into internal energy of the absorber. A notable effect (attenuation) is to gradually reduce the intensity of light waves as they propagate through a medium. Although the absorption of waves does not usually depend on their intensity, in certain conditions (optics) the medium's transparency changes by a factor that varies as a function of wave intensity, and saturable absorption occurs.

In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calculated by integrating radiant flux with respect to time. The symbol Qe is often used throughout literature to denote radiant energy. In branches of physics other than radiometry, electromagnetic energy is referred to using E or W. The term is used particularly when electromagnetic radiation is emitted by a source into the surrounding environment. This radiation may be visible or invisible to the human eye.

## Contents

Human activities since the beginning of the Industrial Revolution (around 1750) have produced a 45% increase in the atmospheric concentration of carbon dioxide (CO
2
), from 280 ppm in 1750 to 406 ppm in early 2017. This increase has occurred despite the uptake of more than half of the emissions by various natural "sinks" involved in the carbon cycle. [6] [7] The vast majority of anthropogenic carbon dioxide emissions (i.e., emissions produced by human activities) come from combustion of fossil fuels, principally coal, oil, and natural gas, with additional contributions coming from deforestation, changes in land use, soil erosion and agriculture (including livestock). [8] [9]

Human behavior is the response of individuals or groups of humans to internal and external stimuli. It refers to the array of every physical action and observable emotion associated with individuals, as well as the human race. While specific traits of one's personality and temperament may be more consistent, other behaviors will change as one moves from birth through adulthood. In addition to being dictated by age and genetics, behavior, driven in part by thoughts and feelings, is an insight into individual psyche, revealing among other things attitudes and values. Social behavior, a subset of human behavior, study the considerable influence of social interaction and culture. Additional influences include ethics, social environment, authority, persuasion and coercion.

The Industrial Revolution, now also known as the First Industrial Revolution, was the transition to new manufacturing processes in Europe and the US, in the period from about 1760 to sometime between 1820 and 1840. This transition included going from hand production methods to machines, new chemical manufacturing and iron production processes, the increasing use of steam power and water power, the development of machine tools and the rise of the mechanized factory system. The Industrial Revolution also led to an unprecedented rise in the rate of population growth.

Carbon dioxide is an important trace gas in Earth's atmosphere. It is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds. CO
2
absorbs and emits infrared radiation at wavelengths of 4.26 µm and 14.99 µm and consequently is a greenhouse gas that plays a significant role in influencing Earth's surface temperature through the greenhouse effect.

Should greenhouse gas emissions continue at their rate in 2017, global warming could cause Earth's surface temperature to exceed historical values as early as 2047, with potentially harmful effects on ecosystems, biodiversity and human livelihoods. [10] At current emission rates temperatures could increase by 2 °C, which the United Nations' IPCC designated as the upper limit to avoid "dangerous" levels, by 2036. [11]

Global warming is a long-term rise in the average temperature of the Earth's climate system, an aspect of climate change shown by temperature measurements and by multiple effects of the warming. Though earlier geological periods also experienced episodes of warming, the term commonly refers to the observed and continuing increase in average air and ocean temperatures since 1900 caused mainly by emissions of greenhouse gasses in the modern industrial economy. In the modern context the terms global warming and climate change are commonly used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region. Many of the observed changes in climate since the 1950s are unprecedented in the instrumental temperature record, and in historical and paleoclimate proxy records of climate change over thousands to millions of years.

The effects of global warming are the environmental and social changes caused by human emissions of greenhouse gases. There is a scientific consensus that climate change is occurring, and that human activities are the primary driver. Many impacts of climate change have already been observed, including glacier retreat, changes in the timing of seasonal events, and changes in agricultural productivity. Anthropogenic forcing has likely contributed to some of the observed changes, including sea level rise, changes in climate extremes, declines in Arctic sea ice extent and glacier retreat.

The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations, dedicated to providing the world with an objective, scientific view of climate change, its natural, political and economic impacts and risks, and possible response options.

## Gases in Earth's atmosphere

The main gases in Earth's atmosphere are: Nitrogen (78%), Oxygen (21%), and Argon (0.9%). The other gases are: carbon dioxide, nitrous oxides, methane, and ozone. They are trace gases that account for almost a tenth of 1% of Earth's atmosphere.

### Greenhouse gases

Greenhouse gases are those that absorb and emit infrared radiation in the wavelength range emitted by Earth. [1] In order, the most abundant[ clarification needed ] greenhouse gases in Earth's atmosphere are:[ citation needed ]

Water vapor, water vapour or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Unlike other forms of water, water vapor is invisible. Under typical atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation. It is less dense than air and triggers convection currents that can lead to clouds.

Methane (or ) is a chemical compound with the chemical formula CH4 (one atom of carbon and four atoms of hydrogen). It is a group-14 hydride and the simplest alkane, and is the main constituent of natural gas. The relative abundance of methane on Earth makes it an attractive fuel, although capturing and storing it poses challenges due to its gaseous state under normal conditions for temperature and pressure.

Nitrous oxide, commonly known as laughing gas or nitrous, is a chemical compound, an oxide of nitrogen with the formula N
2
O
. At room temperature, it is a colourless non-flammable gas, with a slight metallic scent and taste. At elevated temperatures, nitrous oxide is a powerful oxidiser similar to molecular oxygen. It is soluble in water.

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). [12] 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. As of 2006 the annual airborne fraction for CO
2
was about 0.45. The annual airborne fraction increased at a rate of 0.25 ± 0.21% per year over the period 1959–2006. [13]

The airborne fraction is a scaling factor defined as the ratio of the annual increase in atmospheric CO
2
to the CO
2
emissions from anthropogenic sources. It represents the proportion of human emitted CO
2
that remains in the atmosphere. The fraction averages about 45%, meaning that approximately half the human-emitted CO
2
is absorbed by ocean and land surfaces. There is some evidence for a recent increase in airborne fraction, which would imply a faster increase in atmospheric CO
2
for a given rate of human fossil-fuel burning. Changes in carbon sinks can affect the airborne fraction.

### Non-greenhouse gases

The major atmospheric constituents, nitrogen (N
2
), oxygen (O
2
), and argon (Ar), are not greenhouse gases because molecules containing two atoms of the same element such as N
2
and O
2
have no net change in the distribution of their electrical charges when they vibrate, and monatomic gases such as Ar do not have vibrational modes. Hence they are almost totally unaffected by infrared radiation. Some molecules containing just two atoms of different elements, such as carbon monoxide (CO) and hydrogen chloride (HCl), do absorb infrared radiation, but these molecules are short-lived in the atmosphere owing to their reactivity and solubility. Therefore they do not contribute significantly to the greenhouse effect and often are omitted when discussing greenhouse gases.

Some gases have indirect radiative effects (whether or not they are greenhouse gases themselves). This happens in two main ways. One way is that when they break down in the atmosphere they produce another greenhouse gas. For example, methane and carbon monoxide (CO) are oxidized to give carbon dioxide (and methane oxidation also produces water vapor). Oxidation of CO to CO
2
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 CO
2
(15 microns, or 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 CO
2
, which requires reactions with the OH radical, produces an instantaneous reduction in radiative absorption and emission since CO
2
is a weaker greenhouse gas than methane. However, the oxidations of CO and CH
4
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. [15]

Methane has indirect effects in addition to forming CO
2
. The main chemical that reacts with methane in the atmosphere is the hydroxyl radical (OH), thus more methane means that the concentration of OH goes down. Effectively, methane increases its own atmospheric lifetime and therefore its overall radiative effect. The oxidation of methane can produce both ozone and water; and is a major source of water vapor in the normally dry stratosphere. CO and NMVOCs produce CO
2
when they are oxidized. They remove OH from the atmosphere, and this leads to higher concentrations of methane. The surprising effect of this is that the global warming potential of CO is three times that of CO
2
. [16] 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 CH
4
increases as well as producing stratospheric water vapor. [15]

### Contribution of clouds to Earth's greenhouse effect

The major non-gas contributor to Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on greenhouse gas radiative properties. Clouds are water droplets or ice crystals suspended in the atmosphere. [17] [18]

## Impacts on the overall greenhouse effect

The contribution of each gas to the 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 [21] but it is present in much smaller concentrations so that its total direct radiative effect is smaller, in part due to its shorter atmospheric lifetime. On the other hand, in addition to its direct radiative impact, methane has a large, indirect radiative effect because it contributes to ozone formation. Shindell et al. (2005) [22] argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect. [23]

When ranked by their direct contribution to the greenhouse effect, the most important are: [17] [ not in citation given ]

Compound

Formula

Concentration in
atmosphere [24] (ppm)
Contribution
(%)
Water vapor and cloudsH
2
O
10–50,000(A)36–72%
Carbon dioxide CO
2
~4009–26%
Methane CH
4
~1.84–9%
Ozone O
3
2–8(B)3–7%
notes:

(A) Water vapor strongly varies locally [25]
(B) The concentration in stratosphere. About 90% of the ozone in Earth's atmosphere is contained in the stratosphere.

In addition to the main greenhouse gases listed above, other greenhouse gases include sulfur hexafluoride, hydrofluorocarbons and perfluorocarbons (see IPCC list of greenhouse gases). Some greenhouse gases are not often listed. For example, nitrogen trifluoride has a high global warming potential (GWP) but is only present in very small quantities. [26]

### Proportion of direct effects at a given moment

It is not possible to state that a certain gas causes an exact percentage of the greenhouse effect. This is because some of the gases absorb and emit radiation at the same frequencies as others, so that the total greenhouse effect is not simply the sum of the influence of each gas. The higher ends of the ranges quoted are for each gas alone; the lower ends account for overlaps with the other gases. [17] [18] In addition, some gases, such as methane, are known to have large indirect effects that are still being quantified. [27]

Aside from water vapor, which has a residence time of about nine days, [28] major greenhouse gases are well mixed and take many years to leave the atmosphere. [29] Although it is not easy to know with precision how long it takes greenhouse gases to leave the atmosphere, there are estimates for the principal greenhouse gases. Jacob (1999) [30] defines the lifetime ${\displaystyle \tau }$ of an atmospheric species X in a one-box model as the average time that a molecule of X remains in the box. Mathematically ${\displaystyle \tau }$ can be defined as the ratio of the mass ${\displaystyle m}$ (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box (${\displaystyle F_{out}}$), chemical loss of X (${\displaystyle L}$), and deposition of X (${\displaystyle D}$) (all in kg/s): ${\displaystyle \tau ={\frac {m}{F_{out}+L+D}}}$. [30] If output of this gas into the box ceased, then after time ${\displaystyle \tau }$, its concentration would decrease by about 63%.

The atmospheric lifetime of a species therefore measures 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.

Carbon dioxide has a variable atmospheric lifetime, and cannot be specified precisely. [31] The atmospheric lifetime of CO
2
is estimated of the order of 30–95 years. [32] This figure accounts for CO
2
molecules being removed from the atmosphere by mixing into the ocean, photosynthesis, and other processes. However, this excludes the balancing fluxes of CO
2
into the atmosphere from the geological reservoirs, which have slower characteristic rates. [33] Although more than half of the CO
2
emitted is removed from the atmosphere within a century, some fraction (about 20%) of emitted CO
2
remains in the atmosphere for many thousands of years. [34] [35] [36] Similar issues apply to other greenhouse gases, many of which have longer mean lifetimes than CO
2
, e.g. N2O has a mean atmospheric lifetime of 121 years. [21]

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. [37] Earth's surface temperature depends on this balance between incoming and outgoing energy. [37] If this energy balance is shifted, Earth's surface becomes warmer or cooler, leading to a variety of changes in global climate. [37]

A number of natural and man-made mechanisms can affect the global energy balance and force changes in Earth's climate. [37] Greenhouse gases are one such mechanism. [37] Greenhouse gases absorb and emit some of the outgoing energy radiated from Earth's surface, causing that heat to be retained in the lower atmosphere. [37] As explained above, some greenhouse gases remain in the atmosphere for decades or even centuries, and therefore can affect Earth's energy balance over a long period. [37] Radiative forcing quantifies the effect of factors that influence Earth's energy balance, including changes in the concentrations of greenhouse gases. [37] Positive radiative forcing leads to warming by increasing the net incoming energy, whereas negative radiative forcing leads to cooling. [37]

### Global warming potential

The global warming potential (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same mass of CO
2
and evaluated for a specific timescale. Thus, if a gas has a high (positive) radiative forcing but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than CO
2
its GWP will increase when the timescale is considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.

Methane has an atmospheric lifetime of 12 ± 3 years. The 2007 IPCC report lists the GWP as 72 over a time scale of 20 years, 25 over 100 years and 7.6 over 500 years. [38] A 2014 analysis, however, states that although methane's initial impact is about 100 times greater than that of CO
2
, because of the shorter atmospheric lifetime, after six or seven decades, the impact of the two gases is about equal, and from then on methane's relative role continues to decline. [39] The decrease in GWP at longer times is because methane is degraded to water and CO
2
through chemical reactions in the atmosphere.

Examples of the atmospheric lifetime and GWP relative to CO
2
for several greenhouse gases are given in the following table:

Atmospheric lifetime and GWP relative to CO
2
at different time horizon for various greenhouse gases
Gas nameChemical
formula
(years) [21]
Global warming potential (GWP) for given time horizon
20-yr [21] 100-yr [21] 500-yr [38]
Carbon dioxide CO
2
30–95111
Methane CH
4
1284287.6
Nitrous oxide N
2
O
121264265153
CFC-12 CCl
2
F
2
10010 80010 2005 200
HCFC-22 CHClF
2
125 2801 760549
Tetrafluoromethane        CF
4
50 0004 8806 63011 200
Hexafluoroethane C
2
F
6
10 0008 21011 10018 200
Sulfur hexafluoride SF
6
3 20017 50023 50032 600
Nitrogen trifluoride NF
3
50012 80016 10020 700

The use of CFC-12 (except some essential uses) has been phased out due to its ozone depleting properties. [40] The phasing-out of less active HCFC-compounds will be completed in 2030. [41]

Carbon dioxide in Earth's atmosphere if half of global-warming emissions [42] [43] are not absorbed.
(NASA simulation; 9 November 2015)
Nitrogen dioxide 2014 – global air quality levels
(released 14 December 2015). [44]

## Natural and anthropogenic sources

Aside from purely human-produced synthetic halocarbons, most greenhouse gases have both natural and human-caused 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. [46] [47]

The 2007 Fourth Assessment Report compiled by the IPCC (AR4) noted that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century". [48] In AR4, "most of" is defined as more than 50%.

Abbreviations used in the two tables below: ppm = parts-per-million; ppb = parts-per-billion; ppt = parts-per-trillion; W/m2 = watts per square metre

Current greenhouse gas concentrations [49]
GasPre-1750
tropospheric
concentration [50]
Recent
tropospheric
concentration [51]
Absolute increase
since 1750
Percentage
increase
since 1750
Increased
(W/m2) [52]
Carbon dioxide (CO
2
)
280  ppm [53] 395.4 ppm [54] 115.4 ppm41.2%1.88
Methane (CH
4
)
700 ppb [55] 1893 ppb / [56] [57]
1762 ppb [56]
1193 ppb /
1062 ppb
170.4% /
151.7%
0.49
Nitrous oxide (N
2
O
)
270 ppb [52] [58] 326 ppb / [56]
324 ppb [56]
56 ppb /
54 ppb
20.7% /
20.0%
0.17
Tropospheric
ozone (O
3
)
237 ppb [50] 337 ppb [50] 100 ppb42%0.4 [59]
Relevant to radiative forcing and/or ozone depletion; all of the following have no natural sources and hence zero amounts pre-industrial [49]
GasRecent
tropospheric
concentration
Increased
(W/m2)
CFC-11
(trichlorofluoromethane)
(CCl
3
F
)
236 ppt /
234 ppt
0.061
CFC-12 (CCl
2
F
2
)
527 ppt /
527 ppt
0.169
CFC-113 (Cl
2
FC-CClF
2
)
74 ppt /
74 ppt
0.022
HCFC-22 (CHClF
2
)
231 ppt /
210 ppt
0.046
HCFC-141b (CH
3
CCl
2
F
)
24 ppt /
21 ppt
0.0036
HCFC-142b (CH
3
CClF
2
)
23 ppt /
21 ppt
0.0042
Halon 1211 (CBrClF
2
)
4.1 ppt /
4.0 ppt
0.0012
Halon 1301 (CBrClF
3
)
3.3 ppt /
3.3 ppt
0.001
HFC-134a (CH
2
FCF
3
)
75 ppt /
64 ppt
0.0108
Carbon tetrachloride (CCl
4
)
85 ppt /
83 ppt
0.0143
Sulfur hexafluoride (SF
6
)
7.79 ppt / [60]
7.39 ppt [60]
0.0043
Other halocarbons Varies by
substance
collectively
0.02
Halocarbons in total0.3574

Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the following section). Both CO
2
and CH
4
vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates CO
2
mole fractions stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxies and modeling suggests larger variations in past epochs; 500 million years ago CO
2
levels were likely 10 times higher than now. [61] Indeed, higher CO
2
concentrations are thought to have prevailed throughout most of the Phanerozoic eon, with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the Devonian period, about 400 Ma. [62] [63] [64] The spread of land plants is thought to have reduced CO
2
concentrations during the late Devonian, and plant activities as both sources and sinks of CO
2
have since been important in providing stabilising feedbacks. [65] Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator (Snowball Earth) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the CO
2
concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as limestone at the rate of about 1 mm per day. [66] This episode marked the close of the Precambrian eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are approximately 0.645 billion tons of CO
2
per year, whereas humans contribute 29 billion tons of CO
2
each year. [67] [66] [68] [69]

### Ice cores

Measurements from Antarctic ice cores show that before industrial emissions started atmospheric CO
2
mole fractions were about 280 parts per million (ppm), and stayed between 260 and 280 during the preceding ten thousand years. [70] Carbon dioxide mole fractions in the atmosphere have gone up by approximately 35 percent since the 1900s, rising from 280 parts per million by volume to 387 parts per million in 2009. One study using evidence from stomata of fossilized leaves suggests greater variability, with carbon dioxide mole fractions above 300 ppm during the period seven to ten thousand years ago, [71] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO
2
variability. [72] [73] Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the firn) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels.

### Changes since the Industrial Revolution

Since the beginning of the Industrial Revolution, the concentrations of most of the greenhouse gases have increased. For example, the mole fraction of carbon dioxide has increased from 280 ppm to 400 ppm, or 120 ppm over modern pre-industrial levels. The first 30 ppm increase 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. [74] [75]

Recent data also shows that the concentration is increasing at a higher rate. In the 1960s, the average annual increase was only 37% of what it was in 2000 through 2007. [76]

Total cumulative emissions from 1870 to 2017 were 425±20 GtC (1539 GtCO2) from fossil fuel s and industry, and 180±60 GtC (660 GtCO2) from land use change. Land-use change, such as deforestation, caused about 31% of cumulative emissions over 1870–2017, coal 32%, oil 25%, and gas 10%. [77]

Today,[ when? ] the stock of carbon in the atmosphere increases by more than 3 million tonnes per annum (0.04%) compared with the existing stock.[ clarification needed ] This increase is the result of human activities by burning fossil fuels, deforestation and forest degradation in tropical and boreal regions. [78]

The other greenhouse gases produced from human activity show similar increases in both amount and rate of increase. Many observations are available online in a variety of Atmospheric Chemistry Observational Databases.

## Anthropogenic greenhouse gases

This graph shows changes in the annual greenhouse gas index (AGGI) between 1979 and 2011. [79] The AGGI measures the levels of greenhouse gases in the atmosphere based on their ability to cause changes in Earth's climate. [79]
This bar graph shows global greenhouse gas emissions by sector from 1990 to 2005, measured in carbon dioxide equivalents. [80]
Modern global CO2 emissions from the burning of fossil fuels.

Since about 1750 human activity has increased the concentration of carbon dioxide and other greenhouse gases. Measured atmospheric concentrations of carbon dioxide are currently 100 ppm higher than pre-industrial levels. [81] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity, [82] but over periods longer than a few years natural sources are closely balanced by natural sinks, mainly photosynthesis of carbon compounds by plants and marine plankton. As a result of this balance, the atmospheric mole fraction of carbon dioxide remained between 260 and 280 parts per million for the 10,000 years between the end of the last glacial maximum and the start of the industrial era. [83]

It is likely that anthropogenic (i.e., human-induced) warming, such as that due to elevated greenhouse gas levels, has had a discernible influence on many physical and biological systems. [84] Future warming is projected to have a range of impacts, including sea level rise, [85] increased frequencies and severities of some extreme weather events, [85] loss of biodiversity, [86] and regional changes in agricultural productivity. [86]

The main sources of greenhouse gases due to human activity are:

• burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations in the air. Land use change (mainly deforestation in the tropics) account for up to one third of total anthropogenic CO
2
emissions. [83]
• livestock enteric fermentation and manure management, [87] paddy rice farming, land use and wetland changes, man-made lakes, [88] pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are sources of atmospheric methane.
• use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and halons in fire suppression systems and manufacturing processes.
• agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide (N
2
O
) concentrations.

The seven sources of CO
2
from fossil fuel combustion are (with percentage contributions for 2000–2004): [89]

Seven main fossil fuel
combustion sources
Contribution
(%)
Liquid fuels (e.g., gasoline, fuel oil)36%
Solid fuels (e.g., coal)35%
Gaseous fuels (e.g., natural gas)20%
Cement production 3 %
Flaring gas industrially and at wells< 1%
Non-fuel hydrocarbons< 1%
"International bunker fuels" of transport
not included in national inventories [90]
4 %

Carbon dioxide, methane, nitrous oxide (N
2
O
) and three groups of fluorinated gases (sulfur hexafluoride (SF
6
), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs)) are the major anthropogenic greenhouse gases, [91] :147 [92] and are regulated under the Kyoto Protocol international treaty, which came into force in 2005. [93] Emissions limitations specified in the Kyoto Protocol expired in 2012. [93] The Cancún agreement, agreed on in 2010, includes voluntary pledges made by 76 countries to control emissions. [94] At the time of the agreement, these 76 countries were collectively responsible for 85% of annual global emissions. [94]

Although CFCs are greenhouse gases, they are regulated by the Montreal Protocol, which was motivated by CFCs' contribution to ozone depletion rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming, though the two processes often are confused in the media. On 15 October 2016, negotiators from over 170 nations meeting at the summit of the United Nations Environment Programme reached a legally binding accord to phase out hydrofluorocarbons (HFCs) in an amendment to the Montreal Protocol. [95] [96] [97]

### Sectors

#### Tourism

According to UNEP global tourism is closely linked to climate change. Tourism is a significant contributor to the increasing concentrations of greenhouse gases in the atmosphere. Tourism accounts for about 50% of traffic movements. Rapidly expanding air traffic contributes about 2.5% of the production of CO
2
. The number of international travelers is expected to increase from 594 million in 1996 to 1.6 billion by 2020, adding greatly to the problem unless steps are taken to reduce emissions. [98]

#### Trucking and haulage

The trucking and haulage industry plays a part in production of CO
2
, contributing around 20% of the UK's total carbon emissions a year, with only the energy industry having a larger impact at around 39%. [99] Average carbon emissions within the haulage industry are falling—in the thirty-year period from 1977 to 2007, the carbon emissions associated with a 200-mile journey fell by 21 percent; NOx emissions are also down 87 percent, whereas journey times have fallen by around a third. [100] Due to their size, HGVs often receive criticism regarding their CO2 emissions; however, rapid development in engine technology and fuel management is having a largely positive effect.

#### Plastic

Plastic is produced mainly from Fossil fuels. Plastic manufacturing is estimated to use 8 percent of yearly global oil production. The EPA estimates as many as five ounces of carbon dioxide are emitted for each ounce of polyethylene terephthalate (PET) produced—the type of plastic most commonly used for beverage bottles, [101] the transportation produce greenhouse gases also. [102] Plastic waste emits carbon dioxide when it degrades. In 2018 research claimed that some of the most common plastics in the environment release the greenhouse gases Methane and Ethylene when exposed to sunlight in an amount that can affect the earth climate. [103] [104]

From the other side, if it is placed in a landfill, it becomes a carbon sink [105] although biodegradable plastics have caused methane emissions. [106] Due to the lightness of plastic versus glass or metal, plastic may reduce energy consumption. For example, packaging beverages in PET plastic rather than glass or metal is estimated to save 52% in transportation energy, if the glass or metal package is single use, of course.

In 2019 a new report "Plastic and Climate" was published. According to the report plastic wiil contribute Greenhouse gases in the equivalent of 850 million tons of Carbon dioxide (CO2) to the atmosphere in 2019. In current trend, annual emissions will grow to 1.34 billion tons by 2030. By 2050 plastic could emit 56 billion tons of Greenhouse gas emissions, as much as 14 percent of the earth’s remaining carbon budget [107] . The report says that only solutions which involve a reduction in consumption can solve the problem, while others like biodegradable plastic, ocean cleanup, using renewable energy in plastic industry can do little, and in some cases may even worsen it [108] .

## Role of water vapor

Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for clear sky conditions and between 66% and 85% when including clouds. [18] Water vapor concentrations fluctuate regionally, but human activity does not directly affect water vapor concentrations except at local scales, such as near irrigated fields. Indirectly, human activity that increases global temperatures will increase water vapor concentrations, a process known as water vapor feedback. [109] The atmospheric concentration of vapor is highly variable and depends largely on temperature, from less than 0.01% in extremely cold regions up to 3% by mass in saturated air at about 32 °C. [110] (See Relative humidity#other important facts.)

The average residence time of a water molecule in the atmosphere is only about nine days, compared to years or centuries for other greenhouse gases such as CH
4
and CO
2
. [111] Thus, water vapor responds to and amplifies effects of the other greenhouse gases. The Clausius–Clapeyron relation establishes that more water vapor will be present per unit volume at elevated temperatures. This and other basic principles indicate that warming associated with increased concentrations of the other greenhouse gases also will increase the concentration of water vapor (assuming that the relative humidity remains approximately constant; modeling and observational studies find that this is indeed so). Because water vapor is a greenhouse gas, this results in further warming and so is a "positive feedback" that amplifies the original warming. Eventually other earth processes offset these positive feedbacks, stabilizing the global temperature at a new equilibrium and preventing the loss of Earth's water through a Venus-like runaway greenhouse effect. [112]

## Direct greenhouse gas emissions

Between the period 1970 to 2004, greenhouse gas emissions (measured in CO
2
-equivalent
) [113] increased at an average rate of 1.6% per year, with CO
2
emissions from the use of fossil fuels growing at a rate of 1.9% per year. [114] [115] Total anthropogenic emissions at the end of 2009 were estimated at 49.5 gigatonnes CO
2
-equivalent. [116] :15 These emissions include CO
2
from fossil fuel use and from land use, as well as emissions of methane, nitrous oxide and other greenhouse gases covered by the Kyoto Protocol.

At present, the primary source of CO
2
emissions is the burning of coal, natural gas, and petroleum for electricity and heat. [117]

### Regional and national attribution of emissions

According to the Environmental Protection Agency (EPA), GHG emissions in the United States can be traced from different sectors.

There are several different ways of measuring greenhouse gas emissions, for example, see World Bank (2010) [118] :362 for tables of national emissions data. Some variables that have been reported [119] include:

• Definition of measurement boundaries: Emissions can be attributed geographically, to the area where they were emitted (the territory principle) or by the activity principle to the territory produced the emissions. These two principles result in different totals when measuring, for example, electricity importation from one country to another, or emissions at an international airport.
• Time horizon of different gases: Contribution of a given greenhouse gas is reported as a CO
2
equivalent. The calculation to determine this takes into account how long that gas remains in the atmosphere. This is not always known accurately and calculations must be regularly updated to reflect new information.
• What sectors are included in the calculation (e.g., energy industries, industrial processes, agriculture etc.): There is often a conflict between transparency and availability of data.
• The measurement protocol itself: This may be via direct measurement or estimation. The four main methods are the emission factor-based method, mass balance method, predictive emissions monitoring systems, and continuous emissions monitoring systems. These methods differ in accuracy, cost, and usability.

These different measures are sometimes used by different countries to assert various policy/ethical positions on climate change (Banuri et al., 1996, p. 94). [120] The use of different measures leads to a lack of comparability, which is problematic when monitoring progress towards targets. There are arguments for the adoption of a common measurement tool, or at least the development of communication between different tools. [119]

Emissions may be measured over long time periods. This measurement type is called historical or cumulative emissions. Cumulative emissions give some indication of who is responsible for the build-up in the atmospheric concentration of greenhouse gases (IEA, 2007, p. 199). [121]

The national accounts balance would be positively related to carbon emissions. The national accounts balance shows the difference between exports and imports. For many richer nations, such as the United States, the accounts balance is negative because more goods are imported than they are exported. This is mostly due to the fact that it is cheaper to produce goods outside of developed countries, leading the economies of developed countries to become increasingly dependent on services and not goods. We believed that a positive accounts balance would means that more production was occurring in a country, so more factories working would increase carbon emission levels. [122]

Emissions may also be measured across shorter time periods. Emissions changes may, for example, be measured against a base year of 1990. 1990 was used in the United Nations Framework Convention on Climate Change (UNFCCC) as the base year for emissions, and is also used in the Kyoto Protocol (some gases are also measured from the year 1995). [91] :146, 149 A country's emissions may also be reported as a proportion of global emissions for a particular year.

Another measurement is of per capita emissions. This divides a country's total annual emissions by its mid-year population. [118] :370 Per capita emissions may be based on historical or annual emissions (Banuri et al., 1996, pp. 106–07). [120]

While cities are sometimes considered to be disproportionate contributors to emissions, per-capita emissions tend to be lower for cities than the averages in their countries. [123]

### From land-use change

Land-use change, e.g., the clearing of forests for agricultural use, can affect the concentration of greenhouse gases in the atmosphere by altering how much carbon flows out of the atmosphere into carbon sinks. [124] Accounting for land-use change can be understood as an attempt to measure "net" emissions, i.e., gross emissions from all sources minus the removal of emissions from the atmosphere by carbon sinks (Banuri et al., 1996, pp. 92–93). [120]

There are substantial uncertainties in the measurement of net carbon emissions. [125] Additionally, there is controversy over how carbon sinks should be allocated between different regions and over time (Banuri et al., 1996, p. 93). [120] For instance, concentrating on more recent changes in carbon sinks is likely to favour those regions that have deforested earlier, e.g., Europe.

### Greenhouse gas intensity

 Greenhouse gas intensity in the year 2000, including land-use change. Carbon intensity of GDP (using PPP) for different regions, 1982–2011. Carbon intensity of GDP (using MER) for different regions, 1982–2011.

Greenhouse gas intensity is a ratio between greenhouse gas emissions and another metric, e.g., gross domestic product (GDP) or energy use. The terms "carbon intensity" and "emissions intensity" are also sometimes used. [126] Emission intensities may be calculated using market exchange rates (MER) or purchasing power parity (PPP) (Banuri et al., 1996, p. 96). [120] Calculations based on MER show large differences in intensities between developed and developing countries, whereas calculations based on PPP show smaller differences.

### Cumulative and historical emissions

Cumulative energy-related CO
2
emissions between the years 1850–2005 grouped into low-income, middle-income, high-income, the EU-15, and the OECD countries.
Cumulative energy-related CO
2
emissions between the years 1850–2005 for individual countries.
Map of cumulative per capita anthropogenic atmospheric CO
2
emissions by country. Cumulative emissions include land use change, and are measured between the years 1950 and 2000.
Regional trends in annual CO
2
emissions from fuel combustion between 1971 and 2009.
Regional trends in annual per capita CO
2
emissions from fuel combustion between 1971 and 2009.

Cumulative anthropogenic (i.e., human-emitted) emissions of CO
2
from fossil fuel use are a major cause of global warming, [127] and give some indication of which countries have contributed most to human-induced climate change. [128] :15

Top-5 historic CO
2
contributors by region over the years 1800 to 1988 (in %)
RegionIndustrial
CO
2
Total
CO
2
OECD North America33.229.7
OECD Europe26.116.6
Former USSR14.112.5
China  5.5  6.0
Eastern Europe  5.5  4.8

The table above to the left is based on Banuri et al. (1996, p. 94). [120] Overall, developed countries accounted for 83.8% of industrial CO
2
emissions over this time period, and 67.8% of total CO
2
emissions. Developing countries accounted for industrial CO
2
emissions of 16.2% over this time period, and 32.2% of total CO
2
emissions. The estimate of total CO
2
emissions includes biotic carbon emissions, mainly from deforestation. Banuri et al. (1996, p. 94) [120] calculated per capita cumulative emissions based on then-current population. The ratio in per capita emissions between industrialized countries and developing countries was estimated at more than 10 to 1.

Including biotic emissions brings about the same controversy mentioned earlier regarding carbon sinks and land-use change (Banuri et al., 1996, pp. 93–94). [120] The actual calculation of net emissions is very complex, and is affected by how carbon sinks are allocated between regions and the dynamics of the climate system.

Non-OECD countries accounted for 42% of cumulative energy-related CO
2
emissions between 1890 and 2007. [129] :179–80 Over this time period, the US accounted for 28% of emissions; the EU, 23%; Russia, 11%; China, 9%; other OECD countries, 5%; Japan, 4%; India, 3%; and the rest of the world, 18%. [129] :179–80

### Changes since a particular base year

Between 1970 and 2004, global growth in annual CO
2
emissions was driven by North America, Asia, and the Middle East. [130] The sharp acceleration in CO
2
emissions since 2000 to more than a 3% increase per year (more than 2 ppm per year) from 1.1% per year during the 1990s is attributable to the lapse of formerly declining trends in carbon intensity of both developing and developed nations. China was responsible for most of global growth in emissions during this period. Localised plummeting emissions associated with the collapse of the Soviet Union have been followed by slow emissions growth in this region due to more efficient energy use, made necessary by the increasing proportion of it that is exported. [89] In comparison, methane has not increased appreciably, and N
2
O
by 0.25% y−1.

Using different base years for measuring emissions has an effect on estimates of national contributions to global warming. [128] :17–18 [131] This can be calculated by dividing a country's highest contribution to global warming starting from a particular base year, by that country's minimum contribution to global warming starting from a particular base year. Choosing between different base years of 1750, 1900, 1950, and 1990 has a significant effect for most countries. [128] :17–18 Within the G8 group of countries, it is most significant for the UK, France and Germany. These countries have a long history of CO
2
emissions (see the section on Cumulative and historical emissions).

### Annual emissions

Annual per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries. [91] :144 Due to China's fast economic development, its annual per capita emissions are quickly approaching the levels of those in the Annex I group of the Kyoto Protocol (i.e., the developed countries excluding the US). [132] Other countries with fast growing emissions are South Korea, Iran, and Australia (which apart from the oil rich Persian Gulf states, now has the highest percapita emission rate in the world). On the other hand, annual per capita emissions of the EU-15 and the US are gradually decreasing over time. [132] Emissions in Russia and Ukraine have decreased fastest since 1990 due to economic restructuring in these countries. [133]

Energy statistics for fast growing economies are less accurate than those for the industrialized countries. For China's annual emissions in 2008, the Netherlands Environmental Assessment Agency estimated an uncertainty range of about 10%. [132]

The greenhouse gas footprint refers to the emissions resulting from the creation of products or services. It is more comprehensive than the commonly used carbon footprint, which measures only carbon dioxide, one of many greenhouse gases.

2015 was the first year to see both total global economic growth and a reduction of carbon emissions. [134]

### Top emitter countries

#### Annual

In 2009, the annual top ten emitting countries accounted for about two-thirds of the world's annual energy-related CO
2
emissions. [135]

Top-10 annual energy-related CO
2
emitters for the year 2009[ citation needed ]
Country% of global total
annual emissions
Tonnes of GHG
per capita
China 23.65.1
United States 17.916.9
India 5.51.4
Russia 5.310.8
Japan 3.88.6
Germany 2.69.2
Iran 1.87.3
South Korea 1.810.6
United Kingdom 1.67.5

#### Cumulative

Top-10 cumulative energy-related CO
2
emitters between 1850 and 2008 [ citation needed ]
Country% of world
total
Metric tonnes
CO
2
per person
United States 28.51,132.7
China 9.3685.4
Russia 7.95677.2
Germany 6.78998.9
United Kingdom 5.731,127.8
Japan 3.88367
France 2.73514.9
India 2.5226.7
Ukraine 2.13556.4

### Embedded emissions

One way of attributing greenhouse gas (GHG) emissions is to measure the embedded emissions (also referred to as "embodied emissions") of goods that are being consumed. Emissions are usually measured according to production, rather than consumption. [136] For example, in the main international treaty on climate change (the UNFCCC), countries report on emissions produced within their borders, e.g., the emissions produced from burning fossil fuels. [129] :179 [137] :1 Under a production-based accounting of emissions, embedded emissions on imported goods are attributed to the exporting, rather than the importing, country. Under a consumption-based accounting of emissions, embedded emissions on imported goods are attributed to the importing country, rather than the exporting, country.

Davis and Caldeira (2010) [137] :4 found that a substantial proportion of CO
2
emissions are traded internationally. The net effect of trade was to export emissions from China and other emerging markets to consumers in the US, Japan, and Western Europe. Based on annual emissions data from the year 2004, and on a per-capita consumption basis, the top-5 emitting countries were found to be (in tCO
2
per person, per year): Luxembourg (34.7), the US (22.0), Singapore (20.2), Australia (16.7), and Canada (16.6). [137] :5 Carbon Trust research revealed that approximately 25% of all CO
2
emissions from human activities 'flow' (i.e., are imported or exported) from one country to another. Major developed economies were found to be typically net importers of embodied carbon emissions—with UK consumption emissions 34% higher than production emissions, and Germany (29%), Japan (19%) and the US (13%) also significant net importers of embodied emissions. [138]

### Effect of policy

Governments have taken action to reduce greenhouse gas emissions (climate change mitigation). Assessments of policy effectiveness have included work by the Intergovernmental Panel on Climate Change, [139] International Energy Agency, [140] [141] and United Nations Environment Programme. [142] Policies implemented by governments have included [143] [144] [145] national and regional targets to reduce emissions, promoting energy efficiency, and support for renewable energy such as Solar energy as an effective use of renewable energy because solar uses energy from the sun and does not release pollutants into the air.

Countries and regions listed in Annex I of the United Nations Framework Convention on Climate Change (UNFCCC) (i.e., the OECD and former planned economies of the Soviet Union) are required to submit periodic assessments to the UNFCCC of actions they are taking to address climate change. [145] :3 Analysis by the UNFCCC (2011) [145] :8 suggested that policies and measures undertaken by Annex I Parties may have produced emission savings of 1.5 thousand Tg CO
2
-eq
in the year 2010, with most savings made in the energy sector. The projected emissions saving of 1.5 thousand Tg CO
2
-eq is measured against a hypothetical "baseline" of Annex I emissions, i.e., projected Annex I emissions in the absence of policies and measures. The total projected Annex I saving of 1.5 thousand CO
2
-eq does not include emissions savings in seven of the Annex I Parties. [145] :8

### Projections

A wide range of projections of future emissions have been produced. [146] Rogner et al. (2007) [147] assessed the scientific literature on greenhouse gas projections. Rogner et al. (2007) [114] concluded that unless energy policies changed substantially, the world would continue to depend on fossil fuels until 2025–2030. Projections suggest that more than 80% of the world's energy will come from fossil fuels. This conclusion was based on "much evidence" and "high agreement" in the literature. [114] Projected annual energy-related CO
2
emissions in 2030 were 40–110% higher than in 2000, with two-thirds of the increase originating in developing countries. [114] Projected annual per capita emissions in developed country regions remained substantially lower (2.8–5.1 tonnes CO
2
) than those in developed country regions (9.6–15.1 tonnes CO
2
). [148] Projections consistently showed increase in annual world emissions of "Kyoto" gases, [149] measured in CO
2
-equivalent
) of 25–90% by 2030, compared to 2000. [114]

### Relative CO2 emission from various fuels

One liter of gasoline, when used as a fuel, produces 2.32 kg (about 1300 liters or 1.3 cubic meters) of carbon dioxide, a greenhouse gas. One US gallon produces 19.4 lb (1,291.5 gallons or 172.65 cubic feet) [150] [151] [152]

Mass of carbon dioxide emitted per quantity of energy for various fuels [153]
Fuel nameCO
2

emitted
(lbs/106 Btu)
CO
2

emitted
(g/MJ)
CO
2

emitted
(g/kWh)
Natural gas 11750.30181.08
Liquefied petroleum gas 13959.76215.14
Propane 13959.76215.14
Aviation gasoline 15365.78236.81
Automobile gasoline 15667.07241.45
Kerosene 15968.36246.10
Fuel oil 16169.22249.19
Tires/tire derived fuel 18981.26292.54
Wood and wood waste19583.83301.79
Coal (bituminous) 20588.13317.27
Coal (sub-bituminous) 21391.57329.65
Coal (lignite) 21592.43332.75
Petroleum coke 22596.73348.23
Tar-sand Bitumen [ citation needed ][ citation needed ][ citation needed ]
Coal (anthracite) 22797.59351.32

## Life-cycle greenhouse-gas emissions of energy sources

A 2011 IPCC report included a literature review of numerous energy sources' total life cycle CO
2
emissions. Below are the CO
2
emission values that fell at the 50th percentile of all studies surveyed. [154]

Lifecycle greenhouse gas emissions by electricity source.
TechnologyDescription50th percentile
(g CO
2
/kWh e )
Hydroelectric reservoir4
Ocean Energy wave and tidal8
Wind onshore 12
Nuclear various generation II reactor types16
Biomass various18
Solar thermal parabolic trough 22
Geothermal hot dry rock 45
Solar PV Polycrystalline silicon 46
Natural gas various combined cycle turbines without scrubbing469
Coal various generator types without scrubbing1001

## Removal from the atmosphere

### Natural processes

Greenhouse gases can be removed from the atmosphere by various processes, as a consequence of:

• a physical change (condensation and precipitation remove water vapor from the atmosphere).
• a chemical reaction within the atmosphere. For example, methane is oxidized by reaction with naturally occurring hydroxyl radical, OH· and degraded to CO
2
and water vapor (CO
2
from the oxidation of methane is not included in the methane Global warming potential). Other chemical reactions include solution and solid phase chemistry occurring in atmospheric aerosols.
• a physical exchange between the atmosphere and the other compartments of the planet. An example is the mixing of atmospheric gases into the oceans.
• a chemical change at the interface between the atmosphere and the other compartments of the planet. This is the case for CO
2
, which is reduced by photosynthesis of plants, and which, after dissolving in the oceans, reacts to form carbonic acid and bicarbonate and carbonate ions (see ocean acidification).
• a photochemical change. Halocarbons are dissociated by UV light releasing Cl· and F· as free radicals in the stratosphere with harmful effects on ozone (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).

### Negative emissions

A number of technologies remove greenhouse gases emissions from the atmosphere. Most widely analysed are those that remove carbon dioxide from the atmosphere, either to geologic formations such as bio-energy with carbon capture and storage [155] [156] [157] and carbon dioxide air capture, [157] or to the soil as in the case with biochar. [157] The IPCC has pointed out that many long-term climate scenario models require large-scale manmade negative emissions to avoid serious climate change. [158]

## History of scientific research

In the late 19th century scientists experimentally discovered that N
2
and O
2
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 CO
2
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. 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, [159] with consequences for the environment and for human health.

## Related Research Articles

Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for recent global warming and related climate changes on Earth. The effort has focused on changes observed during the period of instrumental temperature record, particularly in the last 50 years. This is the period when human activity has grown fastest and observations of the atmosphere above the surface have become available. According to the Intergovernmental Panel on Climate Change (IPCC), it is "extremely likely" that human influence was the dominant cause of global warming between 1951 and 2010. The best estimate is that observed warming since 1951 has been entirely human caused.

The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without its atmosphere.

Global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere up to a specific time horizon, relative to carbon dioxide. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide and is expressed as a factor of carbon dioxide.

Scientific opinion on climate change is a judgment by a scientist, or by group of scientists, regarding the degree to which global warming is occurring, its likely causes, and its probable consequences.

Radiative forcing or climate forcing is the difference between insolation (sunlight) absorbed by the Earth and energy radiated back to space. The influences that cause changes to the Earth’s climate system altering Earth’s radiative equilibrium, forcing temperatures to rise or fall, are called climate forcings. Positive radiative forcing means Earth receives more incoming energy from sunlight than it radiates to space. This net gain of energy will cause warming. Conversely, negative radiative forcing means that Earth loses more energy to space than it receives from the sun, which produces cooling.

Climate change mitigation consists of actions to limit the magnitude or rate of long-term global warming and its related effects. Climate change mitigation generally involves reductions in human (anthropogenic) emissions of greenhouse gases (GHGs). Mitigation may also be achieved by increasing the capacity of carbon sinks, e.g., through reforestation. Mitigation policies can substantially reduce the risks associated with human-induced global warming.

A runaway greenhouse effect is a state in which a net positive feedback between surface temperature and atmospheric opacity increases the strength of the greenhouse effect on a planet until its oceans boil away.An example of this is believed to have happened in the early history of Venus. On the Earth the IPCC states that "a 'runaway greenhouse effect'—analogous to [that of] Venus—appears to have virtually no chance of being induced by anthropogenic activities."

Carbon dioxide equivalent (CDE) and equivalent carbon dioxide are two related but distinct measures for describing how much global warming a given type and amount of greenhouse gas may cause, using the functionally equivalent amount or concentration of carbon dioxide as the reference.

A low-carbon diet refers to making lifestyle choices to reduce the greenhouse gas emissions (GHGe) resulting from consumption decisions. It is estimated that the U.S. food system is responsible for at least 20 percent of U.S. greenhouse gases. This estimate may be low, as it counts only direct sources of GHGe. Indirect sources, such as demand for products from other countries, are often not counted. A low-carbon diet minimizes the emissions released from the production, packaging, processing, transport, preparation and waste of food. Major tenets of a low-carbon diet include eating less industrial meat and dairy, eating less industrially produced food in general, eating food grown locally and seasonally, eating less processed and packaged foods and reducing waste from food by proper portion size, recycling or composting.

Climate change mitigation scenarios are possible futures in which global warming is reduced by deliberate actions, such as a comprehensive switch to energy sources other than fossil fuels. These are actions that minimize emissions so atmospheric greenhouse gas concentrations are stabilized at levels that restrict the adverse consequences of climate change. Using these scenarios, the examination of the impacts of different carbon prices on an economy is enabled within the framework of different levels of global aspirations.

Greenhouse gas removal projects are a type of climate engineering that seek to remove greenhouse gases from the atmosphere, and thus they tackle the root cause of global warming. These techniques either directly remove greenhouse gases, or alternatively seek to influence natural processes to remove greenhouse gases indirectly. The discipline overlaps with carbon capture and storage and carbon sequestration, and some projects listed may not be considered to be climate engineering by all commentators, instead being described as mitigation.

Carbon dioxide removal (CDR) refers to a number of technologies of which the objective is the large-scale removal of carbon dioxide from the atmosphere. Among such technologies are bio-energy with carbon capture and storage, biochar, ocean fertilization, enhanced weathering, and direct air capture when combined with storage. CDR is a different approach than removing CO
2
from the stack emissions of large fossil fuel point sources, such as power stations. The latter reduces emission to the atmosphere but cannot reduce the amount of carbon dioxide already in the atmosphere. As CDR removes carbon dioxide from the atmosphere, it creates negative emissions, offsetting emissions from small and dispersed point sources such as domestic heating systems, airplanes and vehicle exhausts. It is regarded by some as a form of climate engineering, while other commentators describe it as a form of carbon capture and storage or extreme mitigation. Whether CDR would satisfy common definitions of "climate engineering" or "geoengineering" usually depends upon the scale on which it would be undertaken.

Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising.

The history of the scientific discovery of climate change began in the early 19th century when ice ages and other natural changes in paleoclimate were first suspected and the natural greenhouse effect first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could change the climate. Many other theories of climate change were advanced, involving forces from volcanism to solar variation. In the 1960s, the warming effect of carbon dioxide gas became increasingly convincing. Some scientists also pointed out that human activities that generated atmospheric aerosols could have cooling effects as well. During the 1970s, scientific opinion increasingly favored the warming viewpoint. By the 1990s, as a result of improving fidelity of computer models and observational work confirming the Milankovitch theory of the ice ages, a consensus position formed: greenhouse gases were deeply involved in most climate changes and human-caused emissions were bringing discernible global warming. Since the 1990s, scientific research on climate change has included multiple disciplines and has expanded. Research has expanded our understanding of causal relations, links with historic data and ability to model climate change numerically. Research during this period has been summarized in the Assessment Reports by the Intergovernmental Panel on Climate Change.

Climate change feedback is important in the understanding of global warming because feedback processes may amplify or diminish the effect of each climate forcing, and so play an important part in determining the climate sensitivity and future climate state. Feedback in general is the process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback amplifies the change in the first quantity while negative feedback reduces it.

The Greenhouse gas footprint, or GHG footprint, refers to the amount of greenhouse gases that are emitted during the creation of products or services.

The atmosphere is one of the Earth's major carbon reservoirs and an important component of the global carbon cycle, holding approximately 720 gigatons of carbon. Atmospheric carbon plays an important role in the greenhouse effect. The most important carbon compound in this respect is the gas carbon dioxide. Although it is a small percentage of the atmosphere, it plays a vital role in retaining heat in the atmosphere and thus in the greenhouse effect. Other gases with effects on the climate containing carbon in the atmosphere are methane and chlorofluorocarbons. Emissions by humans in the past 200 years have almost doubled the amount carbon dioxide in the atmosphere.

## References

1. "IPCC AR4 SYR Appendix Glossary" (PDF). Retrieved 14 December 2008.
2. "NASA GISS: Science Briefs: Greenhouse Gases: Refining the Role of Carbon Dioxide". www.giss.nasa.gov. Retrieved 26 April 2016.
3. Karl TR, Trenberth KE (2003). "Modern global climate change". Science. 302 (5651): 1719–23. Bibcode:2003Sci...302.1719K. doi:10.1126/science.1090228. PMID   14657489.
4. Le Treut H.; Somerville R.; Cubasch U.; Ding Y.; Mauritzen C.; Mokssit A.; Peterson T.; Prather M. Historical overview of climate change science (PDF). Retrieved 14 December 2008.
5. "NASA Science Mission Directorate article on the water cycle". Nasascience.nasa.gov. Archived from the original on 17 January 2009. Retrieved 16 October 2010.
6. ESRL Web Team (14 January 2008). "Trends in carbon dioxide". Esrl.noaa.gov. Retrieved 11 September 2011.
7. EPA,OA, US. "Global Greenhouse Gas Emissions Data - US EPA". US EPA.
8. "AR4 SYR Synthesis Report Summary for Policymakers – 2 Causes of change". ipcc.ch. Archived from the original on 28 February 2018. Retrieved 9 October 2015.
9. Mora, C (2013). "The projected timing of climate departure from recent variability". Nature. 502 (7470): 183–87. Bibcode:2013Natur.502..183M. doi:10.1038/nature12540. PMID   24108050.
10. Mann, Michael E. (1 April 2014). "Earth Will Cross the Climate Danger Threshold by 2036". Scientific American. Retrieved 30 August 2016.
11. "FAQ 7.1". p. 14.
12. Canadell, J.G.; Le Quere, C.; Raupach, M.R.; Field, C.B.; Buitenhuis, E.T.; Ciais, P.; Conway, T.J.; Gillett, N.P.; Houghton, R.A.; Marland, G. (2007). "Contributions to accelerating atmospheric CO
2
growth from economic activity, carbon intensity, and efficiency of natural sinks"
. Proc. Natl. Acad. Sci. USA. 104 (47): 18866–70. Bibcode:2007PNAS..10418866C. doi:10.1073/pnas.0702737104. PMC  . PMID   17962418.
13. Forster, P.; et al. (2007). "2.10.3 Indirect GWPs". Changes in Atmospheric Constituents and in Radiative Forcing. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. Retrieved 2 December 2012.
14. MacCarty, N. "Laboratory Comparison of the Global-Warming Potential of Six Categories of Biomass Cooking Stoves" (PDF). Approvecho Research Center. Archived from the original (PDF) on 11 November 2013.
15. Kiehl, J.T.; Kevin E. Trenberth (1997). "Earth's annual global mean energy budget" (PDF). Bulletin of the American Meteorological Society. 78 (2): 197–208. Bibcode:1997BAMS...78..197K. doi:10.1175/1520-0477(1997)078<0197:EAGMEB>2.0.CO;2. Archived from the original (PDF) on 30 March 2006. Retrieved 1 May 2006.
16. "Water vapour: feedback or forcing?". RealClimate. 6 April 2005. Retrieved 1 May 2006.
17. Schmidt, G.A.; R. Ruedy; R.L. Miller; A.A. Lacis (2010), "The attribution of the present-day total greenhouse effect" (PDF), J. Geophys. Res., 115 (D20), pp. D20106, Bibcode:2010JGRD..11520106S, doi:10.1029/2010JD014287, archived from the original (PDF) on 22 October 2011, D20106. Web page
18. Lacis, A. (October 2010), NASA GISS: CO2: The Thermostat that Controls Earth's Temperature, New York: NASA GISS
19. "Appendix 8.A" (PDF). Intergovernmental Panel on Climate Change Fifth Assessment Report. p. 731.
20. Shindell, Drew T. (2005). "An emissions-based view of climate forcing by methane and tropospheric ozone". Geophysical Research Letters. 32 (4): L04803. Bibcode:2005GeoRL..32.4803S. doi:10.1029/2004GL021900.
21. "Methane's Impacts on Climate Change May Be Twice Previous Estimates". Nasa.gov. 30 November 2007. Retrieved 16 October 2010.
22. "Climate Change Indicators: Atmospheric Concentrations of Greenhouse Gases". Climate Change Indicators. United States Environmental Protection Agency. Retrieved 20 January 2017.
23. Wallace, John M. and Peter V. Hobbs. Atmospheric Science; An Introductory Survey. Elsevier. Second Edition, 2006. ISBN   978-0127329512. Chapter 1
24. Prather, Michael J.; J Hsu (2008). "NF
3
, the greenhouse gas missing from Kyoto". Geophysical Research Letters . 35 (12): L12810. Bibcode:2008GeoRL..3512810P. doi:10.1029/2008GL034542.
25. Isaksen, Ivar S.A.; Michael Gauss; Gunnar Myhre; Katey M. Walter Anthony; Carolyn Ruppel (20 April 2011). "Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions" (PDF). Global Biogeochemical Cycles. 25 (2): n/a. Bibcode:2011GBioC..25B2002I. doi:10.1029/2010GB003845 . Retrieved 29 July 2011.
26. "AGU Water Vapor in the Climate System". Eso.org. 27 April 1995. Retrieved 11 September 2011.
27. Betts (2001). "6.3 Well-mixed Greenhouse Gases". Chapter 6 Radiative Forcing of Climate Change. Working Group I: The Scientific Basis IPCC Third Assessment Report – Climate Change 2001. UNEP/GRID-Arendal – Publications. Archived from the original on 29 June 2011. Retrieved 16 October 2010.
28. Jacob, Daniel (1999). Introduction to atmospheric chemistry. Princeton University Press. pp. 25–26. ISBN   978-0691001852. Archived from the original on 2 September 2011.
29. "How long will global warming last?". RealClimate. Retrieved 12 June 2012.
30. Jacobson, M.Z. (2005). "Correction to "Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming."". J. Geophys. Res. 110. p. D14105. Bibcode:2005JGRD..11014105J. doi:10.1029/2005JD005888.
31. Archer, David (2009). "Atmospheric lifetime of fossil fuel carbon dioxide". Annual Review of Earth and Planetary Sciences. 37. pp. 117–34. Bibcode:2009AREPS..37..117A. doi:10.1146/annurev.earth.031208.100206.
32. "Frequently Asked Question 10.3: If emissions of greenhouse gases are reduced, how quickly do their concentrations in the atmosphere decrease?". Global Climate Projections. Retrieved 1 June 2011.
2
in geologic time"
(PDF). Journal of Geophysical Research . 110 (C9): C09S05.1–6. Bibcode:2005JGRC..11009S05A. doi:10.1029/2004JC002625 . Retrieved 27 July 2007.
34. See also: Caldeira, Ken; Wickett, Michael E. (2005). "Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean" (PDF). Journal of Geophysical Research . 110 (C9): C09S04.1–12. Bibcode:2005JGRC..11009S04C. doi:10.1029/2004JC002671. Archived from the original (PDF) on 10 August 2007. Retrieved 27 July 2007.
35. Edited quote from public-domain source: "Climate Change Indicators in the United States". U.S. Environmental Protection Agency (EPA). 2010. Greenhouse Gases: Figure 1. The Annual Greenhouse Gas Index, 1979–2008: Background.. PDF (p. 18)
36. "Table 2.14" (PDF). IPCC Fourth Assessment Report. p. 212.
37. Chandler, David L. "How to count methane emissions". MIT News. Retrieved 20 August 2018. Referenced paper is Trancik, Jessika; Edwards, Morgan (25 April 2014). "Climate impacts of energy technologies depend on emissions timing" (PDF). Nature Climate Change. 4: 347. Archived from the original (PDF) on 16 January 2015. Retrieved 15 January 2015.
38. Vaara, Miska (2003), Use of ozone depleting substances in laboratories, TemaNord, p. 170, ISBN   978-9289308847, archived from the original on 6 August 2011
39. St. Fleur, Nicholas (10 November 2015). "Atmospheric Greenhouse Gas Levels Hit Record, Report Says". New York Times . Retrieved 11 November 2015.
40. Ritter, Karl (9 November 2015). "UK: In 1st, global temps average could be 1 degree C higher". AP News . Retrieved 11 November 2015.
41. Cole, Steve; Gray, Ellen (14 December 2015). "New NASA Satellite Maps Show Human Fingerprint on Global Air Quality". NASA . Retrieved 14 December 2015.
42. Canadell, J.G.; et al. (20 November 2007), "Contributions to Accelerating Atmospheric CO2 Growth from Economic Activity, Carbon Intensity, and Efficiency of Natural Sinks (Results and Discussion: Growth in Atmospheric CO2)", Proceedings of the National Academy of Sciences of the United States of America, 104 (47): 18866–70, Bibcode:2007PNAS..10418866C, doi:10.1073/pnas.0702737104, PMC  , PMID   17962418
43. "Chapter 3, IPCC Special Report on Emissions Scenarios, 2000" (PDF). Intergovernmental Panel on Climate Change. 2000. Retrieved 16 October 2010.
44. Intergovernmental Panel on Climate Change (17 November 2007). "Climate Change 2007: Synthesis Report" (PDF). p. 5. Retrieved 20 January 2017.
45. Ehhalt, D.; et al., "Table 4.1", Atmospheric Chemistry and Greenhouse Gases, archived from the original on 3 January 2013, in IPCC TAR WG1 2001 , pp. 244–45. Referred to by: Blasing, T.J. (February 2013), Current Greenhouse Gas Concentrations, doi:10.3334/CDIAC/atg.032 , on Blasing, T.J. 2013. Based on Blasing et al. (2013): Pre-1750 concentrations of CH4,N2O and current concentrations of O3, are taken from Table 4.1 (a) of the IPCC Intergovernmental Panel on Climate Change), 2001. Following the convention of IPCC (2001), inferred global-scale trace-gas concentrations from prior to 1750 are assumed to be practically uninfluenced by human activities such as increasingly specialized agriculture, land clearing, and combustion of fossil fuels. Preindustrial concentrations of industrially manufactured compounds are given as zero. The short atmospheric lifetime of ozone (hours-days) together with the spatial variability of its sources precludes a globally or vertically homogeneous distribution, so that a fractional unit such as parts per billion would not apply over a range of altitudes or geographical locations. Therefore a different unit is used to integrate the varying concentrations of ozone in the vertical dimension over a unit area, and the results can then be averaged globally. This unit is called a Dobson Unit (D.U.), after G.M.B. Dobson, one of the first investigators of atmospheric ozone. A Dobson unit is the amount of ozone in a column that, unmixed with the rest of the atmosphere, would be 10 micrometers thick at standard temperature and pressure.
46. Because atmospheric concentrations of most gases tend to vary systematically over the course of a year, figures given represent averages over a 12-month period for all gases except ozone (O3), for which a current global value has been estimated (IPCC, 2001, Table 4.1a). CO
2
averages for year 2012 are taken from the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, web site: www.esrl.noaa.gov/gmd/ccgg/trends maintained by Dr. Pieter Tans. For other chemical species, the values given are averages for 2011. These data are found on the CDIAC AGAGE web site: http://cdiac.ornl.gov/ndps/alegage.html or the AGAGE home page: http://agage.eas.gatech.edu.
47. Forster, P.; et al., "Table 2.1", Changes in Atmospheric Constituents and in Radiative Forcing , in IPCC AR4 WG1 2007 , p. 141. Referred to by: Blasing, T.J. 2013
48. Prentice, I.C.; et al. "Executive summary". The Carbon Cycle and Atmospheric Carbon Dioxide. Archived from the original on 7 December 2009., in IPCC TAR WG1 2001 , p. 185. Referred to by: Blasing, T.J. (February 2013), Current Greenhouse Gas Concentrations, doi:10.3334/CDIAC/atg.032
49. Recent CO
2
concentration (395.4 ppm) is the 2013 average taken from globally averaged marine surface data given by the National Oceanic and Atmospheric Administration Earth System Research Laboratory, website: http://www.esrl.noaa.gov/gmd/ccgg/trends/index.html#global. Please read the material on that web page and reference Dr. Pieter Tans when citing this average (Dr. Pieter Tans, NOAA/ESRL http://www.esrl.noaa.gov/gmd/ccgg/trends). The oft-cited Mauna Loa average for 2012 is 393.8 ppm, which is a good approximation although typically about 1 ppm higher than the spatial average given above. Refer to http://www.esrl.noaa.gov/gmd/ccgg/trends for records back to the late 1950s.
50. ppb = parts-per-billion
51. The first value in a cell represents Mace Head, Ireland, a mid-latitude Northern-Hemisphere site, while the second value represents Cape Grim, Tasmania, a mid-latitude Southern-Hemisphere site. "Current" values given for these gases are annual arithmetic averages based on monthly background concentrations for year 2011. The SF
6
values are from the AGAGE gas chromatography – mass spectrometer (gc-ms) Medusa measuring system.
52. "Advanced Global Atmospheric Gases Experiment (AGAGE)". Data compiled from finer time scales in the Prinn; etc (2000). "ALE/GAGE/AGAGE database".
53. The pre-1750 value for N
2
O
is consistent with ice-core records from 10,000 BCE through 1750 CE: "Summary for policymakers", Figure SPM.1, IPCC, in IPCC AR4 WG1 2007 , p. 3. Referred to by: Blasing, T.J. (February 2013), Current Greenhouse Gas Concentrations, doi:10.3334/CDIAC/atg.032
54. Changes in stratospheric ozone have resulted in a decrease in radiative forcing of 0.05 W/m2: Forster, P.; et al., "Table 2.12", Changes in Atmospheric Constituents and in Radiative Forcing , in IPCC AR4 WG1 2007 , p. 204. Referred to by: Blasing, T.J. 2013
55. "SF
6
data from January 2004"
.
"Data from 1995 through 2004". National Oceanic and Atmospheric Administration (NOAA), Halogenated and other Atmospheric Trace Species (HATS).
56. Berner, Robert A. (January 1994). "GEOCARB II: a revised model of atmospheric CO
2
over Phanerozoic time"
(PDF). American Journal of Science. 294 (1): 56–91. Bibcode:1994AmJS..294...56B. doi:10.2475/ajs.294.1.56.
57. Royer, D.L.; R.A. Berner; D.J. Beerling (2001). "Phanerozoic atmospheric CO
2
change: evaluating geochemical and paleobiological approaches". Earth-Science Reviews. 54 (4): 349–92. Bibcode:2001ESRv...54..349R. doi:10.1016/S0012-8252(00)00042-8.
58. Berner, Robert A.; Kothavala, Zavareth (2001). "GEOCARB III: a revised model of atmospheric CO
2
over Phanerozoic time"
(PDF). American Journal of Science. 301 (2): 182–204. Bibcode:2001AmJS..301..182B. CiteSeerX  . doi:10.2475/ajs.301.2.182. Archived from the original (PDF) on 6 August 2004.
59. Beerling, D.J.; Berner, R.A. (2005). "Feedbacks and the co-evolution of plants and atmospheric CO
2
"
. Proc. Natl. Acad. Sci. USA. 102 (5): 1302–05. Bibcode:2005PNAS..102.1302B. doi:10.1073/pnas.0408724102. PMC  . PMID   15668402.
60. Hoffmann, PF; AJ Kaufman; GP Halverson; DP Schrag (1998). "A neoproterozoic snowball earth". Science. 281 (5381): 1342–46. Bibcode:1998Sci...281.1342H. doi:10.1126/science.281.5381.1342. PMID   9721097.
61. Siegel, Ethan. "How Much CO2 Does A Single Volcano Emit?". Forbes. Retrieved 6 September 2018.
62. Gerlach, TM (1991). "Present-day CO
2
emissions from volcanoes". Transactions of the American Geophysical Union. 72 (23): 249–55. Bibcode:1991EOSTr..72..249.. doi:10.1029/90EO10192.
63. See also: "U.S. Geological Survey". 14 June 2011. Retrieved 15 October 2012.
64. Flückiger, Jacqueline (2002). "High-resolution Holocene N
2
O
ice core record and its relationship with CH
4
and CO
2
". Global Biogeochemical Cycles. 16: 1010. Bibcode:2002GBioC..16a..10F. doi:10.1029/2001GB001417.
65. Friederike Wagner; Bent Aaby; Henk Visscher (2002). "Rapid atmospheric CO
2
changes associated with the 8,200-years-B.P. cooling event"
. Proc. Natl. Acad. Sci. USA. 99 (19): 12011–14. Bibcode:2002PNAS...9912011W. doi:10.1073/pnas.182420699. PMC  . PMID   12202744.
66. Andreas Indermühle; Bernhard Stauffer; Thomas F. Stocker (1999). "Early Holocene Atmospheric CO
2
Concentrations". Science. 286 (5446): 1815. doi:10.1126/science.286.5446.1815a.
IndermÜhle, A (1999). "Early Holocene atmospheric CO
2
concentrations". Science. 286 (5446): 1815a–15. doi:10.1126/science.286.5446.1815a.
67. H. J. Smith; M. Wahlen; D. Mastroianni (1997). "The CO
2
concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition". Geophysical Research Letters. 24 (1): 1–4. Bibcode:1997GeoRL..24....1S. doi:10.1029/96GL03700.
68. Charles J. Kibert (2016). "Background". Sustainable Construction: Green Building Design and Delivery. Wiley. ISBN   978-1119055327.
69. "Full Mauna Loa CO2 record". Earth System Research Laboratory. 2005. Retrieved 6 May 2017.
70. Tans, Pieter (3 May 2008). "Annual CO
2
mole fraction increase (ppm) for 1959–2007"
. National Oceanic and Atmospheric Administration Earth System Research Laboratory, Global Monitoring Division.
"additional details".; see also Masarie, K.A.; Tans, P.P. (1995). "Extension and integration of atmospheric carbon dioxide data into a globally consistent measurement record". J. Geophys. Res. 100 (D6): 11593–610. Bibcode:1995JGR...10011593M. doi:10.1029/95JD00859.
71. "Global Carbon Project (GCP)". www.globalcarbonproject.org. Retrieved 19 May 2019.
72. Dumitru-Romulus Târziu; Victor-Dan Păcurar (January 2011). "Pădurea, climatul și energia". Rev. pădur. (in Romanian). 126 (1): 34–39. ISSN   1583-7890. 16720. Archived from the original on 16 April 2013. Retrieved 11 June 2012.(webpage has a translation button)
73. "Climate Change Indicators in the United States". NOAA. 2012. Figure 4. The Annual Greenhouse Gas Index, 1979–2011.
74. "Climate Change Indicators in the United States". US Environmental Protection Agency (EPA). 2010. Figure 2. Global Greenhouse Gas Emissions by Sector, 1990–2005.
75. "Climate Change 2001: Working Group I: The Scientific Basis: figure 6-6". Archived from the original on 14 June 2006. Retrieved 1 May 2006.
76. "The present carbon cycle – Climate Change". Grida.no. Retrieved 16 October 2010.
77. Couplings Between Changes in the Climate System and Biogeochemistry (PDF). Retrieved 13 May 2008.
78. IPCC (2007d). "6.1 Observed changes in climate and their effects, and their causes". 6 Robust findings, key uncertainties. Climate Change 2007: Synthesis Report. A Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Geneva: IPCC.
79. "6.2 Drivers and projections of future climate changes and their impacts". 6 Robust findings, key uncertainties. Climate Change 2007: Synthesis Report. A Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Geneva, Switzerland: IPCC. 2007d.
80. "3.3.1 Impacts on systems and sectors". 3 Climate change and its impacts in the near and long term under different scenarios. Climate Change 2007: Synthesis Report. A Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Geneva: IPCC. 2007d. Archived from the original on 3 November 2018. Retrieved 31 August 2012.
81. Steinfeld, H.; Gerber, P.; Wassenaar, T.; Castel, V.; Rosales, M.; de Haan, C. (2006). "Livestock's long shadow". FAO Livestock, Environment and Development (LEAD) Initiative.
82. Ciais, Phillipe; Sabine, Christopher; et al. "Carbon and Other Biogeochemical Cycles" (PDF). In Stocker Thomas F.; et al. (eds.). Climate Change 2013: The Physical Science Basis. IPCC. p. 473.
83. Raupach, M.R.; et al. (2007). "Global and regional drivers of accelerating CO
2
emissions"
(PDF). Proc. Natl. Acad. Sci. USA. 104 (24): 10288–93. Bibcode:2007PNAS..10410288R. doi:10.1073/pnas.0700609104. PMC  . PMID   17519334.
84. Schrooten, L; De Vlieger, Ina; Int Panis, Luc; Styns, R. Torfs, K; Torfs, R (2008). "Inventory and forecasting of maritime emissions in the Belgian sea territory, an activity based emission model". Atmospheric Environment. 42 (4): 667–76. Bibcode:2008AtmEn..42..667S. doi:10.1016/j.atmosenv.2007.09.071.
85. Grubb, M. (July – September 2003). "The economics of the Kyoto protocol" (PDF). World Economics. 4 (3). Archived from the original (PDF) on 17 July 2011.
86. Lerner & K. Lee Lerner, Brenda Wilmoth (2006). "Environmental issues: essential primary sources". Thomson Gale. Retrieved 11 September 2006.
87. "Kyoto Protocol". United Nations Framework Convention on Climate Change. Home > Kyoto Protocol.
88. King, D.; et al. (July 2011), "Copenhagen and Cancún", International climate change negotiations: Key lessons and next steps, Oxford: Smith School of Enterprise and the Environment, University of Oxford, p. 12, doi:10.4210/ssee.pbs.2011.0003 (inactive 18 February 2019), archived from the original on 1 August 2013 "PDF available" (PDF). Archived from the original (PDF) on 13 January 2012.
89. Johnston, Chris; Milman, Oliver; Vidal, John (15 October 2016). "Climate change: global deal reached to limit use of hydrofluorocarbons". The Guardian . Retrieved 21 August 2018.
90. "Climate change: 'Monumental' deal to cut HFCs, fastest growing greenhouse gases". BBC News. 15 October 2016. Retrieved 15 October 2016.
91. "Nations, Fighting Powerful Refrigerant That Warms Planet, Reach Landmark Deal". New York Times. 15 October 2016. Retrieved 15 October 2016.
92. "A Cheaper and More Efficient Freight Industry In and Out of the UK". freightbestpractice.org.uk. Retrieved 13 September 2015.
93. Newbold, Richard (19 May 2014), A practical guide for fleet operators, returnloads.net, retrieved 20 January 2017.
94. Glazner, Elizabeth. "Plastic Pollution and Climate Change". Plastic Pollution Coalition. Plastic Pollution Coalition. Retrieved 6 August 2018.
95. Luise Blue, Marie-. "What Is the Carbon Footprint of a Plastic Bottle?". Sciencing. Leaf Group Ltd. Retrieved 6 August 2018.
96. Jeanne Royer, Sarah-; Ferrón, Sara; T. Wilson, Samuel; M. Karl, David (1 August 2018). "Production of methane and ethylene from plastic in the environment". PLOS One. 13 (Plastic, Climate Change): e0200574. doi:10.1371/journal.pone.0200574. PMC  . PMID   30067755.
97. Rosane, Olivia (2 August 2018). "Study Finds New Reason to Ban Plastic: It Emits Methane in the Sun" (Plastic, Climate Change). Ecowatch. Retrieved 6 August 2018.
98. EPA (2012). "Landfilling" (PDF).
99. Levis, James W.; Barlaz, Morton A. (July 2011). "Is Biodegradability a Desirable Attribute for Discarded Solid Waste? Perspectives from a National Landfill Greenhouse Gas Inventory Model". Environmental Science & Technology. 45 (13): 5470–5476. Bibcode:2011EnST...45.5470L. doi:10.1021/es200721s. PMID   21615182.
100. "Sweeping New Report on Global Environmental Impact of Plastics Reveals Severe Damage to Climate". Center for International Environmental Law (CIEL). Retrieved 16 May 2019.
101. Plastic & Climate The Hidden Costs of a Plastic Planet (PDF). Center for International Environmental Law, Environmental Integrity Project, FracTracker Alliance, Global Alliance for Incinerator Alternatives, 5 Gyres, and Break Free From Plastic. May 2019. pp. 82–85. Retrieved 20 May 2019.
102. Held, I.M. and Soden, B.J., 2000. Water vapor feedback and global warming. Annual review of energy and the environment, 25(1), pp.441–475.
103. Evans, Kimberly Masters (2005). "The greenhouse effect and climate change". The environment: a revolution in attitudes. Detroit: Thomson Gale. ISBN   978-0787690823.
104. "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2010" (PDF). U.S. Environmental Protection Agency. 15 April 2012. p. 1.4. Retrieved 2 June 2012.
105. Held, I.M.; Soden, B.J. (2000). "Water Vapor Feedback and Global Warming1". Annual Review of Energy and the Environment. 25: 441–75. CiteSeerX  . doi:10.1146/annurev.energy.25.1.441.
106. Includes the Kyoto "basket" of GHGs
107. "Introduction". 1.3.1 Review of the last three decades. in Rogner et al. 2007 This citation clarifies the time period (1970–2004) for the observed emissions trends.
108. Bridging the Emissions Gap: A UNEP Synthesis Report (PDF), Nairobi, Kenya: United Nations Environment Programme (UNEP), November 2011, ISBN   978-9280732290 UNEP Stock Number: DEW/1470/NA
109. "Global Greenhouse Gas Emissions Data". EPA . Retrieved 4 March 2014. The burning of coal, natural gas, and oil for electricity and heat is the largest single source of global greenhouse gas emissions.
110. "Selected Development Indicators" (PDF). World Development Report 2010: Development and Climate Change (PDF). Washington, DC: The International Bank for Reconstruction and Development / The World Bank. 2010. Tables A1 and A2. doi:10.1596/978-0-8213-7987-5. ISBN   978-0821379875.
111. Bader, N.; Bleichwitz, R. (2009). "Measuring urban greenhouse gas emissions: The challenge of comparability. S.A.P.I.EN.S.2 (3)". Sapiens.revues.org. Retrieved 11 September 2011.
112. Banuri, T. (1996). Equity and social considerations. In: Climate change 1995: Economic and social dimensions of climate change. Contribution of Working Group III to the Second Assessment Report of the Intergovernmental Panel on Climate Change (J.P. Bruce et al. Eds.) (PDF). This version: Printed by Cambridge University Press, Cambridge and New York. PDF version: IPCC website. doi:10.2277/0521568544. ISBN   978-0521568548.
113. World energy outlook 2007 edition – China and India insights. International Energy Agency (IEA), Head of Communication and Information Office, 9 rue de la Fédération, 75739 Paris Cedex 15, France. 2007. p. 600. ISBN   978-9264027305. Archived from the original on 15 June 2010. Retrieved 4 May 2010.
114. Holtz-Eakin, D. (1995). "Stoking the fires? CO
2
emissions and economic growth". Journal of Public Economics . 57 (1): 85–101. doi:10.1016/0047-2727(94)01449-X.
115. Dodman, David (April 2009). "Blaming cities for climate change? An analysis of urban greenhouse gas emissions inventories". Environment and Urbanization. 21 (1): 185–201. doi:10.1177/0956247809103016. ISSN   0956-2478.
116. B. Metz; O.R. Davidson; P.R. Bosch; R. Dave; L.A. Meyer (eds.), Annex I: Glossary J–P, archived from the original on 3 May 2010
117. Markandya, A. (2001). "7.3.5 Cost Implications of Alternative GHG Emission Reduction Options and Carbon Sinks". In B. Metz; et al. (eds.). Costing Methodologies. Climate Change 2001: Mitigation. Contribution of Working Group III to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Print version: Cambridge University Press, Cambridge and New York. This version: GRID-Arendal website. doi:10.2277/0521015022 (inactive 18 February 2019). ISBN   978-0521015028. Archived from the original on 5 August 2011. Retrieved 11 April 2011.
118. Herzog, T. (November 2006). Yamashita, M.B. (ed.). Target: intensity – an analysis of greenhouse gas intensity targets (PDF). World Resources Institute. ISBN   978-1569736388 . Retrieved 11 April 2011.
119. Botzen, W.J.W.; et al. (2008). "Cumulative CO
2
emissions: shifting international responsibilities for climate debt". Climate Policy. 8 (6): 570. doi:10.3763/cpol.2008.0539.
120. Höhne, N.; et al. (24 September 2010). "Contributions of individual countries' emissions to climate change and their uncertainty" (PDF). Climatic Change. 106 (3): 359–91. doi:10.1007/s10584-010-9930-6. Archived from the original (PDF) on 26 April 2012.
121. World Energy Outlook 2009 (PDF), Paris: International Energy Agency (IEA), 2009, pp. 179–80, ISBN   978-9264061309, archived from the original (PDF) on 24 September 2015, retrieved 27 December 2011
122. "Introduction", 1.3.1 Review of the last three decades
123. The cited paper uses the term "start date" instead of "base year."
124. "Global CO
2
emissions: annual increase halves in 2008"
. Netherlands Environmental Assessment Agency (PBL) website. 25 June 2009. Retrieved 5 May 2010.
125. "Global Carbon Mechanisms: Emerging lessons and implications (CTC748)". Carbon Trust. March 2009. p. 24. Retrieved 31 March 2010.
126. Vaughan, Adam (7 December 2015). "Global emissions to fall for first time during a period of economic growth". The Guardian. ISSN   0261-3077 . Retrieved 23 December 2016.
127. CO
2
Emissions From Fuel Combustion: Highlights (2011 edition)
, Paris, France: International Energy Agency (IEA), 2011, p. 9, archived from the original on 17 March 2017, retrieved 7 March 2012
128. Helm, D.; et al. (10 December 2007). Too Good To Be True? The UK's Climate Change Record (PDF). p. 3. Archived from the original (PDF) on 15 July 2011.
129. Davis, S.J.; K. Caldeira (8 March 2010). "Consumption-based Accounting of CO
2
Emissions"
(PDF). Proceedings of the National Academy of Sciences of the United States of America. 107 (12): 5687–5692. Bibcode:2010PNAS..107.5687D. doi:10.1073/pnas.0906974107. PMC  . PMID   20212122 . Retrieved 18 April 2011.
130. "International Carbon Flows". Carbon Trust. May 2011. Retrieved 12 November 2012.
131. e.g., Gupta et al. (2007) assessed the scientific literature on climate change mitigation policy: Gupta, S.; et al. Policies, instruments, and co-operative arrangements.
132. "Energy Policy". Paris: International Energy Agency (IEA). 2012.
133. "IEA Publications on 'Energy Policy'". Paris: Organization for Economic Co-operation and Development (OECD) / International Energy Agency (IEA). 2012.
134. Bridging the Emissions Gap: A UNEP Synthesis Report (PDF), Nairobi, Kenya: United Nations Environment Programme (UNEP), November 2011, ISBN   978-9280732290 UNEP Stock Number: DEW/1470/NA
135. "4. Energizing development without compromising the climate" (PDF). World Development Report 2010: Development and Climate Change (PDF). Washington, DC: The International Bank for Reconstruction and Development / The World Bank. 2010. p. 192, Box 4.2: Efficient and clean energy can be good for development. doi:10.1596/978-0-8213-7987-5. ISBN   978-0821379875.
136. Sixth compilation and synthesis of initial national communications from Parties not included in Annex I to the Convention. Note by the secretariat. Executive summary (PDF). Geneva, Switzerland: United Nations Framework Convention on Climate Change (UNFCCC). 2005. pp. 10–12.
137. Compilation and synthesis of fifth national communications. Executive summary. Note by the secretariat (PDF). Geneva (Switzerland): United Nations Framework Convention on Climate Change (UNFCCC). 2011. pp. 9–10.
138. Fisher, B.; et al. "3.1 Emissions scenarios". Issues related to mitigation in the long-term context.
139. "1.3.2 Future outlook". Introduction.
140. "1.3.2.4 Total GHG emissions". Introduction.
141. carbon dioxide, methane, nitrous oxide, sulfur hexafluoride
142. "Greenhouse Gas Emissions from a Typical Passenger Vehicle" (PDF). Epa.gov. US Environment Protection Agency. Retrieved 11 September 2011.
143. Engber, Daniel (1 November 2006). "How gasoline becomes CO
2
, Slate Magazine"
. Slate Magazine . Retrieved 11 September 2011.
144. "Volume calculation for carbon dioxide". Icbe.com. Retrieved 11 September 2011.
145. "Voluntary Reporting of Greenhouse Gases Program". Energy Information Administration. Archived from the original on 1 November 2004. Retrieved 21 August 2009.
146. Moomaw, W.; P. Burgherr; G. Heath; M. Lenzen; J. Nyboer; A. Verbruggen (2011). "Annex II: Methodology" (PDF). IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation: 10. Archived from the original (PDF) on 22 September 2014. Retrieved 17 June 2016.
147. Obersteiner M; Azar C; Kauppi P; et al. (October 2001). "Managing climate risk". Science. 294 (5543): 786–87. doi:10.1126/science.294.5543.786b. PMID   11681318.
148. Azar, C.; Lindgren, K.; Larson, E.D.; Möllersten, K. (2006). "Carbon capture and storage from fossil fuels and biomass – Costs and potential role in stabilising the atmosphere" (PDF). Climatic Change. 74 (1–3): 47–79. doi:10.1007/s10584-005-3484-7.
149. "Geoengineering the climate: science, governance and uncertainty". The Royal Society. 2009. Archived from the original on 7 September 2009. Retrieved 12 September 2009.
150. Fischer, B.S.; Nakicenovic, N.; Alfsen, K.; Morlot, J. Corfee; de la Chesnaye, F.; Hourcade, J.-Ch.; Jiang, K.; Kainuma, M.; La Rovere, E.; Matysek, A.; Rana, A.; Riahi, K.; Richels, R.; Rose, S.; van Vuuren, D.; Warren, R., Issues related to mitigation in the long term context (PDF)
151. Cook, J.; Nuccitelli, D.; Green, S.A.; Richardson, M.; Winkler, B.R.; Painting, R.; Way, R.; Jacobs, P.; Skuce, A. (2013). "Quantifying the consensus on anthropogenic global warming in the scientific literature". Environmental Research Letters. 8 (2): 024024. Bibcode:2013ERL.....8b4024C. doi:10.1088/1748-9326/8/2/024024.