Greenhouse gas

The major constituents of Earth's atmosphere, nitrogen (N
₂)(78%), oxygen (O
₂)(21%), and argon (Ar)(0.9%), 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 or solubility. Therefore, they do not contribute significantly to the greenhouse effect and often are omitted when discussing greenhouse gases.

Atmospheric absorption and scattering at different wavelengths of electromagnetic waves. The largest absorption band of carbon dioxide is not far from the maximum in the thermal emission from ground, and it partly closes the window of transparency of water; hence its major effect.

Greenhouse gases are those that absorb and emit infrared radiation in the wavelength range emitted by Earth.[1] Carbon dioxide (0.04%), nitrous oxide, methane and ozone are trace gases that account for almost 0.1% of Earth's atmosphere and have an appreciable greenhouse effect.

In order, the most abundant greenhouse gases in Earth's atmosphere are:

• Water vapor (H
  ₂O)
• Carbon dioxide (CO
  ₂)
• Methane (CH
  ₄)
• Nitrous oxide (N
  ₂O)
• Ozone (O
  ₃)
• Chlorofluorocarbons (CFCs)
• Hydrofluorocarbons (includes HCFCs and HFCs)

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).[16] 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.[17]

The false colors in this image represent concentrations of carbon monoxide in the lower atmosphere, ranging from about 390 parts per billion (dark brown pixels), to 220 parts per billion (red pixels), to 50 parts per billion (blue pixels).[18]

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 (wavelength 15 microns, or wavenumber 667 cm⁻¹). On the other hand, the single CO vibrational band only absorbs IR at much shorter wavelengths (4.7 microns, or 2145 cm⁻¹), where the emission of radiant energy from Earth's surface is at least a factor of ten lower. Oxidation of methane to 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.[19]

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.[20] 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.[19]

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.[21][22]

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.[21][22] In addition, some gases, such as methane, are known to have large indirect effects that are still being quantified.[31]

Aside from water vapor, which has a residence time of about nine days,[32] major greenhouse gases are well mixed and take many years to leave the atmosphere.[33] 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)[34] 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): ${\displaystyle \tau ={\frac {m}{F_{out}+L+D}}}$.[34] If input 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.[35][25] 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.[36][37][38] 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.[25]

The radiative forcing (warming influence) of long-lived greenhouse gases in earth's atmosphere is undergoing accelerating growth. Nearly one-third of the industrial-era increase ending year 2019 accumulated over just the prior 30 years.[39][40]

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. Earth's surface temperature depends on this balance between incoming and outgoing energy. If this energy balance is shifted, Earth's surface becomes warmer or cooler, leading to a variety of changes in global climate.[41]

A number of natural and man-made mechanisms can affect the global energy balance and force changes in Earth's climate. Greenhouse gases are one such mechanism. 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.[41] 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. Radiative forcing quantifies (in Watts per square meter) the effect of factors that influence Earth's energy balance; including changes in the concentrations of greenhouse gases. Positive radiative forcing leads to warming by increasing the net incoming energy, whereas negative radiative forcing leads to cooling.[42]

The Annual Greenhouse Gas Index (AGGI) is defined by atmospheric scientists at NOAA as the ratio of total direct radiative forcing due to long-lived and well-mixed greenhouse gases for any year for which adequate global measurements exist, to that present in year 1990.[40][43] These radiative forcing levels are relative to those present in year 1750 (i.e. prior to the start of the industrial era). 1990 is chosen because it is the baseline year for the Kyoto Protocol, and is the publication year of the first IPCC Scientific Assessment of Climate Change. As such, NOAA states that the AGGI "measures the commitment that (global) society has already made to living in a changing climate. It is based on the highest quality atmospheric observations from sites around the world. Its uncertainty is very low."[44]

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.[45] 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.[46] 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:

The use of CFC-12 (except some essential uses) has been phased out due to its ozone depleting properties.[47] The phasing-out of less active HCFC-compounds will be completed in 2030.[48]

Carbon dioxide in Earth's atmosphere if half of global-warming emissions[49][50] are not absorbed.
(NASA simulation; 9 November 2015)

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.[75] 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,[76] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO
2 variability.[77][78] 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.

Recent year-to-year increase of atmospheric CO
2.

Major greenhouse gas trends.

Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased. For example, the mole fraction of carbon dioxide has increased from 280 ppm to 415 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.[79][80]

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.[81]

Total cumulative emissions from 1870 to 2017 were 425±20 GtC (1539 GtCO₂) from fossil fuels 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%.[82]

Today, the stock of carbon in the atmosphere increases by more than 3 million tonnes per annum (0.04%) compared with the existing stock. This increase is the result of human activities by burning fossil fuels, deforestation and forest degradation in tropical and boreal regions.[83]

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.

Chart showing 2016 global greenhouse gas emissions by sector.[106] Percentages are calculated from estimated global emissions of all Kyoto Greenhouse Gases, converted to CO₂ equivalent quantities (GtCO₂e).

Global greenhouse gas emissions can be attributed to different sectors of the economy. This provides a picture of the varying contributions of different types of economic activity to global warming, and helps in understanding the changes required to mitigate climate change.

Manmade greenhouse gas emissions can be divided into those that arise from the combustion of fuels to produce energy, and those generated by other processes. Around two thirds of greenhouse gas emissions arise from the combustion of fuels.[107]

Energy may be produced at the point of consumption, or by a generator for consumption by others. Thus emissions arising from energy production may be categorised according to where they are emitted, or where the resulting energy is consumed. If emissions are attributed at the point of production, then electricity generators contribute about 25% of global greenhouse gas emissions.[108] If these emissions are attributed to the final consumer then 24% of total emissions arise from manufacturing and construction, 17% from transportation, 11% from domestic consumers, and 7% from commercial consumers.[109] Around 4% of emissions arise from the energy consumed by the energy and fuel industry itself.

The remaining third of emissions arise from processes other than energy production. 12% of total emissions arise from agriculture, 7% from land use change and forestry, 6% from industrial processes, and 3% from waste.[107] Around 6% of emissions are fugitive emissions, which are waste gases released by the extraction of fossil fuels.

As of 2020 Secunda CTL is the world's largest single emitter, at 56.5 million tonnes CO2 a year.[110]

Electricity generation emits over a quarter of global greenhouse gases.[111] Coal-fired power stations are the single largest emitter, with over 10 Gt CO
2 in 2018.[112] Although much less polluting than coal plants, natural gas-fired power plants are also major emitters.[113]

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.[114]

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%.[115] 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.[116]

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 mass units of carbon dioxide are emitted for each mass unit of polyethylene terephthalate (PET) produced—the type of plastic most commonly used for beverage bottles,[117] the transportation produce greenhouse gases also.[118] 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.[119][120]

From the other side, if it is placed in a landfill, it becomes a carbon sink[121] although biodegradable plastics have caused methane emissions.[122] 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 will contribute greenhouse gases in the equivalent of 850 million tonnes of carbon dioxide (CO₂) to the atmosphere in 2019. In current trend, annual emissions will grow to 1.34 billion tonnes by 2030. By 2050 plastic could emit 56 billion tonnes of Greenhouse gas emissions, as much as 14 percent of the Earth's remaining carbon budget.[123] 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.[124]

The pharmaceutical industry emitted 52 megatonnes of carbon dioxide into the atmosphere in 2015. This is more than the automotive sector. However this analysis used the combined emissions of conglomerates which produce pharmaceuticals as well as other products.[125]

Approximately 3.5% of the overall human impact on climate are from the aviation sector. The impact of the sector on climate in the late 20 years had doubled, but the part of the contribution of the sector in comparison to other sectors did not change because other sectors grew as well.[126]

In 2017 the digital sector produced 3.3% of global GHG emissions, above civil aviation (2%). In 2020 this is expected to reach 4%, the equivalent emissions of India in 2015.[127][128]

Wastewater as well as sanitation systems are known to contribute to greenhouse-gas emissions (GHG) mainly through the breakdown of excreta during the treatment process. This results in the generation of methane gas, that is then released into the environment. Emissions from the sanitation and wastewater sector have been focused mainly on treatment systems, particularly treatment plants, and this accounts for the bulk of the carbon footprint for the sector.[129]

In as much as climate impacts from wastewater and sanitation systems present global risks, low-income countries experience greater risks in many cases.  In recent years, attention to adaptation needs within the sanitation sector is just beginning to gain momentum.[130]

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

There are several ways of measuring greenhouse gas emissions, for example, see World Bank (2010)[131]^:362 for tables of national emissions data. Some variables that have been reported[132] 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 measures are sometimes used by countries to assert various policy/ethical positions on climate change (Banuri et al., 1996, p. 94).[133] 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.[132]

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).[134]

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.[135]

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).[99]^{: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.[131]^:370 Per capita emissions may be based on historical or annual emissions (Banuri et al., 1996, pp. 106–07).[133]

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.[136]

Greenhouse gas emissions from agriculture, forestry and other land use, 1970–2010.

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.[137] 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).[133]

There are substantial uncertainties in the measurement of net carbon emissions.[138] Additionally, there is controversy over how carbon sinks should be allocated between different regions and over time (Banuri et al., 1996, p. 93).[133] For instance, concentrating on more recent changes in carbon sinks is likely to favour those regions that have deforested earlier, e.g., Europe.

Carbon intensity of GDP (using PPP) 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.[139] Emission intensities may be calculated using market exchange rates (MER) or purchasing power parity (PPP) (Banuri et al., 1996, p. 96).[133] Calculations based on MER show large differences in intensities between developed and developing countries, whereas calculations based on PPP show smaller differences.

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,[140] and give some indication of which countries have contributed most to human-induced climate change.[141]^:15 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)[133] 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).[133] 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.[142]^: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%.[142]^:179–80

Between 1970 and 2004, global growth in annual CO
2 emissions was driven by North America, Asia, and the Middle East.[143] 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.[98] In comparison, methane has not increased appreciably, and N
₂O by 0.25% y⁻¹.

Using different base years for measuring emissions has an effect on estimates of national contributions to global warming.[141]^:17–18[144] 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 base years of 1750, 1900, 1950, and 1990 has a significant effect for most countries.[141]^: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).

Per capita anthropogenic greenhouse gas emissions by country for the year 2000 including land-use change.

Annual per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries.[99]^: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).[145] 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 per capita 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.[145] Emissions in Russia and Ukraine have decreased fastest since 1990 due to economic restructuring in these countries.[146]

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%.[145]

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.[147]

In 2009, the annual top ten emitting countries accounted for about two-thirds of the world's annual energy-related CO
2 emissions.[148]

Top-10 annual CO
2 emitters for the year 2017[149]

Country	% of global total annual emissions	Total 2017 CO2 Emissions (kilotons)[150]	Tonnes of GHG per capita[151]
China	29.3	10,877,217	7.7
United States	13.8	5,107,393	15.7
India	6.6	2,454,773	1.8
Russia	4.8	1,764,865	12.2
Japan	3.6	1,320,776	10.4
Germany	2.1	796,528	9.7
South Korea	1.8	673,323	13.2
Iran	1.8	671,450	8.2
Saudi Arabia	1.7	638,761	19.3
Canada	1.7	617,300	16.9

One way of attributing greenhouse gas 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.[152] 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.[142]^:179[153]^: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)[153]^: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).[153]^: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.[154]

Governments have taken action to reduce greenhouse gas emissions to mitigate climate change. Assessments of policy effectiveness have included work by the Intergovernmental Panel on Climate Change,[155] International Energy Agency,[156][157] and United Nations Environment Programme.[158] Policies implemented by governments have included[159][160][161] national and regional targets to reduce emissions, promoting energy efficiency, and support for a renewable energy transition 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.[161]^:3 Analysis by the UNFCCC (2011)[161]^: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.[161]^:8

A wide range of projections of future emissions have been produced.[162] Rogner et al. (2007)[163] assessed the scientific literature on greenhouse gas projections. Rogner et al. (2007)[164] 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.[164] 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.[164] 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).[165] Projections consistently showed increase in annual world emissions of "Kyoto" gases,[166] measured in CO
2-equivalent) of 25–90% by 2030, compared to 2000.[164]

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).[167][168][169]

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 components 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 components 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).

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 and carbon dioxide air capture,[172] or to the soil as in the case with biochar.[172] The IPCC has pointed out that many long-term climate scenario models require large-scale man-made negative emissions to avoid serious climate change.[173]

• NOAA CMDL CCGG – Interactive Atmospheric Data Visualization NOAA CO
  2 data
• Carbon Dioxide Information Analysis Center (CDIAC)
• NASA's Orbiting Carbon Observatory