Greenhouse gas

A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect.[1] The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. In the Solar System, the atmospheres of Venus, Mars, and Titan also contain gases that cause greenhouse effects. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth's surface would be on average about 33 °C (59 °F)[note 1] colder than at present.[2][3][4]

Since the beginning of the Industrial revolution, the burning of fossil fuels has increased the levels of carbon dioxide in the atmosphere from 280ppm to 390ppm.[5][6]
Contents
[hide]

* 1 Greenhouse effects in Earth's atmosphere
* 2 Natural and anthropogenic sources
* 3 Anthropogenic greenhouse gases
* 4 Role of water vapor
* 5 Greenhouse gas emissions
o 5.1 Regional and national attribution of emissions
+ 5.1.1 Cumulative emissions
+ 5.1.2 Changes since a particular base year
+ 5.1.3 Annual and per capita emissions
+ 5.1.4 Top emitters
+ 5.1.5 Effect of policy
+ 5.1.6 Projections
o 5.2 Relative CO2 emission from various fuels
* 6 Removal from the atmosphere and global warming potential
o 6.1 Natural processes
o 6.2 Atmospheric lifetime
o 6.3 Global warming potential
o 6.4 Airborne fraction
o 6.5 Negative emissions
* 7 Related effects
* 8 See also
* 9 Notes
* 10 References
* 11 External links

[edit] Greenhouse effects in Earth's atmosphere
Main article: Greenhouse effect
Modern global anthropogenic Carbon emissions.

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

* water vapor
* carbon dioxide
* methane
* nitrous oxide
* ozone
* chlorofluorocarbons

The contribution to the greenhouse effect by a gas is affected by both the characteristics of the gas and its abundance. For example, on a molecule-for-molecule basis methane is about eighty times stronger greenhouse gas than carbon dioxide ,[7] but it is present in much smaller concentrations so that its total contribution is smaller. When these gases are ranked by their contribution to the greenhouse effect, the most important are:[8]
Gas
Formula
Contribution
(%)
Water Vapor H2O 36 – 72 %
Carbon Dioxide CO2 9 – 26 %
Methane CH4 4 – 9 %
Ozone O3 3 – 7 %

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.[8][9] The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the greenhouse gases.[8][9]

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.[10]
Atmospheric absorption and scattering at different electromagnetic wavelengths. The largest absorption band of carbon dioxide is in the infrared.

Scientists who have elaborated on Arrhenius' theory of global warming are concerned that increasing concentrations of greenhouse gases in the atmosphere are causing an unprecedented rise in global temperatures, with potentially harmful consequences for the environment and human health.[11] Although contributing to many other physical and chemical reactions, the major atmospheric constituents, nitrogen (N2), oxygen (O2), and argon (Ar), are not greenhouse gases. This is because molecules containing two atoms of the same element such as N2 and O2 and monatomic molecules such as Ar have no net change in their dipole moment when they vibrate and hence are almost totally unaffected by infrared light. Although molecules containing two atoms of different elements such as carbon monoxide (CO) or hydrogen chloride (HCl) absorb IR, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. As a consequence they do not contribute significantly to the greenhouse effect and are not often included when discussing greenhouse gases.

Late 19th century scientists experimentally discovered that N2 and O2 do not absorb infrared radiation (called, at that time, "dark radiation") while, at the contrary, water, as true vapour or condensed in the form of microscopic droplets suspended in clouds, CO2 and other poly-atomic gaseous molecules do absorb infrared radiation. It was recognized in the early 20th century that the greenhouse gases in the atmosphere caused the Earth's overall temperature to be higher than what it would be without them.
[edit] Natural and anthropogenic sources
400,000 years of ice core data.
Top: Increasing atmospheric carbon dioxide levels as measured in the atmosphere and reflected in ice cores. Bottom: The amount of net carbon increase in the atmosphere, compared to carbon emissions from burning fossil fuel.

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. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.[12][13]

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".[14] In AR4, "most of" is defined as more than 50%.
Gas Preindustrial level Current level Increase since 1750 Radiative forcing (W/m2)
Carbon dioxide 280 ppm 388 ppm 108 ppm 1.46
Methane 700 ppb 1745 ppb 1045 ppb 0.48
Nitrous oxide 270 ppb 314 ppb 44 ppb 0.15
CFC-12 0 533 ppt 533 ppt 0.17

Ice cores provide evidence for variation in greenhouse gas concentrations over the past 800,000 years. Both CO2 and CH4 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 which indicates CO2 levels staying within a range of between 180ppm and 280ppm 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 CO2 levels were likely 10 times higher than now.[15] Indeed higher CO2 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.[16][17][18] The spread of land plants is thought to have reduced CO2 concentrations during the late Devonian, and plant activities as both sources and sinks of CO2 have since been important in providing stabilising feedbacks.[19] 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 which raised the CO2 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.[20] 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 only about 1% of emissions from human sources.[20][21]
[edit] Anthropogenic greenhouse gases
Global anthropogenic greenhouse gas emissions broken down into 8 different sectors for the year 2000.
Per capita anthropogenic greenhouse gas emissions by country for the year 2000 including land-use change.

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 ppmv higher than pre-industrial levels.[22] Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,[23] 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 concentration 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.[24]

It is likely that anthropogenic warming, such as that due to elevated greenhouse gas levels, has had a discernible influence on many physical and biological systems. Warming is projected to affect various issues such as freshwater resources, industry, food and health.[25]

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

* burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations. Land use change (mainly deforestation in the tropics) account for up to one third of total anthropogenic CO2 emissions.[24]
* livestock enteric fermentation and manure management,[26] paddy rice farming, land use and wetland changes, 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 (N2O) concentrations.

The seven sources of CO2 from fossil fuel combustion are (with percentage contributions for 2000–2004):[27]
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 4 %

The US Environmental Protection Agency (EPA) ranks the major greenhouse gas contributing end-user sectors in the following order: industrial, transportation, residential, commercial and agricultural.[28] Major sources of an individual's greenhouse gas include home heating and cooling, electricity consumption, and transportation. Corresponding conservation measures are improving home building insulation, installing geothermal heat pumps and compact fluorescent lamps, and choosing energy-efficient vehicles.

Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases (sulfur hexafluoride, HFCs, and PFCs) are the major greenhouse gases and the subject of the Kyoto Protocol, which came into force in 2005.[29]

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 December 7, 2009, the US Environmental Protection Agency released its final findings on greenhouse gases, declaring that "greenhouse gases (GHGs) threaten the public health and welfare of the American people". The finding applied to the same "six key well-mixed greenhouse gases" named in the Kyoto Protocol: carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.[30][31]
[edit] Role of water vapor
Main article: water vapor
Increasing water vapor in the stratosphere at Boulder, Colorado.

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.[9] Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields. According to the Environmental Health Center of the National Safety Council, water vapor constitutes as much as 2% of the atmosphere.[32]

The Clausius-Clapeyron relation establishes that air can hold more water vapor per unit volume when it warms. 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. Because water vapor is a greenhouse gas this results in further warming, a "positive feedback" that amplifies the original warming. This positive feedback does not result in runaway global warming because it is offset by other processes that induce negative feedbacks, which stabilizes average global temperatures.
[edit] Greenhouse gas emissions
Main articles: List of countries by carbon dioxide emissions, List of countries by carbon dioxide emissions per capita, List of countries by greenhouse gas emissions, and List of countries by greenhouse gas emissions per capita

Measurements from Antarctic ice cores show that before industrial emissions started atmospheric CO2 levels were about 280 parts per million by volume (ppmv), and stayed between 260 and 280 during the preceding ten thousand years.[33] Carbon dioxide concentrations 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 levels above 300 ppm during the period seven to ten thousand years ago,[34] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO2 variability.[35][36] 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 CO2.

Since the beginning of the Industrial Revolution, the concentrations of most of the greenhouse gases have increased. For example, the concentration of carbon dioxide has increased by about 36% to 380 ppmv, or 100 ppmv over modern pre-industrial levels. The first 50 ppmv increase took place in about 200 years, from the start of the Industrial Revolution to around 1973; however the next 50 ppmv increase took place in about 33 years, from 1973 to 2006.[37]

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.[38]

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.
Relevant to radiative forcing Gas Current (1998)
Amount by volume Increase
(absolute, ppm)
over pre-industrial (1750) Increase
(relative, %)
over pre-industrial (1750) Radiative
forcing
(W/m2)
Carbon dioxide 365 ppm
(383 ppm, 2007.01) 87 ppm
(105 ppm, 2007.01) 31 %
(38 %, 2007.01) 1.46
(~1.53, 2007.01)
Methane 1745 ppb 1045 ppb 67 % 0.48
Nitrous oxide 314 ppb 44 ppb 16 % 0.15
Relevant to both radiative forcing and ozone depletion; all of the following have no natural sources and hence zero amounts pre-industrial Gas Current (1998)
Amount by volume Radiative forcing
(W/m2)
CFC-11 268 ppt 0.07
CFC-12 533 ppt 0.17
CFC-113 84 ppt 0.03
Carbon tetrachloride 102 ppt 0.01
HCFC-22 69 ppt 0.03

(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table 6.1[39][40] ).
[edit] Regional and national attribution of emissions
Greenhouse gas intensity in 2000 including land-use change.
Per capita responsibility for current anthropogenic atmospheric CO2.
Major greenhouse gas trends.
See also: Kyoto Protocol and government action

There are several different ways of measuring GHG emissions (see World Bank (2010, p. 362) for a table of national emissions data).[41]

Some variables that have been reported[42] 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 that caused the emissions to be produced. These two principles would result in different totals when measuring for example the importation of electricity from one country to another or the emissions at an international airport.
* The time horizon of different GHGs. Contribution of a given GHG is reported as a CO2 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 take into account new information.
* What sectors are included in the calculation (e.g. energy industries, industrical 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, the mass balance method, the predictive emissions monitoring system and the continuing emissions monitoring systems. The methods differ in accuracy, but also in cost and usability.

The different measures are sometimes used by different countries in asserting various policy/ethical positions to do with climate change (Banuri et al., 1996, p. 94).[43] This 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[44].


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 GHGs (IEA, 2007, p. 199).[45]

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) (Grubb, 2003, pp. 146, 149).[46] 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 (World Bank, 2010, p. 370). Per capita emissions may be based on historical or annual emissions (Banuri et al., 1996, pp. 106–107).
[edit] Cumulative emissions

Over the 1900-2005 period, the US was the world's largest cumulative emitter of energy-related CO2 emissions, and accounted for 30% of total cumulative emissions (IEA, 2007, p. 201).[45] The second largest emitter was the EU, at 23%; the third largest was China, at 8%; fourth was Japan, at 4%; fifth was India, at 2%. The rest of the world accounted for 33% of global, cumulative, energy-related CO2 emissions.
[edit] Changes since a particular base year

In total, Annex I Parties managed a cut of 3.3% in GHG emissions between 1990 and 2004 (UNFCCC, 2007, p. 11).[47] Annex I Parties are those countries listed in Annex I of the UNFCCC, and are the industrialized countries. For non-Annex I Parties, emissions in several large developing countries and fast growing economies (China, India, Thailand, Indonesia, Egypt, and Iran) GHG emissions have increased rapidly over this period (PBL, 2009).[48]

The sharp acceleration in CO2 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.[27] In comparison, methane has not increased appreciably, and N2O by 0.25% y−1.
[edit] Annual and per capita emissions

At the present time, total annual emissions of GHGs are rising (Rogner et al., 2007).[49] Between the period 1970 to 2004, emissions increased at an average rate of 1.6% per year, with CO2 emissions from the use of fossil fuels growing at a rate of 1.9% per year.

Per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries (Grubb, 2003, p. 144).[46] Due to China's fast economic development, its per capita emissions are quickly approaching the levels of those in the Annex I group of the Kyoto Protocol (PBL, 2009).[50] Other countries with fast growing emissions are South Korea, Iran, and Australia. On the other hand, per capita emissions of the EU-15 and the USA are gradually decreasing over time. Emissions in Russia and the Ukraine have decreased fastest since 1990 due to economic restructuring in these countries (Carbon Trust, 2009, p. 24).[51]

Energy statistics for fast growing economies are less accurate than those for the industrialized countries. For China's annual emissions in 2008, PBL (2008) estimated an uncertainty range of about 10%.
[edit] Top emitters

In 2005, the world's top-20 emitters comprised 80% of total GHG emissions (PBL, 2010. See notes for the following table).[52] Tabulated below are the top-5 emitters for the year 2005 (MNP, 2007).[53] The second column is the country's or region's share of the global total of annual emissions. The third column is the country's or region's average annual per capita emissions, in tonnes of GHG per head of population:
Top-5 emitters for the year 2005 Country or region % of global total
annual emissions Tonnes of GHG
per capita
Chinab 17 % 5.8
United Statesa 16 % 24.1
European Union-27a 11 % 10.6
Indonesiac 6 % 12.9
India 5 % 2.1

Table footnotes:

* These values are for the GHG emissions from fossil fuel use and cement production. Calculations are for carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and gases containing fluorine (the F-gases HFCs, PFCs and SF6).
* These estimates are subject to large uncertainties regarding CO2 emissions from deforestation; and the per country emissions of other GHGs (e.g., methane). There are also other large uncertainties which mean that small differences between countries are not significant. CO2 emissions from the decay of remaining biomass after biomass burning/deforestation are not included.
* a Industrialised countries: official country data reported to UNFCCC.
* b Excluding underground fires.
* c Including an estimate of 2000 million tonnes CO2 from peat fires and decomposition of peat soils after draining. However, the uncertainty range is very large.

[edit] Effect of policy

Rogner et al. (2007) assessed the effectiveness of policies to reduce emissions (mitigation of climate change).[49] They concluded that mitigation policies undertaken by UNFCCC Parties were inadequate to reverse the trend of increasing GHG emissions. The impacts of population growth, economic development, technological investment, and consumption had overwhelmed improvements in energy intensities and efforts to decarbonize (energy intensity is a country's total primary energy supply (TPES) per unit of GDP (Rogner et al., 2007).[54] TPES is a measure of commercial energy consumption (World Bank, 2010, p. 371)).[41]
[edit] Projections

Based on then-current energy policies, Rogner et al. (2007) projected that energy-related CO2 emissions in 2030 would be 40-110% higher than in 2000.[49] Two-thirds of this increase was projected to come from non-Annex I countries. Per capita emissions in Annex I countries were still projected to remain substantially higher than per capita emissions in non-Annex I countries. Projections consistently showed a 25-90% increase in the Kyoto gases (carbon dioxide, methane, nitrous oxide, sulphur hexafluoride) compared to 2000.

process improvement method
IIT Coaching

Since the beginning of the Industrial revolution, the burning of fossil fuels has increased the levels of carbon dioxide in the atmosphere from 280ppm to 390ppm.[5][6]








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