Climate change: Greenhouse gases (CO2, CH4, N2O. CFCs : Source, trends and role),

GREENHOUSE GASES

Many chemical compounds present in Earth's atmosphere behave as 'greenhouse gases'. These are gases which allow direct sunlight (relative shortwave energy) to reach the Earth's surface unimpeded. As the shortwave energy (that in the visible and ultraviolet portion of the spectra) heats the surface, longer-wave (infrared) energy (heat) is re-radiated to the atmosphere. Greenhouse gases absorb this energy, thereby allowing less heat to escape back to space, and 'trapping' it in the lower atmosphere.

Many greenhouse gases occur naturally in the atmosphere, such as carbon dioxide, methane, 

water vapor, and nitrous oxide, while others are synthetic. Those that are man-made include the chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and Perfluorocarbons (PFCs), as well as sulfur hexafluoride (SF₆). Atmospheric concentrations of both the natural and man-made gases have been rising over the last few centuries due to the industrial revolution. As the global population has increased and our reliance on fossil fuels (such as coal, oil and natural gas) has been firmly solidified, so emissions of these gases have risen. While gases such as carbon dioxide occur naturally in the atmosphere, through our interference with the carbon cycle (through burning forest lands, or mining and burning coal), we artificially move carbon from solid storage to its gaseous state, thereby increasing atmospheric concentrations.

Greenhouse gases which reduce the loss of heat into space and therefore contribute to global temperatures through the greenhouse effect. Greenhouse gases are essential to maintaining the temperature of the Earth; without them the planet would be so cold as to be uninhabitable. However, an excess of greenhouse gases can raise the temperature of a planet to lethal levels, as on Venus where the 96.5% carbon dioxide (CO₂) atmosphere results in surface temperatures of about 467 °C (872 °F). Greenhouse gases are produced by many natural and industrial processes, which currently result in CO₂ levels of 380 ppmv (parts per million by volume) in the atmosphere.

Greenhouse Effect

The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's lower atmosphere and surface.

When sunlight reaches the surface of the Earth, some of it is absorbed and warms the surface. Because the Earth's surface is much cooler than the sun, it radiates energy at much longer wavelengths than the sun does, peaking in the infrared at about 10 µm. The atmosphere absorbs these longer wavelengths more effectively than it does the shorter wavelengths from the sun. The absorption of this longwave radiant energy warms the atmosphere; the atmosphere is also warmed by transfer of sensible and latent heat from the surface. Greenhouse gases also emit longwave radiation both upward to space and downward to the surface. The downward part of this longwave radiation emitted by the atmosphere is the "greenhouse effect". The term is a misnomer though, as this process is not the mechanism that warms greenhouses.

On earth, the most abundant greenhouse gases are, in order of relative abundance:

§  Water vapor, which causes about 36–70% of the greenhouse effect on Earth. (Note clouds typically affect climate differently from other forms of atmospheric water.)

§  Carbon dioxide, which causes 9–26%

§  Methane, which causes 4–9%

§  Ozone, which causes 3–7%

§  Other gases nitrous oxide, CFCs, etc.

(Note that this is a combination of the strength of the greenhouse effect of the gas and its abundance. For example, methane is a much stronger greenhouse gas than CO₂, but present in much smaller concentrations).

It is not possible to state that a certain gas causes a certain percentage of the greenhouse effect, because the influences of the various gases are not additive. (The higher ends of the ranges quoted are for the gas alone; the lower ends, for the gas counting overlaps.) Other greenhouse gases include, but are not limited to, nitrous oxide, sulfur hexafluoride, hydrofluorocarbons, perfluorocarbons and chlorofluorocarbons.

The major atmospheric constituents (nitrogen, N₂ and oxygen, O₂) are not greenhouse gases. This is because homonuclear diatomic molecules such as N₂ and O₂ neither absorb nor emit infrared radiation, as there is no net change in the dipole moment of these molecules when they vibrate. Molecular vibrations occur at energies that are of the same magnitude as the energy of the photons on infrared light. Heteronuclear diatomics such as CO or HCl absorb IR; however, 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.

Late 19th century scientists experimentally discovered that N₂ and O₂ did not absorb infrared radiation (called, at that time, "dark radiation") and that CO₂ and many other gases did absorb such radiation. It was recognized in the early 20th century that the known major greenhouse gases in the atmosphere caused the earth's temperature to be higher than it would have been without the greenhouse gases.

Greenhouse Gases : Trends And Role

1. Carbon Dioxide(CO2)

The natural production and absorption of carbon dioxide (CO₂) is achieved through the terrestrial biosphere and the ocean. However, humankind has altered the natural carbon cycle by burning coal, oil, natural gas and wood and since the industrial revolution began in the mid 1700s, each of these actvities has increased in scale and distribution. Carbon dioxide was the first greenhouse gas demonstrated to be increasing in atmospheric concentration with the first conclusive measurements being made in the last half of the 20th century. Prior to the industrial revolution, concentrations were fairly stable at 280 ppm. Today, they are around 370 ppm, an increase of well over 30 %. The atmospheric concentration has a marked seasonal oscillation that is mostly due to the greater extent of landmass in the northern hemisphere and its vegetation. A greater drawdown of CO₂ occurs in the northern hemisphere spring and summer as plants convert CO₂ to plant material through photosynthesis. It is then released again in the fall and winter as the plants decompose.

2. Methane (CH4)

Methane is an extrememly effective absorber of radiation, though its atmospheric concentration is less than CO₂ and its lifetime in the atmosphere is brief (10-12 years), compared to some other greenhouse gases (such as CO₂, N₂O, CFCs). Methane(CH₄) has both natural and anthropogenic sources. It is released as part of the biological processes in low oxygen environments, such as in swamplands or in rice production (at the roots of the plants). Over the last 50 years, human activities such as growing rice, raising cattle, using natural gas and mining coal have added to the atmospheric concentration of methane. Direct atmospheric measurement of atmospheric methane has been possible since the late 1970s and its conentration rose from 1.52 ppmv in 1978 by around 1 % per year to 1990, since when there has been little sustained increase. The current atmospheric concentration is ~1.77 ppmv, and there is no scientific consensus on why methane has not risen much since around 1990.

3. Tropospheric Ozone (O3)

Ultraviolet radiation and oxygen interact to form ozone in the stratosphere. Existing in a broad band, commonly called the 'ozone layer', a small fraction of this ozone naturally descends to the surface of the Earth. However, during the 20^th century, this tropospheric ozone has been supplemented by ozone created by human processes. The exhaust emissions from automobiles and pollution from factories (as well as burning vegetation) leads to greater concentrations of carbon and nitrogen molecules in the lower atmosphere which, when it they are acted on by sunlight, produce ozone. Consequently, ozone has higher concentrations in and around cities than in sparsely populated areas, though there is some transport of ozone downwind of major urban areas. Ozone is an important contributor to photochemical smog. Concentrations of ozone have risen by around 30% since the pre-industrial era, and is now considered by the IPCC to be the third most important greenhouse gas after carbon dioxide and methane. An additional complication of ozone is that it also interacts with and is modulated by concentrations of methane.

4. Nitrous Oxide

Concentrations of nitrous oxide also began to rise at the beginning of the industrial revolution and is understood to be produced by microbial processes in soil and water, including those reactions which occur in fertilizer containing nitrogen. Increasing use of these fertilizers has been made over the last century. Global concentration for N₂O in 1998 was 314 ppb, and in addition to agricultural sources for the gas, some industrial processes (fossil fuel-fired power plants, nylon production, nitric acid production and vehicle emissions) also contribute to its atmospheric load.

5. CFCs etc.

CFCs (chlorofluorocarbons) have no natural source but were entirely synthesized for such diverse uses as refrigerants, aerosol propellants, and cleaning solvents. Their creation was in 1928 and since then concentrations of CFCs in the atmosphere have been rising. Due to the discovery that they are able to destroy stratospheric ozone, a global effort to halt their production was undertaken and was extremely successful. So much so that levels of the major CFCs are now remaining level or declining. However, their long atmospheric lifetimes determine that some concentration of the CFCs will remain in the atmosphere for over 100 years. Since they are also greenhouse gas, along with such other long-lived synthesized gases as CF₄ (carbon tetrafluoride), SF₆ (sulfur hexafluoride), they are of concern. Another set of synthesized compounds called HFCs (hydrofluorocarbons) are also greenhouse gases, though they are less stable in the atmosphere and therefore have a shorter lifetime and less of an impact as a greenhouse gas.

6. Carbon Monoxide (CO) and other reactive gases

Carbon monoxide (CO) is not considered a direct greenhouse gas, mostly because it does not absorb terrestrial thermal IR energy strongly enough. However, CO is able to modulate the production of methane and tropospheric ozone. The Northern Hemisphere contains about twice as much CO as the Southern Hemisphere because as much as half of the global burden of CO is derived from human activity, which is predominantly located in the NH. Due to the spatial variability of CO, it is difficult to ascertain global concentrations, however, it appears as though they were generally increasing until the late 1980s, and have since begun to decline somewhat. One possible explanation is the reduction in vehicle emissions of CO since greater use of catalytic converters has been made.

Volatile Organic Compounds (VOCs) also have a small direct impact as greenhouse gases, as well being involved in chemical processes which modulate ozone production. VOCs include non-methane hydrocarbons (NMHC), and oxygenated NMHCs (eg. alcohols and organic acids), and their largest source is natural emissions from vegetation. However, there are some anthropogenic sources such as vehicle emissions, fuel production, and biomass burning. Though the measurement of VOCs is extremely difficult, it is expected that most anthropogenic emissions of these compounds have increased in recent decades.

Sources of the Greenhouse Gases

Natural and anthropogenic

Most greenhouse gases have both natural and anthropogenic sources. During the pre-industrial Holocene, concentrations of these gases were roughly constant. Since the industrial revolution, concentrations of all the long-lived greenhouse gases have increased due to human actions.

Gas	Preindustrial Level	Current Level	Increase since 1750	Radiative forcing (W/m2)
CO2	280 ppm	384ppm	104 ppm	1.46
CH4	700 ppb	1,745 ppb	1,045 ppb	0.48
N2O	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. Before the ice core record, direct measurements do not exist. Various proxies and modeling suggest large variations; 500 Myr ago CO₂ levels were likely 10 times higher than now. Indeed higher CO₂ concentrations are thought to have prevailed throughout most of the Phanerozoic ion, 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 Mya. 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 stabilizing feedbacks. 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 Mya, 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 1mm per day. 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

Anthropogenic greenhouse gases

Since about 1750 human activity has increased the concentration of carbon dioxide and of some other important greenhouse gases. Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity, but over periods longer than a few years natural sources are closely balanced by natural sinks such as weathering of continental rocks and 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.

Some of the main sources of greenhouse gases due to human activity include:

§  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.

§  livestock enteric fermentation and manure management, 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 concentrations.

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

§  Solid fuels (e.g. coal): 35%

§  Liquid fuels (e.g. gasoline): 36%

§  Gaseous fuels (e.g. natural gas): 20%

§  Flaring gas industrially and at wells: <1%

§  Cement production: 3%

§  Non-fuel hydrocarbons: <1%

§  The "international bunkers" of shipping and air transport not included in national inventories: 4%

The USEPA (United State Envvironment Protection Agency) ranks the major greenhouse gas contributing end-user sectors in the following order: industrial, transportation, residential, commercial and agricultural. Major sources of an individual's greenhouse gases include home heating and cooling, electricity consumption, and transportation. Corresponding conservation measures are improving home building insulation, 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.

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.

Greenhouse Gas Emissions

Measurements from Antarctic ice cores show that just before industrial emissions started, atmospheric CO₂ levels were about 280 parts per million by volume (ppm; the units µL/L are occasionally used and are identical to parts per million by volume). From the same ice cores it appears that CO₂ concentrations stayed between 260 and 280 ppm during the preceding 10,000 years. Studies using evidence from stomata of fossilized leaves suggest greater variability, with CO₂ levels above 300 ppm during the period 7,000–10,000 years ago, though others have argued that these findings more likely reflect calibration/ contamination problems rather than actual CO₂ variability.

Since the beginning of the Industrial Revolution, the concentrations of many of the greenhouse gases have increased. The concentration of CO₂ has increased by about 100 ppm (i.e., from 280 ppm to 380 ppm). The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973; the next 50 ppm increase took place in about 33 years, from 1973 to 2006. The greenhouse gases with the largest radiative forcing are:

Relevant to radiative forcing
Gas	Current (1998) Amount by volume	Increase over pre-industrial (1750)	Percentage increase	Radiative forcing (W/m²)
CO2	365 ppm {383 ppm (2007.01)}	87 ppm {105 ppm (2007.01)}	31% {37.77 % (2007.01)}	1.46 {~1.532 (2007.01)}
CH4	1,745 ppb	1,045 ppb	150%	0.48
N2O	314 ppb	44 ppb	16%	0.15

(Source: IPCC radiative forcing report 1994 updated (to 1998 & 2003) by IPCC TAR table 6.1).

Recent rates of change and emission

The sharp acceleration in CO₂ emissions since 2000 of >3% y⁻¹ (>2 ppm y⁻¹) from 1.1% y⁻¹ during the 1990s is attributable to the lapse of formerly declining trends in carbon intensity of both developing and developed nations. Although over 3/4 of cumulative anthropogenic CO₂ is still attributable to the developed world, 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. In comparison, methane has not increased appreciably, and N₂O by 0.25% y⁻¹.

Role of Greenhouse Gases on Climate Change

Given the natural variability of the Earth’s climate, it is difficult to determine the extent of change that humans cause. In computer-based models, rising concentrations of greenhouse gases generally produce an increase in the average temperature of the Earth. Rising temperatures may, in turn, produce changes in weather, sea levels, and land use patterns, commonly referred to as “climate change.”

Assessments generally suggest that the Earth’s climate has warmed over the past century and that human activity affecting the atmosphere is likely an important driving factor. A National Research Council study dated May 2001 stated, “Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.”

However, there is uncertainty in how the climate system varies naturally and reacts to emissions of greenhouse gases. Making progress in reducing uncertainties in projections of future climate will require better awareness and understanding of the buildup of greenhouse gases in the atmosphere and the behavior of the climate system.