Greenhouse gas emissions source and sinks- IPCC 1992

Greenhouse gases are the sole reason for the temperature increase in the environment. The Intergovernmental Panel on Climate Change(IPCC) assessed the sources and sink of these gases. This report was published in 1992 which was an update on the IPCC first assessment report. It neither confirmed or justified the first assessment report. But it was important because it led to an increase in a deep understanding of this global problem. Increase in global heat mainly driven by CO2 emissions has fostered widespread concern.

Carbon dioxide

The two main reasons for the increase in CO2 is fossil fuel consumption and land-use change emission. The key issue is to identify the processes controlling carbon in and out of the atmosphere and how to measure them effectively. Global land-use change(especially deforestation) and carbon fluctuation with process controlling release and uptake(particularly in the biosphere and the oceans) are two concerns are dominant. For 1991, the best global estimate is approximately 355ppmv(parts per million by volume), which means an increase of 1.8ppmv/yr. There is a gradient in concentration of carbon from the South pole to the Arctic circle. This is particularly due to Northern Hemisphere, where about 90% of the emissions occur.

Table for sources and sinks

Sources:		Gigatonne/year
	1. Industrial combustion	5.5-6.5
	2. Oil fires	0.01
	3. Land-use change	1.1-3.6
Sinks:
	1. Ocean	1.1-2.8
	2. Terrestrial biosphere	0.2-3.0

Sources

1. Combustion- The industrial emission due to combustion was 5.5-6.5GtC/year in 1989-1990. Specifically, these emissions were due to production of cement, gas flaring and mines. During Kuwaiti oil fires in 1991, approximately 65Tg of carbon as CO2 was released which is approximately equal to 1% of estimated annual global fossil fuel emission.

2. Land-use change- The release of carbon from land-use change during 1990 was 1.1-3.6 GtC. Carbon emission in land-use change depends on carbon intensity per ha, area converted and future of area converted. According to the Food and Agricultural Organization(FAO) during 1981-1990 in the closed and open canopy of 17Mha/yr. Fearnside reported that only in the Brazilian Amazonian forest there is a deforestation rate of 2.1Mha/yr, which is approximately 12.3% of total global deforestation.

Sinks

1. Ocean- IPCC estimates the global annual ocean sink of 1.2-2.8GtC/year. The strength of global sink depends on the sea breeze. Sea breeze depends on wind speed, temperature and partial pressure of gases. Global concern regarding coral reef bleaching has had no impact on the oceanic sink.

2. Terrestrial biosphere- The IPCC analysis showed 0.2-3.0 GtC/year net imbalance during the 1980s. This term is highly unspecific because the scientists during the creation of the report were still unsure whether the carbon goes to the terrestrial biosphere or sink.

Models and prediction

There exist a variety of models, including a 1-D ocean-atmosphere box-diffusion model and 3-D ocean-atmosphere model. All these models are still in the infant stages because the models are subject to a lot of uncertainty and are often unreliable. This problem often has an exponential difference over different time periods. Some of the models at the regional scale-covered a lot but the problem is still the high reliability on these models.

Methane

It is an important greenhouse gas. It also helps in the production of ozone in the upper troposphere and water vapour in the stratosphere which are significant greenhouse gases. CH4 sources are divided into two parts, natural and anthropogenic emissions. Its present concentration is 1.72 ppmv global averaged. There has also been a decrease in the rate of growth of atmospheric concentration of CH4 during the 1980s which saw a decrease of about 10 ppb from the last decade. The reason for the decrease in emission rate is fairly unknown.

Sources:		Teragram/year
	Natural
	1. Wetlands	115
	2. Termites and oceans	30
	3. Methane hydrates(eg. Permafrost)	5
	4. Freshwaters	5
	Anthropogenic
	1. Coal mining, natural gas and pet industry	100
	2. Enteric farming	80
	3. Rice paddies	60
	4. Animal waste and domestic sewage plant	50
	5. Biomass burning	40
	6. Landfill	30
	Total	515
Sinks:
	1. Atmospheric removal	340-500
	2. Soil uptake	15-45
	Total	355-545

Sources

1. Natural methane emissions- Wetlands are the most significant CH4 emitter at about 115 Teragram(Tg), followed by termites and oceans which jointly produce 30Tg. Then there are freshwater and methane hydrates like permafrost which emit 5 Tg CH4 each.

2. Anthropogenic methane emissions- We have coal mining, natural gas and pet industry which are the precursor and produce combined about 100Tg CH4. Enteric fermentation which is a digestive process in ruminant animals(cattle, sheep, goats and buffalo) produce 80Tg CH4. Rice paddies produce 60Tg CH4. Animal waste and domestic sewage plants produce 25 Tg CH4 each. Biomass burning and landfills produce 40 and 30Tg CH4 respectively. This shows clear anthropogenic sources dominate natural emission by 2:1. This makes up a total of 515Tg CH4 emission.

Sinks

The main sink of methane is a reaction with OH radicals in the troposphere so a precise knowledge of rate constant and atmospheric concentration of OH is essential in determining the sink. It current removal of CH4 by OH radicals is estimated at 340-500TgCH4. An average uptake of CH4 by soil is 15-45TTg.

About 20% of total annual CH4 emissions are from the fossil fuel industry, mainly coal-mining and natural gas emission. Enteric fermentation in ruminant animals produces about 16% annual average which could be significantly decreased by changing our food habits. Also, the large burps of permafrost which emit significant CH4 could be catastrophic if not controlled. The atmospheric lifetime of CH4 is estimated to be 9-13 years.

Nitrous oxide

A long-lived greenhouse gas which is a source of oxidation of nitrogen in the stratosphere. It plays an important role in controlling the stratospheric ozone. The concentration of NO2 is about 310ppbv which is 8% greater than the pre-industrial era.

Sources:		Teragram/year
	Natural
	1. Oceans	1.4-2.6
	2. Wet forests	2.2-3.7
	3. Dry savannas	0.5-2.0
	4. Forests	0.5-2.0
	Anthropogenic
	1. Cultivated soils	0.03-3.0
	2. Burning biomass	0.2-1.0
	3. Stationary combustion	0.1-0.3
	4. Mobile sources	0.2-0.6
	5. Adipic acid production	0.4-0.6
	6. Nitric acid production	0.1-0.3
	Total	5.63-16.1
Sinks:
	Photolysis in atmosphere	7-13
	Atmospheric increase	3-4.5
	Total	10-17.5

Its major anthropogenic emissions include cultivated soil and burning biomass which accounts to almost 15%(approx.) of its emissions. Major nitrous oxide production industry is adaptic and nitric acid. Almost 90% of adipic acid is used in the production of nylon 66. The main use of nitric acid is fertilizers and explosives. The impact of tropical deforestation is fairly unclear. Although the sources seem to be more than emissions still there are many aspects that are undefined and uncertain that might play a significant role in emission. It is advocated that agricultural production triggers biological production. There were large changes in land use in the tropics which are unaccounted. The atmospheric lifetime of nitrous oxide is about 130(110-168) years.

Halogenic species

These are groups of gases having characteristic halogenic elements. Major source gases are CFCs, HCFC-22, the halons, CH3CCl3(methyl chloroform)and CCl4(carbon tetrachloride). They have almost equal production in both northern and southern hemisphere. These gases are primarily responsible for ozone depletion but their consumption in recent years has decreased.

Sources:		Parts per trillion volume(WMO(1992))
	CFC-11	9-10
	CFC-12	17-18
	CFC-113	6
	CCl4	1-1.5
	HCFC-22	5-6
	CH3CCl3	4-5
	CBrClF2	0.4-0.7
	CBrF3	0.2-0.4
	Total	42.6 pptv/yr
Sinks:
	Photolysis in stratosphere	?
	Reaction with OH in the troposphere	?

CFCs alone are harmless to the ozone layer. After the photolysis in the stratosphere, the chlorine atom is released which participates in a catalyst chain reaction which destroys ozone. Partially halogenated compounds sink through reaction with OH in the troposphere. There is no significant sink other than these reactions in our atmosphere. Its consumption has decreased after the Montreal protocol and London Amendments to the Montreal protocol. This Amendment called for a reduction of 50% by 1995, 85% by 1997 and a complete phase-out in 2000. (This report was published in 1992).

The CFCs 11,12, and 113 decreased by 40% between 1986-1991. The main source of compounds containing mostly Florine remains to be unknown.

Troposphere Ozone precursor

Tropical ozone is predicted to increase with increasing emission in nitrogen oxides(NOx), carbon monoxide(CO), CH4 and non-methane hydrocarbons(NMHC). Ozone is a useful gas in the atmosphere which plays a key role in absorbing ultraviolet radiations from the sun. About 90% of total column ozone resides in the stratosphere and 10% in the troposphere. Increase in CO, CH4 and NMHC levels leads to a decrease in OH free radical. Whereas increasing NOx emission leads to enhanced OH levels.

Sources and sinks

It is clear that these compounds have natural and anthropogenic sources but their sources and sinks remain uncertain. There have been many studies suggesting the previous knowledge is inadequate and future studies are important to give a quantitative value to these greenhouse gases. These gases have short atmospheric life and no advanced monitoring techniques that measure it which makes it almost impractical to measure it. Available knowledge is given below

Sources and sinks of Carbon monoxide(CO)

Primary sources:		Teragram/year
	1. Fossil fuel	500
	2. Biomass burning	600
	3. Plants	50-200
	4. Oceans	20-80
Secondary sources:
	1. NMHC(non-methane hydrocarbons) oxidation	600
	2. Methane oxidation	630
	Total	2400-2610
Sinks:
	1. OH reaction	2050
	2. Soil uptake	280
	3. Stratospheric oxidation	80-140
	Total	2410-2470

CO is also a very toxic gas which is slightly lighter than air. It is also called the silent killer. Biomass burning and fossil fuels are major precursors for CO emissions.

Sources of Nitrogen oxides

Natural:
1. Soil	5-20
2. Lightning	2-20
3. Transport from stratosphere	1 (approx)
Anthropogenic:
1. Fossil fuel combustion	24
2. Biomass burning	2.5-13
3. Tropospheric aircrafts	0.6
Total	35.1-78.6

Sulphur-containing gases

These gases transform into a sulphur aerosol particle that affects the atmosphere in three ways. First, it scatters sunlight back to space, reducing radiation reaching the earth surface. Secondly, it assists the formation of water droplets in the clouds, thereby changing its physical characteristics. Thirdly affects atmospheric chemical composition.

Sources

Source	Teragram/year
Anthropogenic emissions(mainly SO2)	70-80
Biomass burning(SO2)	0.8-2.5
Oceans(DMS)	10-50
Solid and plants(DMS and H2S)	0.2-4
Volcanic emission(mainly SO2)	7-10

Volcanic eruptions are a major stratospheric aerosol producer. Krueger (1991) used Nimbus 7 TOMS data to estimate that the eruption of Mt.Pinatubo in the Philippines in 1991 added about 20 million tons of SO2 directly to the stratosphere. In addition to that, aeroplane emissions of SO2 have increased during the last decade.

Sinks

Large scale removal occurs during rainfall. These particles are unique and dominant in forming water droplets in the clouds. The quantitative data showing particles required or used during the process of precipitation is unknown and subject to research.

Increased levels of SO2 also results in acid rains. Oxidation of SO2 in aerosol sulphates occurs in the gas phase. All the tropical reactions involving SO2 depend on the concentration of OH. The reactions are also strongly influenced by humidity because oxidation by O3 is always feasible. The lifetime of SO2 and Hydrogen sulphide(H2S) is a few weeks at most. Their short lifetime results in a heterogeneous mixture in the atmosphere and non-influencers in a stratospheric sulphur layer.

There have been many advances in research on this topic and it is fair to say that this article is outdated 28 years. But this document is the bases of understanding all the future phenomenon. So, this is a good place to understand the basics.

Note- All the data given above is from IPCC 1992 report and is their property.