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Greenhouse Gases

1. The issue in brief: What are Greenhouse Gases (GHG)?

Introduction

Greenhouse gases (GHG) are chemical compounds found in the atmosphere that contribute to the Greenhouse effect. These gases allow sunlight to enter the atmosphere freely. Due to the difference in temperature between the Sun and the Earth, the heat (infrared radiation) transferred to the Earth's surface by the sunlight is reflected back in a much longer wavelengths than the Sun does. The atmosphere absorbs these longer wavelengths more effectively than it does the shorter wavelengths from the sun. The incorporation of this longwave radiant energy warms the atmosphere; but the atmosphere is also warmed by transfer of sensible and latent heat from the Earth's 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 the popularly known greenhouse effect. Over time, the amount of energy sent from the sun to the Earth's surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth's surface roughly constant.

Many gases exhibit these greenhouse properties. Some of them occur in nature (water vapour, carbon dioxide, methane, nitrous oxide, and ozone), while others are exclusively human-made (like gases used for aerosols, and chlorofluorocarbons, CFCs). They all are molecules composed of more than two component atoms, bound loosely enough together to be able to vibrate with the absorption of heat. Eventually, the vibrating molecule will emit the radiation again, and it will likely be absorbed by another greenhouse gas molecule. This absorption-emission-absorption cycle serves to keep the heat near the surface, effectively insulating the surface from the cold of space. The major components of the atmosphere (N2 and O2) are two-atom molecules bound too tightly together to vibrate and thus they do not absorb heat and contribute to the greenhouse effect.

According to the Intergovernmental Panel on Global Warming and Climate Change Fourth Assessment Report (IPCC, 2007), most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. Changes in the atmospheric abundance of greenhouse gases and aerosols, in solar radiation and in land surface properties alter the energy balance of the climate system. These changes are expressed in terms of radiative forcing, which is used to compare how a range of human and natural factors drive warming or cooling influences on global climate.

2. Major Greenhouse Gases

Carbon dioxide (CO2)

Carbon dioxide is a colourless, odourless non-flammable gas and is the most important anthropogenic greenhouse gas. The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores. The annual carbon dioxide concentration growth-rate was larger during the last 10 years (1995 - 2005 average: 1.9 ppm per year), than it has been since the beginning of continuous direct atmospheric measurements (1960 - 2005 average: 1.4 ppm per year) although there is year-to-year variability in growth rates.

The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use, with land use change providing another significant but smaller contribution. Annual fossil carbon dioxide emissions increased from an average of 6.4 to 6.8 GtC (23.5 to 25.0 GtCO2) per year in the 1990s, to 7.2 to 7.5 GtC (26.4 to 27.5 GtCO2) per year in 2000-2005 (2004 and 2005 data are interim estimates). Carbon dioxide emissions associated with land-use change are estimated to be 1.6 to 2.7 GtC (5.9 to 9.9 GtCO2) per year over the 1990s, although these estimates have a large uncertainty (IPCC, 2007).

Methane (CH4)

Methane is a colourless, odourless, flammable gas. It is formed when plants decay and where there is very little air. It is often called swamp gas because it is abundant around water and swamps. Bacteria that breakdown organic matter in wetlands and bacteria that are found in cows, sheep, goats, buffalo, termites, and camels produce methane naturally. Methane comes mainly from landfills, coal mines, oil and gas operations, and agriculture.

The global atmospheric concentration of methane has increased from a pre-industrial value of about 715 ppb to 1732 ppb in the early 1990s, and is 1774 ppb in 2005. The atmospheric concentration of methane in 2005 exceeds by far the natural range of the last 650,000 years (320 to 790 ppb) as determined from ice cores. Growth rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period. It is very likely that the observed increase in methane concentration is due to anthropogenic activities, predominantly agriculture and fossil fuel use, but relative contributions from different source types are not well determined.

Nitrous Oxide (N2O)

Nitrous Oxide is a colourless greenhouse gas, however, it has a sweet odour. This gas is released naturally from oceans and by bacteria in soils. It is also emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes.

Together, temperate and tropical soils dominate natural nitrous oxide emissions on a global scale. The global atmospheric nitrous oxide concentration increased from a pre-industrial value of about 270 ppb to 319 ppb in 2005. The growth rate has been approximately constant since 1980. More than a third of all nitrous oxide emissions are anthropogenic and are primarily due to agriculture.

Ozone (O3)

Ozone is created naturally in the stratosphere by the combining of atomic oxygen (O) with molecular oxygen (O2) -This process is activated by sunlight- and artificially through photochemical smog production. Ozone is destroyed naturally by the absorption of ultraviolet radiation, and by the collision of ozone with other atmospheric atoms and molecules.

Its concentrations varies with latitude and altitude in the atmosphere. Their global levels have generally decreased in the stratosphere and increased near the Earth's surface. Its role in the enhancement of the greenhouse effect has been difficult to determine scientifically.

Chlorofluorocarbons Gases (Aerosols)

Fluorocarbons is a general term for any group of synthetic organic compounds that contain fluorine and carbon. Many of these compounds, such as chlorofluorocarbons(CFC's), can be easily converted from gas to liquid or liquid to gas. Because of these properties, CFC's can be used in aerosol cans, refrigerators, and air conditioners. The amount of aerosols in the air has direct effect on the amount of solar radiation hitting the Earth's surface.

Hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for ozone-depleting substances (i.e., CFCs, HCFCs, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases ("High GWP gases").

Anthropogenic contributions to aerosols (primarily sulphate, organic carbon, black carbon, nitrate and dust) together produce a cooling effect, with a total direct radiative forcing of -0.5 to -0.1 W m-2 and an indirect cloud albedo forcing of -0.7 to -0.3 W m-2.

Other Sources

Significant anthropogenic contributions to radiative forcing come from several other sources. Tropospheric ozone changes due to emissions of ozone-forming chemicals (nitrogen oxides, carbon monoxide, and hydrocarbons) contribute +0.35 to +0.65 W m-2. The direct radiative forcing due to changes in halocarbons is +0.34 to +0.37 W m-2. Changes in surface albedo, due to land-cover changes and deposition of black carbon aerosols on snow, exert respective forcings of -0.2 to 0.0 and +0.1 to +0.2 W m-2.

3. Organisations

Intergovernmental Panel on Global Warming and Climate Change (IPCC)

Recognizing the problem of potential global climate change the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) established the Intergovernmental Panel on Global Warming and Climate Change (IPCC) in 1988. The role of the IPCC is to assess the scientific, technical and socio-economic information relevant for the understanding of the risk of human-induced climate change.

The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation. IPCC reports should be neutral with respect to policy, although they may need to deal objectively with scientific, technical and socio-economic factors relevant to the application of particular policies.

Review is an essential part of the IPCC process. Since the IPCC is an intergovernmental body, review of IPCC documents should involve both peer review by experts and review by governments.

UNEP-WCMC (World Conservation Monitoring Centre)- Biodiversity and Global Warming and Climate Change

The UNEP World Conservation Monitoring Centre was established in 2000 as the world biodiversity information and assessment centre of the United Nations Environment Programme.The Centre's roots go back to 1979 when IUCN established a Cambridge office to monitor endangered species. In 1988 the independent, non-profit World Conservation Monitoring Centre was founded jointly by IUCN, WWF and UNEP.

World Data Centre for Greenhouse Gases (WDCGG)

The World Data Centre for Greenhouse Gases (WDCGG) is established under the Global Atmosphere Watch (GAW) programme to collect, archive and provide data for greenhouse (CO2, CH4, CFCs, N2O, etc.) and related (CO, NOX, SO2, VOC, etc.) gases and surface ozone in the atmosphere and ocean, measured under GAW and other programmes.

From this web site (http://gaw.kishou.go.jp/wdcgg.html), you can obtain information including WDCGG's publications and measurement data that have been contributed by organizations and individual researchers in the world.

Contributors





Last edited by Miriam Hansen .
Page last modified on Wednesday September 29, 2010 09:12:30 GMT.
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