greenhouse gases

Please download to get full document.

View again

of 14
4 views
PDF
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Document Description
A greenhouse gas is one of several gases that can absorb and emit longwave (infrared) radiation in a planetary atmosphere. Of the sunlight that falls on the Earth’s surface, approximately 40% of that energy is reradiated upward into the atmosphere in the form of longwave radiation. Approximately 75% of that upward radiated longwave energy is absorbed by water vapor, carbon dioxide, methane and other greenhouse gases. Since this absorption process is molecular in nature, the subsequent reradiation of energy by these gases is multidirectional. As a result, about 50% of the longwave emission is reradiated back toward the Earth where it is once again turned into heat energy. Through this process, greenhouse gases contribute to the amount of heat energy released at the Earth’s surface and in the lower atmosphere
Document Share
Documents Related
Document Tags
Document Transcript
  INTRODUCTION  A greenhouse gas is a gas in an atmosphere that absorbsand emits radiation within the thermal infrared range.This process isthe fundamental cause of the greenhouse effect.   The primarygreenhouse gases in the Earth’s atmosphere are water vapor,carbondioxide, methane, nitrous oxide, and ozone. In the Solar System, theatmospheres of Venus, Mars, and Titan also contain gases thatcause greenhouse effects. Greenhouse gases greatly affect thetemperature of the Earth; without them, Earth’s surface would be onaverage about 33 °C (59°F) colder than at present.Since the beginning of the Industrial Revolution, the burning of fossilfuels has contributed to the increase in carbon dioxide in theatmosphere from 280 ppm to 390 ppm, despite the uptake of a largeportion of the emissions through various natural “sinks” involved inthe carbon cycle. Anthropogenic carbon dioxide (CO 2 ) emissions(i.e., emissions produced by human activities) come from combustionof carbonaceous fuels, principally wood, coal, oil, and natural gas. Aside from purely human-produced synthetic halocarbons, mostgreenhouse gases have both natural and human-caused sources.During the pre-industrial Holocene, concentrations of existing gaseswere roughly constant. In the industrial era, human activities haveadded greenhouse gases to the atmosphere, mainly through theburning of fossil fuels and clearing of forests.The 2007 Fourth Assessment Report compiled by theIntergovernmental Panel on Climate Change (IPCC)noted that “changes in atmospheric concentrations of greenhouse gases andaerosols, land cover and solar radiation alter the energy balance of the climate system”, and concluded that “increases in anthropogenic(caused by human activities) greenhouse gas concentrations is verylikely to have caused more than 50% of the increases in globalaverage temperatures since the mid-20 th century”.  Greenhouse gases A greenhouse gas   is one of several gases that can absorb and emitlongwave (infrared) radiation in a planetary atmosphere. Of the   sunlight   that falls on the Earth’s surface, approximately 40%of that energy is reradiated upward into the atmosphere in theform of longwave radiation. Approximately 75% of that upwardradiated longwave energy is absorbed by water vapor,   carbondioxide,   methane   and other greenhouse gases. Since thisabsorption process is molecular in nature, the subsequentreradiation of energy by these gases is multidirectional. As aresult, about 50% of the longwave emission is reradiated backtoward the Earth where it is once again turned into heat energy.Through this process, greenhouse gases contribute to theamount of heat energy released at the Earth’s surface and in thelower atmosphere. Impact of a given gas on the overallgreenhouse effect Each gases’ contribution to the greenhouse effect is affected by thecharacteristics of the gas, its abundance, and any indirect effects itmay cause. For example, on a molecule-for-molecule basis the directradiative effects of methane is about 72 times stronger greenhousegas than carbon dioxide over a 20 year time frame but it is present inmuch smaller concentrations so that its total direct radiative effect issmaller.On the other hand, in addition to its direct radiative impact methanehas a large indirect radiative effect because it contributes to ozoneformation. Shindell et al. (2005) argue that the contribution to climatechange from methane is at least double previous estimates as aresult of this effect.  Anthropogenic greenhouse gases Since about 1750 human activity has increased the concentration of carbon dioxide and other greenhouse gases. Measured atmosphericconcentrations of carbon dioxide are currently 100 ppm higher thanpre-industrial levels. Natural sources of carbon dioxide are more than20 times greater than sources due to human activity, but over periods longer than a few years natural sources are closely balancedby natural sinks, mainly photosynthesis of carbon compounds byplants and marine plankton. As a result of this balance, theatmospheric mole fraction of carbon dioxide remained between 260and 280 parts per million for the 10,000 years between the end of thelast glacial maximum and the start of the industrial era.It is likely that anthropogenic warming, such as that due to elevatedgreenhouse gas levels, has had a discernible influence on manyphysical and biological systems. Warming is projected to affectvarious issues such as freshwater resources, industry, food andhealth.The main sources of greenhouse gases due to human activity are:  burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations in the air. Land use change(mainly deforestation in the tropics) account for up to onethird of total anthropogenic CO 2 emissions.  livestock enteric fermentation and manure management,paddy ricefarming, land use and wetland changes, pipelinelosses, and covered vented landfill emissions leading tohigher methane atmospheric concentrations. Many of thenewer style fully vented septic systems that enhance andtarget 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 andmanufacturing processes.  agricultural activities, including the use of fertilizers, that leadto higher nitrous oxide (N 2 O) concentrations.  When these gases are ranked by their direct contribution to thegreenhouse effect, the most important are: Gas FormulaContribution (%) Water vaporH 2 O36 – 72 %Carbon dioxideCO 2 9 – 26 %MethaneCH 4 4 – 9 %OzoneO 3 3 – 7 % On the other hand, in addition to its direct radiative impact methanehas a large indirect radiative effect because it contributes to ozoneformation. Shindell et al. (2005) argue that the contribution to climatechange from methane is at least double previous estimates as aresult of this effect.When these gases are ranked by their direct contribution to thegreenhouse effect, the most important are:In addition to the main greenhouse gases listed above, other greenhouse gases include sulfur hexafluoride, hydrofluorocarbons andperfluorocarbons (see IPCC listof greenhouse gases). Some greenhouse gases are not often listed.For example, nitrogen trifluoride has a high global warmingpotential (GWP) but is only present in very small quantities.
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks