Greenhouse gas emissions —what are they and what do they do?

Carbon dioxide (CO2) is well known as a greenhouse gas (GHG) that contributes to global climate change. But it is not the only one and it is not the most damaging.

Current atmospheric concentration of carbon dioxide (CO2) in the most common and benchmark greenhouse gas (GHG) is at 392 parts per million (ppm), whereas in 1950 it was about 280 ppm according to a National Oceanic Atmospheric Administration chart showing air sample results from glacial ice cores.

Recent impacts of the CO2 concentration increase and the consequent warming include: arctic sea ice minimums (summer ice cover) have been dropping at about 12% per decade; the global sea level has risen from 4-8 inches in the past 100 years; and Greenland’s ice loss is about 100 billion tons per year, double the rate of 1996 to 2005, according to NASA data.

Carbon dioxide is one major greenhouse gas but not the only important one. That is, a number of gases keep infrared radiation, or warmth, caused by sunlight hitting surfaces on earth, from escaping earth’s atmosphere.  Below is a chart showing top GHGs. Global Warming Potential (GWP) is a measurement of a gas’s ability to hold a unit of heat over a set period of time, with CO2 used as the reference or standard gas.

Gas

Global Warming Potential (GWP) 

 CO2

 1

 CH4*

 21

 N2O

 310

 HFCs

 Nine listed, from 140 to 11,700

 CxFx

 Four listed, from 6,500 to 9,200

 SF6

 23,900

Summarized from IPCC, 1996 as shown in Draft Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010, page ES-3 

*CH4 GWP includes direct efects and indirect effects due to the production of tropospheric ozone and stratospheric water vapor. The indirect effect due to the production of C02 is not includeded. (Converts to CO2 over a period of years) 

The largest source of GHG from human activities in the U.S. has been CO2, about 83 percent of total GHG emissions. The largest source of CO2 was from fossil fuel combustion. Electricity generation and transportation accounted for almost 70 percent of the fossil fuel contribution. There are many sources of CO2 and huge sinks in the form of forests and ocean water. Trees sequester carbon dioxide while open waters absorb it and make it into weak acid.  While methane (CH4) and other GHGs are more potent that CO2 in global warming potential, they are less common than CO2 in Earth’s atmosphere. Methane comes chiefly from natural gas production, enteric fermentation in rumen and stomachs as microbial fermentation takes place, landfills, wastewater treatment and manure management. Nitrous oxide (N2O), produced in fossil fuel combustion, soil cultivation and nitric acid production, is a potent GHG.  

HFCs are hydrofluorocarbons that are now used as substitutes for our former use of ozone depleting gases. Perfluorochemicals (PFCs or combinations of fluoride and carbon and sometimes sulfonates) are man-made compounds used as stain resistors or non-stick coatings in various household products and are very resistant to breakdown. Other sources are semiconductor manufacture and aluminum production. Magnesium processing, semiconductor manufacture and electrical transmission account for sulfur hexafluoride (SF6), an extremely powerful GHG.  

Water vapor is not listed in the gases table but is abundant and can have important greenhouse effects.  Water vapor content interacts with temperature increases, affecting heat escape, but also with rainfall and sunlight reaching the earth’s surface in a complex feedback mechanism. 

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