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Climate Change: Contributing Causes of Global Warming
Table of Contents
Acknowledgements
Background and Overview
Contributing Causes of Global Warming
Impacts of Global Warming
Reasons for Action
Climate Change Solutions
The Individual's Role
Where To Go for Help
Complete List of Links

Essential Links:

Assessment of the Impacts of Global Change on Regional U.S. Air Quality: A Synthesis of Climate Chan...
Intended for managers and scientists working on air quality to provide information on the potential ...

The Fourth Assessment Report: The Physical Science Basis
Describes fundamental earth science and the historical overview of climate change science.

US EPA - Inventory of Greenhouse Gas Emissions and Sinks
This report by U.S. EPA summarizes the latest information on U.S. anthropogenic greenhouse gas emiss...


Greenhouse Gas (GHG) Emitting Activities

Globally, citizens and businesses consume fossil-fuel-derived electricity and fuel inefficiently, and at rates that contribute significantly to the total atmospheric concentration of GHGs. Other major contributors include land use (including agriculture and deforestation), electricity, use of fuels for transportation and heat, waste disposal, livestock production, and (mostly industrial) use and production of high global-warming potential (GWP) products or gases. See a helpful flow chart produced by Climate Analysis Indicators Tool, depicting world GHG emissions by sector and end-use activity, and the resulting GHG species from said sectors and end use.

(Note: Many different studies conduct GHG accounting to determine and report a sector or activity's GHG inventory. Published results vary somewhat from study to study, based on use of a particular protocol or standard used, and how the study set boundaries for the sector or activity being analyzed. As such, any GHG emissions or inventories quantified within this document are based on the study linked or cited.)

In the U.S., energy-related activities account for three-quarters of our human-generated GHG emissions. The U.S. Environmental Protection Agency (EPA) Climate Change Program ranks the major GHG contributing end-use sectors (not directly related to energy production and distribution activities) in the following order: industrial, transportation, residential, commercial, and agricultural. EPA also recently issued the 2009 U.S. Greenhouse Gas Inventory Report, an emissions inventory that identifies and quantifies primary anthropogenic sources and sinks of GHGs and major contributing sectors in the U.S.

The National Electrical Manufacturers Association (the trade association for the electrical manufacturing industry) and the EPA 2009 inventory indicate that some of the highest “energy intensive” industries are cement, iron and steel, aluminum, chemicals, forest products, glass, metal casting, mining, and petroleum refining.

According to the U.S. Department of Transportation, the transportation sector directly accounted for about 28 percent of total U.S. GHG emissions in 2006, making it the second largest source of GHG emissions, behind only electricity generation (34 percent). Nearly 97 percent of transportation GHG emissions came through direct combustion of fossil fuels, with the remainder due to CO2 from electricity (for rail) and hydrofluorocarbons (HFCs) emitted from vehicle air conditioners and refrigerated transport [1]. These estimates of GHG emissions do not include additional lifecycle emissions related to transportation, such as the extraction and refining of fuel and the manufacture of vehicles, which also are a significant source of domestic and international GHG emissions.

The Major Greenhouse Gases

The largest contributor to global warming is carbon dioxide (CO2) emissions from burning of fossil fuels. Other significant GHGs include methane and nitrous oxide, along with several flourinated gases.

The atmospheric abundance of these gases is important, but the potency of each gas (with respect to warming) is also very important in understanding climate change impacts. Global warming potential (GWP) is used to understand how much a given mass of a GHG is estimated to contribute to global warming and rates the potency of the gas relative to carbon dioxide (whose GWP is by definition 1). The GWP is a relative scale which compares the gas in question to that of the same mass of carbon dioxide. The GWP value depends on how the gas concentration decays over time in the atmosphere, and often not precisely known. Hence, the values should not be considered exact. Because a GWP rating is calculated over a specific time interval (e.g., 20 years, or 100 years), the time interval must be stated whenever a GWP is quoted or else the value is meaningless.

To illustrate, methane has a GWP of 25 (over 100 years), while sulfur hexafluoride has a GWP of 22,800 (over 100 years). These illustrate a much higher potency than carbon dioxide (with a GWP of 1). However, the sheer quantity of CO2 releases, despites its lower potency, makes it the GHG of greatest concern.

The following briefly discusses GHG emissions that are causing significant impact, along with a few additional emissions and conditions worth mentioning.

Carbon Dioxide (CO2)

Since the Industrial Revolution, global atmospheric concentrations of CO2 have risen about 36 percent [2], principally due to the combustion of fossil fuels. According to the EPA, electricity production consumed 36 percent of U.S. energy from fossil fuels in 2007 and produced 42 percent of the CO2 emissions. Electricity generators rely on coal for more than half of their energy requirements in the U.S. and accounted for 94 percent of the coal consumed for energy in the U.S. In addition, the distribution of electricity is inefficient. Aside from electricity generation, the five major fuel-consuming sectors that contribute to the increase in CO2, in this order, are:

    1) Industrial (especially production of iron and steel, cement, ammonia, lime products, and aluminum),
    2) Transportation

    3) Residential

    4) Commercial

    5) Agriculture (especially rice cultivation, use of nitrogen-based fertilizers, land use patterns, and livestock production)

Another major contributor to CO2 flux has been land use change, particularly deforestation. The United Nations Food and Agriculture Organization (FAO) reported in October 2006 that deforestation accounts for 25 to 30 percent of the release of GHGs. The Environmental Defense Fund states that at current rates, deforestation will increase CO2 concentration by nearly 130 ppm over the next century.

Other sources include waste management, especially from incineration of wastes, and from landfill gas.

Methane (CH4)

Methane, another potent GHG, is produced through anaerobic decomposition of organic matter such as enteric fermentation in animals, wetland rice cultivation, and the decomposition of animal waste and anaerobic decomposition of solid wastes in landfills. Other sources include production and distribution of natural gas, as a by-product of coal mining, landfills, and incomplete combustion of fossil fuels. Atmospheric levels of CH4, have also risen approximately 143 percent since the pre-Industrial Era - from an estimated 722 parts per billion (ppb) to approximately 1,774 ppb in 2005, although the rate of increase in methane emissions has declined [3].

Nitrous Oxide (N2O)

Nitrous Oxide (N2O) levels have increased since the pre-Industrial Era. Anthropogenic sources include agricultural soils (especially the production of nitrogen-fixing crops and forages), the use of synthetic and manure fertilizers, fossil fuel combustion (especially from mobile combustion), wastewater treatment and waste incineration, and biomass burning.

Hydrofluorocarbons (HFCs)

HFCs are man-made chemicals, many of which have been developed as alternatives to ozone-depleting substances (ODS) for industrial, commercial, and consumer products, mostly as refrigerants or propellants. The growing use of refrigerants has led to increased HFC use and release into the atmosphere.

The GWP of HFCs range from 140 to 11,700 (HFC-23). The HFCs with the largest measured atmospheric abundances are (in order): HFC-23 (CHF3), HFC-134a (CF3CH2F), and HFC-152a (CH3CHF2). The only significant emissions of HFCs before 1990 were of the chemical HFC-23, which is generated as a byproduct of the production of HCFC-22.

Perfluorocarbons (PFCs)

Primary aluminum production and semiconductor manufacture are the largest known man-made sources of two perfluorocarbons – CF4 (tetrafluoromethane) and C2F6 (hexafluoroethane). PFCs have extremely stable molecular structures and are largely immune to the chemical processes in the lower atmosphere that break down most atmospheric pollutants. Not until the PFCs reach the mesosphere, about 60 kilometers above Earth, do very high-energy ultraviolet rays from the sun destroy them. This removal mechanism is extremely slow and as a result, PFCs accumulate in the atmosphere and remain there for several thousand years.

The global warming potential of sulfur hexafluoride SF6 is 23,900, making it the most potent GHG the IPCC has evaluated. SF6 is a colorless, odorless, nontoxic, nonflammable gas with excellent dielectric properties. SF6 is used for insulation and current interruption in electric power transmission and distribution equipment, in the magnesium industry to protect molten magnesium from oxidation and potentially violent burning, in semiconductor manufacturing to create circuitry patterns on silicon wafers, and as a tracer gas for leak detection. Additional fluorinated compounds with high GWPs include nitrogen trifluoride and tetrafluoromethane.

Chlorofluorocarbons (CFCs) and other Ozone Depleting Substances

Ozone Depleting Substances (ODS) include anthropogenic chlorine and bromine-containing halocarbons, such as chlorofluorocarbons (CFCs), which were developed for industrial, commercial, and consumer products, mostly as refrigerants or propellants.  Because ODSs have contributed to the destruction of the Ozone Layer in the stratosphere, their use has been phased out under the Montreal Protocol.  There is still, however, a global black market for these substances, which have high GWPs.  CFC-12 has a GWP of 8,500, and CFC-11 has a GWP of 5,000.  An additional warming effect of CFCs and other ODSs comes from their destruction of stratospheric ozone (see below).

Ozone
Ozone is present in both the upper stratosphere where it shields the Earth from harmful levels of ultraviolet (UV) radiation (Ozone Layer), and also at lower concentrations in the troposphere where it is the main component of anthropogenic photochemical “smog.”  The role of ozone in climate change differs depending on whether it is up high in the Ozone Layer (stratospheric) or down low as smog (tropospheric).

  • Stratospheric Ozone.  In the stratosphere, the Ozone Layer protects us from harmful UV rays and reduces incoming radiation that warms the earth.  Ozone Depleting Substances (ODS), however, can act in the stratosphere, and deplete stratospheric ozone.  This depletion contributes to warming, since more UV rays can reach the earth’s surface and warm it.  With the phase-out of ODSs under the Montreal Protocol, the Ozone Layer is expected to recover, yet increased methane and water vapor may hamper the recovery.
  • Tropospheric Ozone.  In the troposphere, ozone acts as a greenhouse gas.  It is formed as a byproduct of volatile organic compounds (VOCs) and nitrogen oxides emitted from motor vehicles and power plants.

Ozone's climate effect is hard to quantify, since it is short-lived.  What is clear, however, is that its effect is significant.  Estimates of its contribution to the increased global warming potential of the atmosphere are around 10-25 percent.Ozone is present in both the upper stratosphere where it shields the Earth from harmful levels of radiation, and also at lower concentrations in the troposphere where it is the main component of anthropogenic photochemical “smog.” In the past, anthropogenic chlorine and bromine-containing halocarbons, such as chlorofluorocarbons (CFC) and other ozone-depleting substances (ODS), depleted stratospheric ozone concentrations, reducing the ability to reflect incoming radiation (thus increasing global warming). It wasn’t until 1973 that chlorine in these compounds was found to be a catalytic agent in ozone destruction. Reduction of the upper stratospheric ozone layer (above 18 km) contributed significantly to global warming. Recovery of the ozone layer has mostly resulted from the global phase out of CFCs and certain other ODS with the institution of the Montreal Protocol on Substances that Deplete the Ozone Layer.

Other GHG Contributors

Emissions from non-energy fossil fuel consumption in the U.S. The fuels used for these purposes are diverse, including natural gas, liquefied petroleum gases (LPG), asphalt (a viscous liquid mixture of heavy crude oil distillates), petroleum coke (manufactured from heavy oil), and coal coke (manufactured from coking coal). The non-energy applications are equally diverse, and include:

  • feedstocks for the manufacture of plastics, rubber, synthetic fibers, and other materials;
  • reducing agents for the production of various metals and inorganic products; and,
  • consumable products used in manufacturing or other industries products such as lubricants, solvents, and waxes.

Some of these non-energy fossil fuel uses in products are also considered VOCs, which impart a small direct impact as GHGs, as well being involved in chemical processes which modulate ozone production. Anthropogenc VOCs include non-methane hydrocarbons (NMHC) and oxygenated NMHCs (eg. alcohols and organic acids), some constituents from vehicle emissions, fuel production, and biomass burning. Though measurement of VOCs is extremely difficult, it is expected that most anthropogenic emissions of these compounds have increased in recent decades. Emissions may occur during the manufacture of a product and/or during the product’s lifetime, such as solvent use.

Water vapor is also a GHG and is the most abundant greenhouse gas in the atmosphere. Water vapor results from fossil fuel combustion, heating of water, and other human activities, but is also naturally occuring. Because the water vapor content of the atmosphere is expected to greatly increase in response to warmer temperatures (not from a direct result of industrialization), there is the potential for a water vapor feedback that could amplify the expected climate warming effect due to increased carbon dioxide alone. However, it is less clear how cloudiness would respond to a warming climate; depending on the nature of the response, clouds could either further amplify or partly mitigate the water vapor feedback. This feedback is important to projecting future climate change, but remains fairly poorly measured and understood [4].

References

[1] U.S. Department of Transportation. 2006. Transportation's Role in Climate Change.

[2] Intergovernmental Panel on Climate Change (IPCC). 2007. Fourth Assessment Report.
[3] US EPA. 2009. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007.

[4] National Oceanic and Atmospheric Administration. Greenhouse Gases - Frequently Asked Questions


 

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