Course: BFM 101 Biofuels Fundamentals

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Greenhouse Gases and Climate Impacts

Greenhouse gases (GHGs) are gases in the atmosphere that absorb and emit thermal radiation in a process known as the greenhouse effect – the mechanism by which solar radiation is captured and earth is warmed to an extent necessary for supporting life (figure 3.4, “Natural Greenhouse Effect”).  The primary GHGs are water vapor, carbon dioxide, methane, nitrous oxide and ozone.  These gases differ in their abundances in the atmosphere as well as their warming power.  The science of global climate change indicates an overall warming trend for the earth as a whole in association with rising levels of GHGs.   While natural sources contribute to GHGs and changes in their concentrations over time, global scientific consensus indicates that anthropogenic (i.e., human) sources of GHGs contribute to global climate change (figure 3.4, “Human Enhanced Greenhouse Effect”).

Link:
(For more information on global warming and climate change see:http://www.epa.gov/climatechange and http://www.scientificamerican.com/article.cfm?id=science-behind-climate-change)

Figure 3.4. The greenhouse effect is the primary mechanism by which solar radiation is captured by earth’s atmosphere.  Human activity has enhanced the warming ability of this mechanism. (Will Elder, U.S. National Park Service.)

Among the various energy use sectors, power plants are the biggest contributor to annual GHG emissions.  Industrial process and transportation fuels are ranked second and third overall, respectively (figure 3.5).  The contributions of power plants, industrial processes and transportation fuels to annual GHG emissions are a result of combustion of fossil fuels, most notably coal, natural gas and petroleum, which releases the carbon that was once stored in these non-renewable energy sources into the atmosphere as carbon dioxide and thereby enriches the greenhouse effect.

Bioenergy has the potential to be carbon-neutral by balancing the amount of carbon released in use of bioenergy products with an equivalent amount put into and stored in soils, plant and animal tissues, or other material such as the ocean floor (figure 3.6).  That is, bioenergy is part of a global carbon cycle where plants take up atmospheric carbon dioxide and convert it into plant tissue (i.e., they sequester carbon). Carbon is released back to the atmosphere when the plant biomass is burned directly or after it has been converted into a fuel and used (i.e., carbon emissions), and from there it is available again for plant up-take (i.e., sequestration).  However, when taking a life-cycle assessment approach, bioenergy may or may not prove to be carbon neutral.  The neutrality of bioenergy with regard to other GHGs such as methane and nitrous oxide is also questionable.  Of importance in evaluating the carbon-neutrality of a bioenergy crop are land use change and feedstock production systems.

Fact Sheet:
Bioenergy and Greenhouse Gas Emissions

Resources provided by the eXtension Farm Energy Community of Practice at http://www.extension.org/ag+energy

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