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7.8: Greenhouse Gases

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    We will be covering the four major categories of greenhouse gases that have been impacted by humans the most. See Table \(\PageIndex{1}\) for a numeric comparison of these greenhouse gases.

    • Carbon dioxide, CO2

    • Methane, CH4

    • Nitrous oxide, N2O

    • Synthetic fluorinated gases, including hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)

    Carbon dioxide (CO2) is the greenhouse gas responsible for most of the human-caused climate change in our atmosphere. It has the highest concentration in the atmosphere of any of the greenhouse gases that we’ll discuss here. Remember that CO2 is a direct product of both combustion and cellular respiration, causing it to be produced in great quantities both naturally and anthropogenically. Any time biomass or fossil fuels are burned, CO2 is released. Major anthropogenic sources include: electricity production from coal-fired and natural gas power plants, transportation, and industry (Chapter 4). To get an idea of how CO2 concentration has changed over time, watch this video compiled by the National Oceanic and Atmospheric Administration (NOAA): This video contains atmospheric CO2 concentrations measured directly, dating back to 1958, as well as atmospheric CO2 concentrations measured indirectly from ice core data, dating back to 800,000 BCE. By 1990, a quantity of over seven billion tons of carbon (equivalent to 26 billion tons of carbon dioxide when the weight of the oxygen atoms are also considered) was being emitted into the atmosphere every year, much of it from industrialized nations. Similar to the action of the naturally existing greenhouse gases, any additional greenhouse gases leads to an increase in the surface temperature of the Earth.

    While CO2 is produced by aerobic cellular respiration, gases such as CH4 and N2O are often the products of anaerobic metabolisms. Agriculture is a major contributor to CH4 emissions, as you saw in section 7.1. In addition to anaerobic bacteria, methane is also a significant component of natural gas, and is commonly emitted through the mining and use of natural gas and petroleum, in addition to coal mining. For a review of how fossil fuels are mined, see Chapter 4. Finally, landfills contribute significantly to CH4 emissions, as the waste put into the landfill largely undergoes anaerobic decomposition as it is buried under many layers of trash and soil. Natural sources of CH4 include swamps and wetlands, and volcanoes.

    The vast majority of N2O production by humans comes from agricultural land management. While some N2O is naturally emitted to the atmosphere from soil as part of the nitrogen cycle, human changes in land management, largely due to agricultural practices, have greatly increased N2O emissions. Some N2O is also emitted from transportation and industry.

    Due to their relatively high concentrations in the atmosphere compared to synthetic gases, CO2, CH4, and N2O, are responsible for most of the human-caused global climate change over the past century. Figure 7.9 shows the increases in all three gases following the industrial revolution. Ice core data shows us that the atmospheric CO2 concentration never exceeded 300 ppm before the industrial revolution. As of early 2015, the current atmospheric CO2 concentration is 400 ppm. Comparing Figure \(\PageIndex{1}\) to Figure 7.3.3, above, what is likely to happen to global temperature following this unprecedented rise in greenhouse gas levels?

    Screenshot (95).pngFigure \(\PageIndex{1}\): Increase in greenhouse gas concentrations in the atmosphere over the last 2,000 years. Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air. Source: USGCRP (2009)

    One class of greenhouse gas chemicals that has no natural sources is the fluorinated gases. These include HFCs, PFCs, and SF6, among others. Because these are synthetic chemicals that are only created by humans, these gases were essentially non-existent before the industrial revolution. These synthetic gases are used for a wide variety of applications, from refrigerants to semiconductor manufacturing, and propellants to fire retardants. They tend to have a long lifetime in the atmosphere, as seen in Table \(\PageIndex{1}\). Some of these chemicals, as well as the older chlorofluorocarbons (CFCs), have been phased out by international environmental legislation under the Montreal Protocol (see Chapter 6). Due to their long lifespan, many of these now banned CFCs remain in the atmosphere. Newer chemical replacements, such as HFCs, provide many of the same industrial applications, but unfortunately have their own environmental consequences.

    Just as greenhouse gases differ in their sources and their residence time in the atmosphere, they also differ in their ability to produce the greenhouse effect. This is measured by the global warming potential, or GWP, of each greenhouse gas. The GWP of a greenhouse gas is based on its ability to absorb and scatter energy, as well as its lifetime in the atmosphere. Since CO2 is the most prevalent greenhouse gas, all other greenhouse gases are measured relative to it. As the reference point, CO2 always has a GWP of 1. Note the very high GWP values of the synthetic fluorinated gases in Table \(\PageIndex{1}\). This is largely due to their very long residence time in the atmosphere. Also note the higher GWP values for CH4 and N2O compared to CO2. How does this impact the comparison of the environmental effects of agricultural practices in lessindustrialized and more-industrialized countries that we completed in section 7.1?

    Greenhouse gas Chemical formula or abbreviation Lifetime in atmosphere Global warming potential (100-year)
    Carbon dioxide CO2 Variable 1
    Methane CH4 12 years 28-36
    Nitrous oxide N2O 114 years 298
    Hydrofluorocarbons Abbreviation: HFCs 1-270 years 12-14,800
    Perfluorocarbons Abbreviation: PFCs 2,600-50,000 years 7,390
    Sulfur hexafluoride SF6 3,200 years 22,800

    Other climate influencers

    In addition to greenhouse gases, other manmade changes may be forcing climate change. Increases in near-surface ozone from internal combustion engines, aerosols such as carbon black, mineral dust and aviation-induced exhaust are acting to raise the surface temperature. This primarily occurs due to a decrease in the albedo of light-colored surfaces by the darker-colored carbon black, soot, dust, or particulate matter. As you know, it is more comfortable to wear a white shirt on a hot summer day than a black shirt. Why is this? Because the lighter-colored material bounces more solar radiation back toward space than the darker-colored material does, allowing it to stay cooler. The darker-colored material absorbs more solar radiation, increasing its temperature. Just as the white shirt has a higher albedo than the black shirt, light-colored objects in nature (such as snow) have a higher albedo than dark-colored objects (such as soot or dust). As humans increase the amount of carbon black, soot, dust, and particulates in the atmosphere, we decrease the albedo of light-colored surfaces, causing them to absorb more solar radiation and become warmer than they would without human influence. An example of this can be seen in the snow on Figure \(\PageIndex{2}\).

    Screenshot (96).pngFigure \(\PageIndex{2}\): A photograph of the extreme dust deposition from the deserts of the Colorado Plateau onto the Colorado Rockies snowpack in 2009. Taken from the high point of the Senator Beck Basin in the San Juan Mountains, it captures the extent of the impact of darkening in which the snow albedo dropped to about 30%, more than doubling the absorption of sunlight. Credit: S. McKenzie Skiles, Snow Optics Laboratory, NASA/JPL