Common Contaminants in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 255

7.8.8 Greenhouse Gases

Greenhouse gases are gases in the atmosphere capable of absorbing and emitting radiation
within the thermal infrared range (Karl and Trenberth 2003; USEPA 2009g). Greenhouse
gases cause the Earth to warm up, and since the start of the industrial revolution in the
early eighteenth century, levels of greenhouse gases in the Earth's atmosphere have
increased (United States Department of State 2006). Figure 7.22 shows the greenhouse gas
effect. The primary effects of greenhouse gases are climate change related and not directly
related to toxicity.
Greenhouse gases are not generally investigated at specific sites of environmental con-
tamination. It is important, however, to discuss greenhouse gases because of their poten-
tial impacts on climate change. Figure 7.23 shows the increase of carbon dioxide in the
atmosphere since 1960 (NOAA 2009). The yearly variation shown in Figure 7.23 is seasonal
and is attributed to extraction of carbon dioxide from plant matter during photosynthesis.
The greenhouse gases include the following compounds (USEPA 2009g):

• Carbon dioxide (CO 2 )
• Methane (CH 4 )
• Nitrous oxide (N 2 O)
• Fluorinated gases including the following:
• Hydrofluorocarbons (HFCs)
• Perfluorocarbons (PFCs)
• Sulfur hexafluoride (SF 6 )—an industrially produced gas with a global warm-
ing potential (GWP) almost 24,000 times that of CO 2 . The GWP measures the
relative contribution of a gas to global warming based on its ability to absorb
infrared radiation, its residence time in the atmosphere, and the specific wave-
lengths of energy absorbed. For comparison purposes, CO 2 has a GWP = 1.

Some solar radiation
is reflected by the
Earth and the
atmosphere.

Some of the infrared radiation
passes through the atmosphere.
Some is absorbed and re-emitted
in all directions by greenhouse
gas molecules. The effect of this
is to warm the Earth's surface
and the lower atmosphere.

Most radiation is absorbed
by the earth's surface
and warms it.

Atmosphere
Earth's surface

Infrared radiation
is emitted by the
Earth's surface.

FIGURE 7.22
Greenhouse gas effect. (From United States Environmental Protection Agency, Greenhouse Gases, http://www.
epa.gov/climatechange/indicators/pdfs/CI-greenhouse-gases.pdf (accessed June 28, 2010), 2010a.)