Greenhouse Gases

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Many of the variable components of the atmosphere are particularly relevant to a discussion about global climate change because they absorb long wave infrared radiation (IR, or heat). These gases are referred to as greenhouse gases (GHGs) because they function similar to a greenhouse, and include H2O, CO2, CH4 N2O, O3, and CFCs.

Greenhouses are used for growing plants in cold seasons, and are made of glass or plastic roofs and walls, which allow in sunlight (see Figure 2). When the sunlight reaches the plants and soil, or any other surface in the greenhouse, it is absorbed and converted into IR or heat. The glass/plastic roof retains this heat so that plants can grow inside during cold winters.

Figure 3. The greenhouse effect. Solar short-wave radiation (light) entering the atmosphere is absorbed by the Earth’s surface and emitted as long-wave infrared radiation (heat) that is then absorbed by greenhouse gases which heats the atmosphere.1

Like the glass or plastic of a greenhouse, the Earth’s atmosphere is nearly transparent to the incoming solar radiation, which is mainly composed of short wavelength radiation (light). As a result, most of the sunlight passes through the atmosphere without being absorbed. When sunlight reaches Earth, it is absorbed by liquid and solid structures such as water, soil, plants, and buildings. Once the light is absorbed, it is converted into long-wavelength infrared radiation (IR), or heat. This heat warms the surface of the planet, making it suitable for life on Earth. In this way, the Earth’s thin layer of atmosphere helps to keep the Earth warm just as greenhouse glass/plastic helps maintain heat within the greenhouse structure.

The Earth’s surface reflects some of the solar radiation that it receives back out into space. This reflectance is referred to as Earth’s albedo, and it varies from region to region. Natural concentrations of atmospheric greenhouse gases absorb this released IR, thereby slowing the loss of heat from the Earth into space (see Figure 3). This phenomenon, called the greenhouse effect, when in balance, helps maintain the range of temperature on Earth suitable for living organisms.

Without the greenhouse effect, Earth’s average surface temperature would be -18°C (0°F), too cold to support most plant and animal life. However, too much greenhouse gas in the atmosphere has caused the global climate change and warming impacts that the Earth is experiencing today.

Different greenhouse gases (GHGs) have different capacities for trapping heat. For example, among the five major greenhouse gases, water vapor is the most abundant and contributes to 90% of the greenhouse effect. Whereas methane (CH4) and nitrous oxide (N2O) are not as abundant as carbon dioxide (CO2), they are 20 and 300 times more potent than CO2 in trapping heat, respectively.

Looking Ahead

Fossil fuels and forests have instrumental value for human life. However, when we use these resources in a way that is not sustainable for Earth’s atmosphere we morally violate the intrinsic value of nature. This issue is discussed in the upcoming Biodiversity and Ethics section.

Human activities such as burning fossil fuels (coal, oil, and natural gas), industrial agricultural practices, and deforestation have caused the concentrations of greenhouse gases in the atmosphere to increase precipitously since the beginning of the Industrial Revolution in the mid-eighteenth century  (Figure 4).

Industrial agricultural practices produce more greenhouse gas emissions than any other single human activity through the manufacture of inorganic fertilizers, operation of field machinery, transportation of supplies to farms and food to market, and the energy required for pump irrigation.

Deforestation world-wide is primarily conducted to expand agriculture and development, and removes an enormously important “sink” of carbon dioxide by removing the dense forest vegetation. By removing the photosynthetic vegetation which “takes up” carbon dioxide, we effectively increase concentrations of greenhouse gases in the atmosphere.

For example, average global atmospheric carbon dioxide has increased from 280 ppm to over 400 ppm since the mid-eighteenth century. Check the US National Oceanic & Atmospheric Administration (NOAA) for the current concentration of CO2 in the atmosphere. Increased carbon dioxide has amplified the greenhouse effect and, as a result, has contributed to a rise in the surface temperature of the Earth. A more detailed discussion of how burning fossil fuels disrupts the natural balance of the Earth’s carbon cycle is described later in this chapter.

Concentrations of Greenhouse Gases from 0 to 2005

Figure 4: Atmospheric concentrations of 3 greenhouse gases (carbon dioxide,methane, and nitrous oxide) over the last 2,000 years. Note that each of these gases has increased since 2005, and CO2 is now hovering around 400 ppm. You can see the most recent concentration values of these greenhouse gases at the Carbon Dioxide Information Analysis Center.1

Questions to Consider

Do you have any elders in your family or among your friends and neighbors who you can talk to about climate? If so, ask them if they have personally experienced a change in weather patterns and climate in the course of their lives.

If they have experienced changes in climate in their lives, what factors do you think might go into explaining these changes?

What meaning do their experiences have for the way you think about weather patterns and climate?