Unit 2: Atmosphere // Section 4: Major Greenhouse Gases
Many GHGs, including water vapor (the most important), ozone, carbon dioxide, methane, and nitrous oxide, are naturally present in the atmosphere. Other GHGs are synthetic chemicals that are emitted only as a result of human activity. Anthropogenic (human) activities are significantly increasing atmospheric concentrations of many GHGs.
- Carbon dioxide (CO2), the most significant GHG directly affected by anthropogenic activity, is the product of the oxidation of carbon in organic matter, either through combustion of carbon-based fuels or the decay of biomass. Natural CO2 sources include volcanic eruptions, respiration of organic matter in natural ecosystems, natural fires, and exchange of dissolved CO2 with the oceans. The main anthropogenic sources are (a) fossil fuel combustion and (b) deforestation and land use changes (such as converting agricultural land or forests to urban development), which release stored organic matter and reduce the ability of natural ecosystems to store carbon.
- Methane (CH4) is produced by anaerobic decay of organic material in landfills, wetlands, and rice fields; enteric fermentation in the digestive tracts of ruminant animals such as cattle, goats, and sheep; manure management; wastewater treatment; fossil fuel combustion; and leaks from natural gas transportation and distribution systems and abandoned coal mines.
- Nitrous oxide (N2O) is produced by fertilizer use, animal waste management, fossil fuel combustion, and industrial activities.
- Hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) are synthetic chemicals that are used in a variety of industrial production processes such as semiconductor manufacturing. PFCs are also produced as a by-product of aluminum smelting. Both groups of chemicals are finding increasing use as substitutes for ozone-depleting chlorofluorocarbons (CFCs), which are being phased out under the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. HFCs and PFCs are replacing CFCs in applications such as refrigeration and foam-blowing for insulation.
When atmospheric GHG concentrations increase, Earth temporarily traps infrared radiation more efficiently, so the natural radiative balance is disturbed until its surface temperature rises to restore equilibrium between incoming and outgoing radiation. It takes many decades for the full effect of greenhouse gases to be realized in higher surface temperatures, because the oceans have a huge capacity to store heat. They must be gradually warmed by excess infrared radiation from the atmosphere. Figure 4 illustrates the relative contributions from man-made emissions of various GHGs to climate change.
Figure 4. Importance of human-produced greenhouse gases
See larger image
Source: Courtesy Marian Koshland Science Museum of the National Academy of Sciences http://www.koshland-science-museum.org.
As we will see in section 8, "The Global Carbon Cycle," CO2 emitted from combustion of fossil fuel cycles between the atmosphere and land and ocean "sinks" (carbon storage reservoirs), which are absorbing a large fraction of anthropogenic carbon emissions. Ultimately, though, there are limits to the amount of carbon that these sinks can absorb. These sinks are more likely to delay than to prevent human actions from altering Earth's radiative balance.
Higher surface temperatures on Earth will have profound impacts on our planet's weather and climate. Before we consider those impacts, however, we need to understand how variables such as pressure, temperature, and moisture combine to create air currents, drive normal atmospheric circulation patterns, and create the overall climate.