Basics of the Carbon Cycle and the Greenhouse Effect – The majority of the Earth’s atmosphere is composed of a mixture of only a few gases-nitrogen, oxygen, and argon; combined these three gases comprise more than 99.5% of all the gas molecules in the atmosphere. These gases which are most abundant within the atmosphere exhibit almost no effect on warming the earth and its atmosphere since they do not absorb visible or infrared radiation. However, there are minor gases which comprise only a small portion of the atmosphere (about 0.43% of all air molecules, most of which are water vapor at 0.39%) that do absorb infrared radiation. These “trace” gases contribute substantially to warming of the Earth’s surface and atmosphere due to their abilities to contain the infrared radiation emitted by the Earth (see below for details on the Greenhouse Effect). Since these trace gases influence the Earth in a manner somewhat similar to a greenhouse, they are referred to as GreenHouse Gases, or GHGs.
Influential Greenhouse Gases
CO2 Molecule / Carbon Cycle
Carbon Dioxide (CO2) is a colorless, odorless gas consisting of molecules made up of two oxygen atoms and one carbon atom. Carbon dioxide is produced when an organic carbon compound (such as wood) or fossilized organic matter, (such as coal, oil, or natural gas) is burned in the presence of oxygen. Carbon dioxide is removed from the atmosphere by carbon dioxide “sinks”, such as absorption by seawater and photosynthesis by ocean-dwelling plankton and land plants, including forests and grasslands. However, seawater is also a source, of CO2 to the atmosphere, along with land plants, animals, and soils, when CO2 is released during respiration.
CH4 Molecule Carbon Cycle
Methane (CH4) is a colorless, odorless non-toxic gas consisting of molecules made up of four hydrogen atoms and one carbon atom. Methane is combustible, and it is the main constituent of natural gas-a fossil fuel. Methane is released when organic matter decomposes in low oxygen environments. Natural sources include wetlands, swamps and marshes, termites, and oceans. Human sources include the mining of fossil fuels and transportation of natural gas, digestive processes in ruminant animals such as cattle, rice paddies and the buried waste in landfills. Most methane is broken down in the atmosphere by reacting with small very reactive molecules called hydroxyl (OH) radicals.
N2O Molecule
Nitrous oxide (N2O) is a colorless, non-flammable gas with a sweetish odor, commonly known as “laughing gas”, and sometimes used as an anesthetic. Nitrous oxide is naturally produced in the oceans and in rainforests. Man-made sources of nitrous oxide include the use of fertilizers in agriculture, nylon and nitric acid production, cars with catalytic converters and the burning of organic matter. Nitrous oxide is broken down in the atmosphere by chemical reactions driven by sunlight.
SF6 Molecule
Sulfur hexafluoride (SF6) is an extremely potent greenhouse gas. SF6 is very persistent, with an atmospheric lifetime of more than a thousand years. Thus, a relatively small amount of SF6 can have a significant long-term impact on global climate change. SF6 is human-made, and the primary user of SF6 is the electric power industry. Because of its inertness and dielectric properties, it is the industry’s preferred gas for electrical insulation, current interruption, and arc quenching (to prevent fires) in the transmission and distribution of electricity. SF6 is used extensively in high voltage circuit breakers and switchgear, and in the magnesium metal casting industry.
The Greenhouse Effect
Many of the atmospheric trace gases, despite their relatively minor abundances, have a significant influence on Earth’s climate, due to a phenomenon called the “Greenhouse Effect”.
The Sun ultimately drives Earth’s climate by emitting energy in the form of sunlight. Sunlight is solar radiation mostly in the form of visible and a smaller portion as ultraviolet (UV) energy. This is also called shortwave radiation. Clouds and the Earth’s surface reflect some of this incoming solar radiation back out to space (approximately 30%), some (mostly UV) is absorbed by the atmosphere (about 20%), and the remaining half is absorbed at the Earth’s surface. Sunlight absorbed by Earth’s surface acts to warm the surface.