Gases that trap heat in the atmosphere are called greenhouse gases. These gases absorb heat energy emitted from Earth’s surface and re-radiate it back to the ground. In this way, they contribute to the greenhouse effect, which keeps the planet from losing all of its heat from the surface at night.
The concentrations of various greenhouse gases in the atmosphere determine how much heat is absorbed by the atmosphere and re-radiated back to the surface. Human activities especially fossil-fuel combustion since the Industrial Revolution are responsible for steady increases in the concentration of greenhouse gases in the atmosphere.
There are three factors that affect the degree to which any greenhouse gas will influence global warming:
- Its abundance in the atmosphere
- How long it stays in the atmosphere
- Its global-warming potential
Carbon Dioxide (Co2)
Carbon Dioxide is a colorless gas that is a by-product of the combustion of organic matter. It makes up less than 0.04 percent of Earth’s atmosphere, most of which was put there by volcanic activity very early in the planet’s life. Today, human activities are pumping huge amounts of CO2 into the atmosphere, through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon cycle. These increased concentrations are considered the primary factor in global warming, because carbon dioxide absorbs infrared radiation. Most of the energy that escapes the Earth’s atmosphere comes in this form, so extra CO2 means more energy absorption and an overall increase in the planet’s temperature.
Water vapor is the most potent of the greenhouse gases in Earth’s atmosphere and is the most abundant greenhouse gas. The amount of water vapor in the atmosphere cannot, in general, be directly modified by human behavior—it’s set by air temperatures. The warmer the surface, the greater the evaporation rate of water from the surface. As a result, increased evaporation leads to a greater concentration of water vapor in the lower atmosphere capable of absorbing infrared radiation and emitting it downward.
Methane is a combustible gas, and it is the main component of natural gas. Methane occurs naturally through the decomposition of organic material and is often encountered in the form of “swamp gas.”
Man-made processes produce methane in several ways. It is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.
Methane acts much like carbon dioxide in the atmosphere, absorbing infrared energy and keeping heat energy on Earth. While there isn’t as much methane as carbon dioxide in the atmosphere, it is thought that methane can absorb and emit twenty times more heat than CO2. Some scientists even speculate that a large-scale venting of methane into the atmosphere (such as from the release of huge chunks of methane ice locked under the oceans) could have created brief periods of intense global warming that led to some of the mass extinctions in the planet’s distant past.
Nitrous Oxide (N2O)
Nitrous oxide (or N2O), is more commonly known as laughing gas. It is another important greenhouse gas. Although the amounts being released by human activities are not as great as the amounts of CO2, nitrous oxide absorbs much more energy than CO2 (about 270 times as much). For this reason, efforts to curb greenhouse gas emissions have focused on N2O as well. The use of large amounts of nitrogen fertilizer on crops releases nitrous oxide in great quantities, and it is also a by-product of combustion.
Ozone is a natural gas composed of three atoms of oxygen. Its chemical symbol is O3. It is blue in color and has a strong odor. Normal oxygen (O2), which we breathe, has two oxygen atoms and is colorless and odorless. The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer between about 10 km and 50 km above the surface (or between about 6 and 31 miles). However, even in this “layer”, the ozone concentrations are only two to eight parts per million, so most of the oxygen there remains of the dioxygen type.
When UV light strikes (Oxygen) O2 molecules, they are split into two individual O atoms — O and O. When one of the O atoms combine with O2 molecule, ozone (O3) is created. Even though Ozone is only a small part of the gases in this layer, it plays a vital role because it shields us from the sun’s harmful UV rays. It is called often referred to as the ‘Good Ozone’, for obvious reasons—because it protects humans, life and animals on earth.
Bad Ozone is also known as Tropospheric Ozone, or ground level ozone. It is an atmospheric pollutant. This gas is found in the troposphere, the layer that forms the immediate atmosphere. Bad Ozone does not exist naturally. Human actions cause chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). Each time there is a reaction of chemicals such as those found in cars, power plants and factory emissions, in the presence of sunlight (UV light), Bad Ozone is created. Bad ozone contaminates the air and contributes to what we typically experience as “smog” or haze. Note that smog from bad ozone is usually in the summer, caused by the action of sunlight on a mixture of hydrocarbons and oxides of nitrogen. It is known as Photochemical or Summer Smog.
Ozone acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth. However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide. Because of its short-lived nature, tropospheric ozone does not have strong global effects but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide according to NASA.
Fluorinated gases (F-gases) are man-made gases that can stay in the atmosphere for centuries and contribute to a global greenhouse effect. Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases.
Chlorofluorocarbons (CFCs), once used as refrigerants and aerosol propellants until they were phased out by international agreement, are also greenhouse gases.