Greenhouse effect is the result of greenhouse gases (e.g., CO2, NO2, CH4, H2O, and others) naturally present in the atmosphere. Sunlight (visible light in the electromagnetic spectrum) passes through the atmosphere and warms the Earth’s surface. In turn, the Earth surface radiates some of this energy as infrared (IR) radiation back to space. As IR radiation passes through the atmosphere, greenhouse gases absorb most of the energy. This energy is then re-emitted in all directions. Some of this IR radiation is trapped by the atmosphere, thus warming the atmosphere and the Earth’s surface. This process slows the loss of heat to space, keeping the Earth’s surface warmer than it would be without the greenhouse gases. Without these greenhouse gases, the Earth’s atmosphere would be much colder and life as we know it would not exist. This process is known as the greenhouse effect because it is similar to how a greenhouse works.
Increase in greenhouse gases from burning of fossil fuels, livestock agriculture, and deforestation has led to global warming. This process is sometimes known as the enhanced greenhouse effect. During the past several ice ages in the last 600 thousand years, the maximum atmospheric CO2 was about 280 parts per million (PPM) by volume. Now it is nearly 410 ppm and is increasing by about 2 ppm per year.
How do greenhouse gases trap IR radiation? The energy from IR photons causes a greenhouse gas molecule to vibrate. A gas molecule would likely bump into several other gas molecules before re-emitting the infrared photon. The gas molecule might transfer the energy it gained from the absorbed photon to another molecule, adding speed to that molecule’s motion. Since the temperature of a gas is a measure of the speed of its molecules, the faster motion of a molecule that eventually results from the absorbed IR photon raises the atmospheric temperature.
This ability to absorb and re-emit IR energy is what makes CO2 an effective heat-trapping greenhouse gas. Not all gas molecules are able to absorb IR radiation. For example, nitrogen (N2) and oxygen (O2), which make up more than 90% of the Earth’s atmosphere, do not absorb IR photons. CO2 molecules can vibrate in ways that simpler nitrogen and oxygen molecules cannot. This property allows CO2 molecules to capture IR photons. Other significant greenhouse gases include water vapor (H2O), methane (CH4), nitrous oxide (N2O) and ozone (O3). The figure below (from the NASA Goddard Institute for Space Studies) shows the absorption at specific outgoing (from the Earth) IR frequencies by specific greenhouse gases.
