The greenhouse effect happens because of certain naturally occurring substances in the atmosphere. Unfortunately, since the Industrial Revolution, humans have been pouring huge amounts of those substances into the air.
Carbon dioxide (CO2) 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, resulting in an overall increase in carbon dioxide concentrations [Source: Keeling, C.D. and T.P. Whorf]. These increased concentrations are considered the primary factor in global warming, because carbon dioxide absorbs infrared radiation. Most of the energy that escapes Earth's atmosphere comes in this form, so extra CO2 means more energy absorption and an overall increase in the planet's temperature.
The Worldwatch Institute reports that carbon emissions worldwide have increased from about 1 billion tons in 1900 to about 7 billion tons in 1995. The Institute also notes that the average surface temperature of Earth has gone from 14.5 degrees C in 1860 to 15.3 degrees C in 1980.
The IPCC says that the pre-industrial amount of CO2 in the Earth's atmosphere was about 280 parts per million (ppm), meaning that for every million molecules of dry air, 280 of them were CO2. In contrast, 2005 levels of CO2 were measured at 379 ppm [Source: IPCC].
Nitrous oxide (N2O) 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 [Source: Soil Conservation Council of Canada]. 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.
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:
- By extracting it from coal
- From large herds of livestock (i.e., digestive gases)
- From the bacteria in rice paddies
- Decomposition of garbage in landfills
Methane acts much like carbon dioxide in the atmosphere, absorbing infrared energy and keeping heat energy on Earth. The IPCC says that methane's concentration in the atmosphere in 2005 was 1,774 parts per billion (ppb) [Source: IPCC]. While there isn't as much methane as carbon dioxide in the atmosphere, methane can absorb and emit twenty times more heat than CO2 [Source: Hopwood, Nick and Cohen, Jordan]. 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 [Source: Discover Magazine].
What will actually happen if the entire planet warms up a few degrees? Read the next section to find out.