2. Global Warming

New heat trapping gases in our atmosphere

During the past few decades, several heat-trapping gases have been accumulating in the atmosphere as the result of human activity (Table GW.1). Although less common than carbon dioxide and water vapor, each molecule is far more powerful and potentially as significant for global warming. These other man-made greenhouse gases include methane, abbreviated CH4, nitrous oxide, or N2O for short, and the chlorofluorocarbons, known as the CFCs. Even though the total emissions of these molecules are quite small when compared with those of carbon dioxide, they are much more efficient at trapping infrared heat radiation. As a result, they can together contribute about as much global warming as carbon dioxide alone.

Table GW.1. Greenhouse gases produced by human activitya

Greenhouse gas Pre-industrial Recent GWPc Lifetime Sources
concentrationb concentrationb
(1860) (2001) (years)
Carbon Dioxide (abbreviated CO2)266 ppm369.4 ppm1120Burning coal, oil, and natural gas, deforestation, cement manufacture
Methane (abbreviated CH4)848 ppb1782.5 ppb2312Livestock, rice growing, natural gas and oil production, coal mining
Nitrous Oxide (abbreviated N2O)285 ppb314.5 ppb296114Nitrogen fertilizers, chemical manufacturing, waste treatment
CFC-11zero262.5 ppt3,80050aerosol cans,
CFC-12zero540.5 ppt 8,100102foam insulation, industrial solvents
Sulfur hexafluoride(abbreviated SF6)zero 4.0 ppt22,2003,200Electrical equipment insulation, magnesium production, medical treatments

a From http://cdiac.esd.ornl.gov/pns/current_ghg.htm. Water vapor produces sixty to seventy percent of the Earth’s global warming, but it is not included here because water vapor is not directly produced by human activity.

b Averages of the measured amounts; ppm = parts per million (106), ppb = parts per billion (109), ppt = parts per trillion (1012).

c The GWP, or the Global Warming Potential, is used to contrast the radiative effects of different greenhouse gases relative to carbon dioxide. Values are the ratio of global warming from one unit mass of a gas to that of one unit mass of carbon dioxide over 100 years.

Methane is the same natural gas that we use at home for cooking and heating. Most of the atmospheric methane does not, however, come from gas wells. It is produced by agricultural activities such as growing rice and raising cattle. The gas is emitted by bacteria that thrive in oxygen-free places like rice paddies and the stomachs of cows.

Since pre-industrial times, the atmospheric concentration of methane has increased more than 110 percent. Over the same period atmospheric carbon dioxide has risen about 30 percent. Although methane is about 200 times less abundant than carbon dioxide, each incremental molecule of methane has about 20 times the heat-trapping power as each additional molecule of carbon dioxide.

When found in swamps, methane is known as marsh gas; it sometimes ignites spontaneously, producing flickering blue flares called will-o’-the-wisps. Some of it also escapes from coal mines, natural gas wells and leaky pipelines.

Nitrous oxide, or laughing gas, is also building up in the air, although not as rapidly as methane. The current rate of increase is about 0.2 percent a year, primarily as the result of nitrogen-based fertilizers but also from burning of fossil fuels in cars and power plants.

The chlorofluorocarbons, abbreviated CFCs, are very effective heat-trapping molecules. The addition of one CFC molecule to the air can have the same greenhouse effect as the addition of 10,000 molecules of carbon dioxide to the present atmosphere. Fortunately, the warming effect of these industrial chemicals may soon be leveling off since they have been banned on the basis of their ozone-destroying capability.

Contrary to popular misconception, however, ozone depletion and global warming are not the same thing. The CFC molecules that destroy ozone also trap heat, but the thinning of the ozone layer does not by itself make the Earth’s surface hotter.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University