Greenhouse Gas Concentrations
The atmospheric concentration of a greenhouse gas is a measure of the abundance of that gas in air, usually defined in terms of the proportion of the total volume that it accounts for. Greenhouse gases are trace gases in the atmosphere and are usually measured in parts per million by volume (ppmv), parts per billion by volume (ppbv) or parts per trillion (million million) by volume (pptv). Despite their very low concentrations, they have the ability to regulate the global climate, by trapping heat that would otherwise escape to space. The natural greenhouse effect keeps the Earth's surface 33°C warmer than it should be. The enhancement of the greenhouse effect by man-made greenhouse gas emissions, including carbon dioxide, methane, nitrous oxide and CFCs, may have caused a global warming during the 20th century of 0.6°C, with another 3°C projected to occur in the 21st century.
The pre-industrial concentration of carbon dioxide in the atmosphere was about 280 ppmv. This concentration had remained fairly constant since the end of the last Ice Age about 14.000 years ago, when it increased from 190 ppmv. Coinciding with this prehistoric rise in atmospheric carbon dioxide was a global rise in average surface temperature of 5°C. The concern today is that the increase in atmospheric carbon dioxide since the late 18th century as a result of man-made carbon dioxide emissions is causing a further global warming. Atmospheric concentration of carbon dioxide is now about 370 ppmv, and is increasing 1.2 ppmv each year. This concentration is higher than at any time during the last 160,000 years. This level of increase over the last 200 years has contributed roughly 60% to the enhancement of the greenhouse effect. To prevent further increases in carbon dioxide concentrations would require a massive 60% reduction of global carbon dioxide emissions.
The present global atmospheric concentration of methane is 1.75 ppmv, more than double the pre-industrial value of about 0.8 ppmv. Since the late 18th century methane has contributed about 20% to the enhancement of the greenhouse effect. In order to stabilise concentrations of methane at present day levels, an immediate reduction in global emissions by 15 to 20% would be needed.
The global atmospheric concentration of nitrous oxide - 314 ppbv - is now about 16% greater than pre-industrial concentrations. Nitrous oxide has been responsible for about 4 to 6% of the enhancement of the greenhouse effect. In order to stabilise concentrations of nitrous oxide at present day levels, an immediate reduction in global emissions by 70 to 80% would be needed.
The concentrations of the CFCs in the atmosphere are very small, measured in parts per trillion (million million). However, they have contributed about 12% to the enhancement of the natural greenhouse effect, because they are very good at trapping heat. Molecule for molecule some CFCs are thousands of times stronger than carbon dioxide as greenhouse gases. Concentrations of some of the CFCs are now beginning to decline since their use was phased out in the aftermath of the Montreal Protocol (1987) to prevent ozone depletion. Concentrations of their replacements, the HCFCs however, are increasing. To date, HCFCs have contributed about 2% to the enhancement of the greenhouse effect.