Basic Information on Greenhouse and Related Gases
The following analytical results for major greenhouse and related gases are excerpted from the WMO WDCGG Data Summary (WMO WDCGG No. 30), March 2006.
1. Carbon Dioxide (CO2)
The level of Carbon Dioxide (CO2), which of all the greenhouse gases contributes most to global warming, has been increasing since the pre-industrial period. Global mean mixing ratio has reached a new high in 2004 at 377.1 ppm, which increased by 1.8 ppm during the last year. This mixing ratio corresponds to 135% of the pre-industrial level. Mixing ratios peak in northern high and mid-latitudes, suggesting strong net sources in these areas.
The global growth rate varies significantly interannually and was 1.9 ppm/year on average for the latest 10 years (1994-2004). The high growth rates in 1987/1988, 1997/1998 and 2002/2003, which exceeded 2 ppm/year, resulted from the warm events related to El Niño-Southern Oscillation (ENSO).
The anomalously strong El Niño event in 1997/1998 brought about worldwide high increases in 1998. The exceptionally low growth rates in 1992, including negative values for northern high and mid-latitudes, were caused by low global temperatures following the eruption of Mt. Pinatubo in 1991.
Amplitudes of the seasonal cycle are clearly large in northern high and mid-latitudes and small in the Southern Hemisphere. The northern seasonal cycle mainly reflects seasonal variation in the absorption/emission in the biosphere there, while the southern cycle reflects oceanic variations and biomass burning in addition to the influence of the biosphere. In southern low latitudes, an annual cycle cannot be seen clearly but a semiannual cycle can. This is probably due to two opposing factors - the direct influence of sources and sinks there and the propagation of the out-of-phase seasonal variation from the Northern Hemisphere.
Image Credit: Global Warming/Robert A. Rohde
2. Methane (CH4)
CH4 is the second most significant greenhouse gas, and its level has been increasing since the beginning of the 19th Century. Global mean mixing ratios reflect an annual increase, and the annual averaged mixing ratio was 1783 ppb in 2004, which was the same as that in 2003. This level was the highest mixing ratio since the beginning of the worldwide observation. The mixing ratio corresponds to 255% of that in the pre-industrial level.
The annual mixing ratios peak in the northernmost latitudes and fall toward the southernmost latitudes, suggesting significant net sources in northern latitudes.
The global growth rate was 11 ppb/year on average for the period 1984-1990, but the rates decreased markedly from the 1980s to the 1990s. The global growth rate for the latest 10 years (1994-2004) was 3.7 ppb/year. Growth rates decreased significantly in some years, including 1992, when negative values were recorded in northern high latitudes, and 1996, when growth almost stopped in many regions. However, both hemispheres experienced high growth rates in 1998, caused by an exceptionally high global mean temperature. The global growth rates decreased to almost zero in 2000-2001, but the growth rates increased again with the occurrence of the 2002 /2003 El-Niño event.
Monthly mean mixing ratios have a seasonal variation with high mixing ratios in winter and low mixing ratios in summer. Unlike CO2, amplitudes of the seasonal cycle are large for CH4, not only in the Northern Hemisphere but also in southern high and mid-latitudes. In southern low latitudes, a distinct semi-annual component with a secondary maximum in boreal winter overlays the annual component. This is attributed to the large-scale transportation of CH4 from the Northern Hemisphere.
3. Nitrous Oxide (N2O)
Nitrous oxide (N2O) is an important greenhouse gas, and its level is increasing on a global scale. Data for N2O submitted to the WDCGG show that mixing ratios are increasing in both hemispheres. Global mean mixing ratio has reached a new high in 2004 at 318.6 ppb, which increased by 0.7 ppb during the last year. The mean growth rate of the global mean mixing ratio during the latest 10 years (1994 - 2004) was 0.8 ppb/year. This mixing ratio corresponds to 118% of the pre-industrial level.
4. Halocarbons
Halocarbons, most of which are anthropogenic, are effective greenhouse gases and some also act as ozone-depleting compounds. Levels of some halocarbons (CFCs, etc.) increased in the 1970s and 1980s, but have now almost ceased increasing as a result of regulation of production and emission under the Montreal Protocol on Substances that Deplete the Ozone Layer and its Adjustments and Amendments.
Mixing ratios of CFC-11 peaked around 1992 and then started decreasing. CFC-12 increased slowly, but growth has almost stopped in recent times. CFC-113 growth stopped in the early 1990s, and over the last decade has shown a trend of decreasing slightly. Mixing ratios of HCFC-141b and HCFC-142b are increasing linearly. Mixing ratios of CCl4 are decreasing slowly. Mixing ratios of CH3CCl3 peaked around 1992 and thereafter clearly started to decrease.
The mixing ratios of HCFCs, which are the industrial replacements of CFCs, are increasing at rapid rates, although their mixing ratios are small.
5. Surface Ozone (O3)
Ozone (O3) plays important roles in the atmospheric environment through radiative and chemical processes. It absorbs UV radiation in the stratosphere, making a temperature profile, and circulates the atmosphere with its absorbed energy. It also absorbs IR radiation, and is thus one of the greenhouse gases.
Variation in the mixing ratio of O3 near the surface, so-called surface ozone, reflects various processes there. While some of the O3 in the troposphere comes from the stratosphere, the rest is chemically produced in the troposphere through oxidation of CO or hydrocarbons in the presence of rich NOx.
Many stations at various locations measure the mixing ratio of surface ozone. As the seasonal and interannual variations are relatively large, it is difficult to identify a global long-term trend.
6. Carbon Monoxide (CO)
Carbon monoxide (CO) is not a greenhouse gas, but brings influences the mixing ratios of greenhouse gases by affecting hydroxyl radicals (OH). Its mixing ratio in northern high latitudes has been increasing since the mid-19th Century. The mean global mixing ratio was 94 ppb in 2004. The mixing ratio is high in the Northern Hemisphere and low in the Southern Hemisphere, suggesting substantial anthropogenic emissions in the Northern Hemisphere.
Although the global mixing ratio of CO was increasing before the mid-1980s, the growth stopped or the mixing ratio subsequently decreased after then (WMO, 1999a). There was large fluctuation in the growth rate, however, with high positive rates followed by high negative rates in northern latitudes and southern low latitudes from 1997 to 1999. The growth rates in the Northern Hemisphere increased again in 2002.
Monthly mean mixing ratios show a seasonal variation with large fluctuations in the Northern Hemisphere and small fluctuations in the Southern Hemisphere. This seasonal cycle is driven by industrial emissions, biomass burning, large-scale transportation, and variations in OH mixing ratio which acts as a sink.
7. Nitrogen Monoxide (NO), and Nitrogen Dioxide (NO2)
Nitrogen oxides (NOx, i.e., NO and NO2) are not greenhouse gases, but influence mixing ratios of important greenhouse gases by affecting OH. In the presence of NOx, CO and hydrocarbons are oxidized to produce ozone (O3), which affects the Earth’s radiative balance as a greenhouse gas and the oxidization capacity of the atmosphere by reproducing OH.
Most of the stations reporting NOx data to the WDCGG are located in Europe. NOx has large temporal and geographic variability, and it is difficult to identify a long-term trend. In Europe, NO2 mixing ratios are generally higher in southern regions than in northern regions.
8. Sulfur Dioxide (SO2)
Sulphur dioxide (SO2) is not a greenhouse gas but a precursor of atmospheric sulphate (H2SO4) aerosol. Sulphate aerosol is produced by SO2 oxidation through photochemical gas-to-particle conversion. SO2 has also been a major source of acid rain and deposition throughout industrial times.
Most of the stations reporting SO2 data to the WDCGG are located in Europe. Generally, in Europe, SO2 mixing ratios are higher in southern regions than in northern regions.
