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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. 41), March 2017.

1. Carbon Dioxide (CO2)

The level of carbon dioxide (CO2), which contributes the most to increases in anthropogenic induced radiative forcing, has been increasing since the beginning of the industrial era. The global average mole fraction of CO2 reached a new high of 400.0±0.1 ppm in 2015, which constitutes 144% of the pre-industrial level (in 1750). The annual increase of 2.3 ppm from 2014 to 2015 was larger than that observed from 2013 to 2014 and that averaged over the past decade (about 2.1 ppm/year).

The global growth rate of CO2 shows significant interannual variability driven by natural processes. Large interannual changes in 1987/1988, 1997/1998, 2002/2003, 2009/2010 and 2014/2015 resulted from warmer conditions caused by El Niño-Southern Oscillation (ENSO) events. The anomalously strong El Niño event in 1997/1998 resulted in greater annual increases in CO2 worldwide in 1998 than during any other one-year period. The high growth rate in 2012/2013 and the smaller one in 2013/2014 were most likely related to changes in fluxes between the atmosphere and terrestrial biosphere, particularly in tropical and subtropical regions (WMO, 2015). The exceptionally low growth rate in 1992, including negative values in northern high latitudes, may have been due to low global temperatures following the eruption of Mount Pinatubo in 1991.

Variations in CO2 mole fraction can be seen on seasonal scales. The seasonal amplitudes are large in northern high and mid-latitudes and small in the Southern Hemisphere. In southern low latitudes, there is no clear annual cycle, but a semiannual cycle can be determined.

2. Methane (CH4)

Methane (CH4) is the second most significant greenhouse gas which is largely influenced by anthropogenic activity and whose level has been increasing since the beginning of the industrial era. The annual average mole fraction was 1845±2 ppb in 2015, an increase of 11 ppb since 2014. The mean annual absolute increase during the last 10 years was 6.0 ppb/year. The mole fraction is now 256% of that in the pre-industrial period.

The latitudinal gradient of CH4 mole fraction is large from the northern mid-latitudes to the tropics, suggesting that the major sources of CH4 are located in the Northern Hemisphere.

In the 1990s, CH4 growth rates decreased significantly in all latitudinal zones. However, both hemispheres experienced high growth rates in 1998, caused by the higher than average global mean temperature. The global growth rates were generally low from 1999 to 2006, except during the El Niño event of 2002/2003, but since 2007 they have been high again. In the last nine years through 2015, the global mole fraction increased by 6.7 ppb/year.

CH4 mole fractions vary seasonally, being relatively high in winter and low in summer. The seasonal amplitudes of CH4 are large, not only in the Northern Hemisphere but also in southern high and mid-latitudes which are associated with methane sinks. In southern low latitudes, a distinct secondary maximum in boreal winter overlies the annual cycle.

3. Nitrous Oxide (N2O)

Nitrous oxide (N2O) is an important greenhouse gas whose level is increasing globally. N2O data submitted to the WDCGG show that mole fractions are increasing in both hemispheres. The global mean mole fraction reached a new high of 328.0±0.1 ppb in 2015, which is 1.0 ppb higher than that in the previous year. This mole fraction corresponds to 121% of that in the pre-industrial period. The mean annual absolute increase during the last 10 years was 0.89 ppb/year and the interhemispheric difference in mole fraction is 1.0 ppb (averaged over the years 1980 to 2015), indicating that the majority of N2O sources are situated in the Northern Hemisphere.

4. Halocarbons and Other Halogenated Species

Halocarbons, most of which are anthropogenic and generated since the 20th century, are potent greenhouse gases, with some also acting as ozone-depleting compounds. Levels of some halocarbons (e.g. CFCs) increased in the 1970s and 1980s, but this increase has almost ceased by now, due to the production and consumption control of halocarbons under the Montreal Protocol on Substances that Deplete the Ozone Layer and its subsequent Adjustments and Amendments. However, some substances targeted by the Kyoto Protocol but not regulated by the Montreal Protocol, such as HFCs and SF6, are increasing.

The mole fraction of CFC-11 peaked around 1992 and then started decreasing. The mole fraction of CFC-12 increased until around 2005 and then started decreasing gradually. The mole fraction of CFC-113 stopped increasing in the 1990s, followed by a slight decrease over about twenty years. The mole fractions of HCFCs, used mainly as substitutes for CFCs, have increased significantly during the last decade. The growth of HCFC-141b decelerated around 2005, but has accelerated over the last several years. The mole fraction of Halon-1211 has decreased since 2005, whereas the mole fraction of Halon-1301 is increasing. The mole fraction of CCl4 was maximal around 1991 and has since decreased slowly. The mole fraction of CH3CCl3 peaked around 1992 and decreased thereafter. The mole fractions of HFC-134a, HFC-152a and SF6 are increasing, but the growth of HFC-152a has decelerated over the last several years.

5. Carbon Monoxide (CO)

Carbon monoxide (CO) is not a greenhouse gas itself but influences the mole fractions of greenhouse gases by affecting hydroxyl radicals (OH). Beginning in 1950, the CO mole fraction increased at a rate of 1% per year but started to decrease in the late 1980s (WMO, 1999). In 2015, the global mean mole fraction of CO was 91±2 ppb. The mole fraction is high in the Northern Hemisphere and low in the Southern Hemisphere, suggesting substantial anthropogenic emissions in the Northern Hemisphere.

There is a large interannual variability of CO growth rates. The growth rate increases are mainly attributed to biomass burning emissions during El Niño conditions.

The monthly mean mole fractions show seasonal variations, with large amplitudes in the Northern Hemisphere and small amplitudes in the Southern Hemisphere with opposite phase.


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