WMO GREENHOUSE GAS BULLETINNo. 15 | 25 November 2019The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2018ISSN2078-0796Measurements of the content of radiocarbon 14C in atmospheric carbon dioxide CO2 provide a unique way to discriminate between fossil fuel combustion and natural sources of CO2. Simultaneous observations of CO2and 14C demonstrate the decline of 14C content in atmospheric CO2caused by CO2addition from fossil fuel combustion. This finding illustrates the importance of long-term measurements of atmospheric composition by laboratories involved in the WMO Global Atmosphere Watch GAW Programme in helping identify greenhouse gas emission sources.Three isotopes of carbon are found in natural systems 12C 99 of all carbon, 13C 1 and 14C 1 part per trillion. All carbon isotopes are present throughout the carbon cycle, but the relative proportion of each isotope in different carbon reservoirs varies, providing unique “fingerprints” for each reservoir. Therefore, measuring the isotopic composition of atmospheric CO2helps identify and quantify its sources and sinks.14CO2is produced in very small amounts in the upper atmosphere by cosmic rays. 14C is radioactive and decays slowly with a half-life of 5 700 years, resulting in a small but measurable 14C content in atmospheric CO2and in plant materials ed from CO2. Fossil fuels were ed from plant material millions of years ago, hence any 14C present when the plants were alive has since decayed during their stay in the Earth’s crust.13C is a stable isotope, meaning that the 13C content of fossil fuels does not change over time. However, the plants from which fossil fuels were ed take up 12C in preference to 13C, so that fossil fuels contain less 13C than the current atmospheric CO2. Fossil fuel combustion, therefore, also results in a decline in the 13C content of atmospheric CO2.The figure on the left shows the development of CO2emissions panel a [1, 2], atmospheric abundance b and isotope ratios c of CO2, since 1760, from air trapped in ice cores and air collected at Cape Grim, Australia [3], and 14C content d of atmospheric CO2from tree rings [4, 5] and air collected at Wellington, New Zealand [6]. As anthropogenic emissions have increased, atmospheric CO2has increased also. At the same time, both the 13C and 14C content of atmospheric CO2have declined, as the fossil fuel CO2emitted into the atmosphere has no 14C and a lower 13C content than the current atmosphere. The simultaneous decline in both 13C and 14C content alongside CO2increases can only be explained by the ongoing release of CO2from fossil fuel burning.The 14C fossil fuel signal in atmospheric CO2was swamped by the near doubling of 14C in the atmosphere in the early 1960s due to 14C produced by atmospheric nuclear weapons testing see panel d in the figure on the left, making 14C unusable for fossil fuel detection since the early 1950s. Yet that human-produced 14C spike has now roughly levelled throughout the carbon cycle. Since the 1990s, 14C has again become useful for detecting fossil fuel CO2and is now the principle for uating emissions of fossil fuel CO2in atmospheric measurements. For example, patterns of fossil fuel CO2hotspots have been observed across much of the world using measurements of atmospheric 14C taken directly in the air and in plant material [7, 8].WEATHER CLIMATE WATERLaw DomeWAISEDMLDMLDE08-2 firnDSSW20K firnSouth Pole firnCape GrimM AK-BHDSQ1981δ13C-CO2[‰]a Anthropogenic CO2emissionsBomb 14C perturbation1760 1810 1860 1910 1960 2010Date [CE]-6.4-6.9-7.4-7.9-8.460050040030020010020100-10-20∆14C-CO2[‰]b13C content of atmospheric CO2d 14C content of atmospheric CO2121086420400390370350330310290270Anthropogenic CO2emissions [GtC/yr]CO2[ppm]c Atmospheric CO2mole fractionIsotopes confirm the dominant role of fossil fuel combustion in increasing levels of atmospheric carbon dioxidecutive summaryThe latest analysis of observations from the WMO GAW Programme shows that globally averaged surface mole fractions1calculated from this in-situ network for carbon dioxide CO2, methane CH4 and nitrous oxide N2O reached new highs in 2018, with CO2at 407.80.1 ppm2, CH4at 18692 ppb3and N2O at 331.10.1 ppb. These values represent, respectively, 147, 259 and 123 of pre-industrial before 1750 levels. The increase in CO2from 2017 to 2018 was very close to that observed from 2016 to 2017, and practically equal to the average yearly increase over the last decade. For CH4, the increase from 2017 to 2018 was higher than both that observed from 2016 to 2017 and the average over the last decade. For N2O, the increase from 2017 to 2018 was also higher than that observed from 2016 to 2017 and the average growth rate over the past 10 years. The National Oceanic and Atmospheric Administration NOAA Annual Greenhouse Gas Index AGGI [9] shows that from 1990 to 2018 radiative forcing by long-lived greenhouse gases LLGHGs increased by 43, with CO2accounting for about 80 of this increase.Overview of the GAW in-situ network observations for 2018This fifteenth WMO Greenhouse Gas Bulletin reports atmospheric abundances and rates of change of the most important LLGHGs – CO2, CH4and nitrous oxide N2O – and provides a summary of the contributions of other gases. These three, together with CFC-12 and CFC-11, account for approximately 964[9] of radiative forcing due to LLGHGs Figure 1.The GAW Programme http//www.wmo.int/gaw coordinates systematic observations and analyses of greenhouse gases GHGs and other trace species. Sites where greenhouse gases have been measured in the last decade are shown in Figure 2. Measurement data are reported by participating countries, and are archived and distributed by the World Data Centre for Greenhouse Gases WDCGG at the Japan Meteorological Agency.The results reported here by WDCGG for the global average and growth rate are slightly different from those reported by NOAA for the same years [10], due to differences in the stations used, in the averaging procedure and a slightly different time period for which the numbers are representative. The World Data Centre for Greenhouse Gases follows the procedure described in detail in the GAW Report No. 184 [11].Table 1 provides globally averaged atmospheric abundances of the three major LLGHGs in 2018 and changes in their abundances since 2017 and 1750. Data from mobile stations blue triangles and orange diamonds in Figure 2, with the exception of NOAA sampling in the eastern Pacific, are not used for this global analysis.The three GHGs shown in Table 1 are closely linked to anthropogenic activities, and interact strongly with the biosphere and the oceans. Predicting the evolution of the atmospheric content of GHGs requires quantitative understanding of their many sources, sinks and chemical transations in the atmosphere. Observations from GAW provide invaluable insights into the budgets of these and other LLGHGs, and are used to improve emission estimates and uate satellite retris of LLGHG column averages. *Assuming a pre-industrial mole fraction of 278 ppm for CO2, 722 ppb for CH4and 270 ppb for N2O.Figure 1. Atmospheric radiative forcing, relative to 1750, of LLGHGs, and the 2018 update of the NOAA AGGI [9].CO2CH4N2O2018 global mean abundance407.80.1 ppm18692 ppb331.10.1 ppb2018 abundance relative to year 1750*147 259 1232017–2018 absolute increase2.3 ppm 10 ppb 1.2 ppb2017–2018 relative increase 0.57 0.54 0.36Mean annual absolute increase over the last 10 years2.26 ppm yr–17.1 ppb yr–10.95 ppb yr–1Table 1. Global annual surface mean abundances 2018 and trends of key greenhouse gases from the GAW global GHG monitoring network. Units are dry-air mole fractions, and uncertainties are 68 confidence limits [12]. The averaging is described in the GAW Report No. 184 [11]. A number of stations are used for the analyses 129 for CO2, 127 for CH4and 96 for N2O.3.02.52.01.50.50.0Radiative Forcing W m-2 1.0AGGI 2018 1.431.41.21.00.80.60.40.20.0Annual Greenhouse Gas Index AGGICO2CH4N2O CFC-12 CFC-11 15-minor1979 1981 1983 1985 1987 1989 1990 1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2015 20172Ground-based Aircraft Ship GHG comparison sitesFigure 2. The GAW global network for CO2in the last decade. The network for CH4is similar.The Integrated Global Greenhouse Gas Ination System IG3IS, ig3is.wmo.int, promoted by WMO, provides further insights into the sources of GHGs on the national and sub-national level.The NOAA AGGI [9] in 2018 was 1.43, representing a 43 increase in total radiative forcing 4 by all LLGHGs since 1990 and, on this scale, a 1.8 increase from 2017 to 2018 Figure 1. The total radiative forcing by all LLGHGs in 2018 3.1 W m-2 corresponds to an equivalent CO2mole fraction of 496 ppm [9].Carbon Dioxide CO2Carbon dioxide is the single most important anthropogenic GHG in the atmosphere, contributing approximately 664 of the radiative forcing by LLGHGs total 3.1 W.m-2. It is responsible for about 824of the increase in radiative forcing over the past decade and about 81 of the increase over the past five years. The pre-industrial level of 278 ppm represented a balance of fluxes among the atmosphere, the oceans and the land biosphere. The globally averaged CO2mole fraction in 2018 was 407.80.1 ppm Figure 3. The increase in annual mean from 2017 to 2018, 2.3 ppm, is nearly the same as the increase from 2016 to 2017 and practically equal to the average growth rate for the past decade 2.26 ppm yr-1. Atmospheric CO2thus reached 147 of the pre-industrial level in 2018, primarily because of emissions from combustion of fossil fuels and cement production the emissions of fossil fuel CO2were projected to reach 36.6 2 GtCO25in 2018 [13], deforestation and other land-use change 5.5 GtCO2.yr-1average for 2009–2018. Of the total emissions from human activities during the period 2009–2018, about 44 accumulated in the atmosphere, 22 in the ocean and 29 on land; the unattributed budget imbalance is 5 [13]. The portion of CO2emitted by fossil fuel combustion that remains in the atmosphere airborne fraction varies inter-annually due to the high natural variability of CO2sinks without a confirmed global trend.Methane CH4Methane contributes about 174of the radiative forcing by LLGHGs. Approximately 40 of methane is emitted into the atmosphere by natural sources e.g., wetlands and termites and about 60 comes from anthropogenic sources e.g., cattle farming, rice agriculture, fossil fuel exploitation, landfills and biomass burning [14]. Globally averaged CH4calculated from in-situ observations reached a new high of 1869 2 ppb in 2018, an increase of 10 ppb with respect to the previous year Figure 4. This increase is higher than the increase of 7 ppb in the period 2016–2017 and the average annual increase over the past decade. The mean annual increase of CH4dropped from approximately 12 ppb yr-1in the late 1980s to near zero during 1999–2006. Atmospheric CH4has been increasing since 2007, reaching 259 of the pre-industrial level 722 ppb due to increased emissions from anthropogenic sources. Studies using GAW CH4measurements indicate that higher CH4emissions from wetlands in the tropics and from anthropogenic sources at mid-latitudes of the northern hemisphere are likely causes of this recent increase see central insert on the supporting isotopic studies.YearN2O growth rate ppb/yr0.00.51.01.52.01985 1990 1995 2000 2005 2010 2015b16001650170017501800185019001985 1990 1995 2000 2005 2010 2015YearCH4mole fraction ppbaYearCH4growth rate ppb/yr-5051015201985 1990 1995 2000 2005 2010 2015bYearCO2growth rate ppm/yr0.01.02.03.04.01985 1990 1995 2000 2005 2010 2015b3003053103153203253303351985 1990 1995 2000 2005 2010 2015YearN2O mole fraction ppba3403503603703803904004101985 1990 1995 2000 2005 2010 2015YearCO2mole fraction ppm420aFigure 3. Globally averaged CO2mole fraction a and its growth rate b from 1984 to 2018. Increases in successive annual means are shown as shaded columns in b. The red line in a is the monthly mean with the seasonal variation removed; the blue dots and line depict the monthly averages. Observations from 129 stations have been used for this analysis.Figure 4. Globally averaged CH4mole fraction a and its growth rate b from 1984 to 2018. Increases in successive annual means are shown as shaded columns in b. The red line in a is the monthly mean with the seasonal variation removed; the blue dots and line depict the monthly averages. Observations from 127 stations have been used for this analysis.Figure 5. Globally averaged N2O mole fraction a and its growth rate b from 1984 to 2018. Increases in successive annual means are shown as shaded columns in b. The red line in a is the monthly mean with the seasonal variation removed; in this plot, it is overlapping with the blue dots and line that depict the monthly averages. Observations from 96 stations have been used for this analysis.3Continued on page 64Atmospheric methane is the second most important anthropogenic greenhouse gas. It has contributed about 17 of the total radiative forcing by LLGHGs since pre-industrial times, as shown in Figure 6.As reported in Dlugokencky et al. [18], isotopic measurements carry powerful ination about sources of atmospheric methane because they are enriched or depleted in carbon and hydrogen isotopes 13C or D relative to ambient background air Figure 7. CH4ed at high temperatures combustion is enriched in the heavier isotope, and CH4from biogenic origin is depleted. Biogenic sources, such as wetlands, have 13C signatures that vary between −70‰ and −60‰ at high northern latitudes, and between −60‰ and −50‰ in tropical climates. Because of different photosynthetic pathways, C3 and C4 plants have very different organic carbon isotope signatures and, when these plants are either burned or digested, the CH4released has different isotopic signatures. Therefore, savannah grassland burning C4 releases CH4with δ13C ranging from −20‰ to −15‰, whereas boreal forest burning releases CH4ranging from −30‰ to −25‰. Similarly, ruminants digesting C4 plants give off CH4ranging from −55‰ to −50‰, whereas those eating C3 plants give off −65‰ to −60‰ CH4. The natural gas industry produces CH4of variable isotopic signature depending on the ation temperature of the gas reservoir biogenic or thermogenic. The resultant gas distribution networks contain gas of approximately −50‰ in the Russian pipelines, around −35‰ for the North Sea and in some cases −25‰.Isotopic measurements can provide some useful insights into the renewed growth of methane that started in 2007. Figure 8 presents in greater detail the recent changes in the global methane level a