The latest data on atmospheric concentrations of greenhouse gases (GHGs) released by the World Meteorological Organization (WMO)’s Global Atmosphere Watch and published in the 25 November 2019 issue (issue #15) of the WMO Greenhouse Gas Bulletin make for chilling reading. Issue #15 can be accessed at the bottom of this post.
The data for 2018 show an increase in concentrations for three key GHGs, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) over 2017 levels (Table 1). For two of the GHGs (CH4 and N2O) the rate of increase 2017-2018 is greater than the average rate of increase over the preceding decade, while for CO2 the increase is around the average rate of change for that same period. Positive radiative forcing attributed to the three key GHGs has increased markedly since pre-industrial times (pre-AD 1750). Thus radiative forcing attributed to increased concentrations of CO2 has increased by 147%, CH4 by 259% and N2O by 123% since pre-AD 1750.
Table 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 with 68% uncertainties shown. The data are based on an average of several stations around the world. There are relatively few stations for tropical latitudes.
Emissions of CO2, CH4 and N2O can persist in the atmosphere for relatively long periods. They are also therefore commonly referred to as “long-lived GHGs” (LLGHGs). One other group of pollutants are also considered as LLGHGs, the ozone-depleting chlorofluorocarbons (CFCs) together with hydrochloroflurocarbons (HCFCs), hydrofluorocarbons (HFCs) and sulphur fluorides, such as “deep voice gas” mentioned previously on the Environmental Pollution blog (see here ). Although atmospheric concentrations of CFCs have stabilised and even reduced in the last decade or so since implementation of the Montreal Protocol, levels of HCFCs, HCFCs and sulphur hexaflouride (SF6, deep voice gas) have increased (HCFCs were meant as an interim replacement for CFCs, with HFCs intended to replace HCFCs) (Figure 1). According to the WMO Greenhouse Gas Bulletin, radiative forcing of all LLGHGs combined has increase by 43% since 1990, with the 2018 level corresponding to an equivalent CO2 mole fraction of 496 ppm!
Figure 1: Changing atmospheric concentrations of halocarbons (CHFs, HCFCs and HFCs) and SF6.
Issue #15 of the WMO Greenhouse Gas Bulletin also refers to some neat work involving the measurement of different isotopes of carbon. Carbon in the environment occurs in the form of three isotopes ~ 12C (the most common), 13C and 14C (the least common). Establishing the relative ratios of these isotopes in the atmosphere provides an indication of the source of the carbon, and knowing the source can help explain the most likely cause of increased concentrations of carbon-containing GHGs in the atmosphere (notably CO2 and CH4). The figure at the top of this blog and repeated below as Figure 2 illustrates changes in anthropogenic (human) emissions, the atmospheric concentration of the 13C isotope and CO2 and the 14C content of the atmosphere since the mid-18th century. Some of the data are from direct measurements, whereas the earlier data relating to atmospheric composition are from carbon preserved in ice cores and tree rings. The figure shows that as human emissions of CO2 have increased (panel a) so too have concentrations of CO2 in the atmosphere (panel c). At the same time, both the relative proportions of 13C and 14C have declined. This is because CO2 emitted from the burning of fossil fuels is relatively rich in 12C, and massive emissions of 12C-enriched CO2 from the burning of coal and oil in particular have caused a relative decline in the proportion of the 14C and 13C isotopes in the atmosphere (the massive spike in atmospheric 14C around the beginning of the 1960s relates to the atmospheric detonation of atomic bombs). Variations in relative concentrations of the three carbon isotopes provide additional evidence that a large part of the increased CO2 in the atmosphere is from the burning of fossil fuels, and is therefore human in origin.
Figure 2: Variations CO2 emissions and atmospheric concentrations, and in relative proportions of 13C and 14C isotopes.
Finally, issue #15 of the WMO Greenhouse Gas Bulletin highlights the interesting trends shown by CH4 concentrations in the atmosphere. Following rapid increases during the 1980s and 1990s, atmospheric concentrations stabilised during the early 2000s. The stabilisation appears to have been short-lived, however, as by the late 2000s levels had started to rise rapidly again (Figure 3). Approximately 60% of CH4 emissions are linked to human activity (e.g. cattle farming, rice agriculture, and the burning of fossil fuels and living biomass – e.g. forests and peatlands). Once again isotopic measurements can help determine the source of CH4 in the atmosphere. Despite this, the cause(s) of the plateau and the renewed increase in atmospheric concentrations is(are) a focus of much debate among scientists at present.
Figure 3: Globally averaged CH4 mole fraction (a) and its growth rate (b) from 1984 to 2018. The red line in both panels is the monthly mean with the seasonal variation removed. The figure is based on observations made at 127 stations globally.
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