11.4: Atmospheric methane
Total global \(CH_4\) emissions are approximately 550 teragrams (Tg) of \(CH_4\) per year (1 Tg \(CH_4\) per year = 1012 grams of \(CH_4\) per year; Saunois et al., 2016). Of this, roughly 40% comes from natural sources. The largest (and most uncertain) natural emissions of \(CH_4\) are from wetlands, defined as regions that are permanently or seasonally waterlogged. Natural wetlands include high-latitude bogs and fens, tropical swamps, and temperate wetlands. Saturated soils in warm tropical environments tend to produce the most \(CH_4\). However, warming Arctic temperatures raise concerns of increasing emissions from high-latitude wetlands and future decomposition of carbon currently stored in frozen Arctic soils (e.g., Schaefer et al., 2011; Schuur et al., 2015). Figure \(\PageIndex{1}\), provides a pictorial representation of the main components of the global methane cycle.
Estimates of global \(CH_4\) emissions from wetlands range from 127 to 227 Tg \(CH_4\) per year (Saunois et al., 2016), with most probable values between 167 and 185 Tg \(CH_4\) per year. Most emissions occur in tropical regions (Matthews 1989; Melton et al., 2013; Saunois et al., 2016). Currently, only about 25 Tg \(CH_4\) per year (i.e., 4% of global emissions) are thought to be emitted from high northern latitudes (AMAP 2015; Saunois et al., 2016). Because emissions are sensitive to temperature and precipitation they exhibit significant seasonal cycles, especially at high latitudes, as well as interannual variability caused by moisture and temperature variability. Smaller amounts of CH4 are emitted from fires, the ocean, and enteric fermentation in termites and wild animals (20 Tg CH4 per year or less for each). In addition, up to 60 Tg CH4 per year may be emitted from geological sources, such as seeps, clathrates, mud volcanoes, and geothermal systems (Etiope et al., 2008; Schwietzke et al., 2016).
Table \(\PageIndex{1}\): Historic a and Decadal b Global Mean Emissions and Their Partitioning to the Carbon Reservoirs of Atmosphere, Ocean, and Land 1750–2011 Cumulative Pg Cc 1980–1989 Pg C per Year 1990–1999 Pg C per Year 2000–2009 Pg C per Year 2007–201
| 1750–2011 Cumulative Pg Cc | 1980–1989 Pg C per Year | 1990–1999 Pg C per Year | 2000–2009 Pg C per Year | 2007–2016 Pg C per Year | 2016 Pg C per Year | |
|---|---|---|---|---|---|---|
| Emissions | ||||||
| Fossil Fuels and Industry | 375 ± 30 | 5.5 ± 0.3 | 6.3 ± 0.3 | 7.8 ± 0.4 | 9.4 ± 0.5 | 9.9 ± 0.5 |
| Land-Use Change | 180 ± 80 | 1.2 ± 0.7 | 1.3 ± 0.7 | 1.2 ± 0.7 | 1.3 ± 0.7 | 1.3 ± 0.7 |
| Partitioning to Carbon Reservoir | ||||||
| Partitioning to Carbon Reservoir | 240 ± 10 | 3.4 ± 0.1 | 3.1 ± 0.1 4 | 4.0 ± 0.1 | 4.7 ± 0.1 | 6.0 ± 0.2 |
| Ocean Uptake | 160 ± 80 | 1.7 ± 0.5 | 1.9 ± 0.5 | 2.1 ± 0.5 2 | 2.4 ± 0.5 | 2.6 ± 0.5 |
| Land Uptake | 155 ± 30 | 2.0 ± 0.6 | 2.5 ± 0.5 | 2.9 ± 0.8 3.0 | 3.0 ± 0.8 | 2.7 ± 0.9 |
|
Unlike CO2, CH4 has an atmospheric chemical sink that nearly balances total global emissions. Removal of atmospheric CH4 by reaction with the hydroxyl radical (OH) results in a CH4 atmospheric lifetime of about 9 to 10 years. Observationally constrained estimates of CH4 lifetime suggest either small decreases of about 2% from 1980 to 2005 (Holmes et al., 2013) or stable CH4 lifetimes with the possibility of interannual variability of about 2% (Montzka et al., 2011). CH4 is a much more powerful greenhouse gas than CO2 (on a per mass basis and over 100 years, CH4 is about 25 times more effective at trapping heat than CO2).
As shown in Figure \(\PageIndex{2}\)b, this page, atmospheric CH4 increased rapidly during the 1980s and early 1990s before its growth leveled off between the mid-1990s and early 2000s. Methane has resumed its increase in the atmosphere since 2006, and observations show that this growth has even accelerated since 2014. The changing atmospheric CH4 growth rate has been the subject of much debate, questioning why growth rate slowed for a decade starting in the mid-1990s. Several studies suggested that this slower rate was due to decreases in fugitive emissions from fossil fuel production (Aydin et al., 2011; Simpson et al., 2012) or to decreased emissions from anthropogenic microbial sources, such as rice agriculture (Kai et al., 2011). On the other hand, Dlugokencky et al. (1998, 2003) proposed that the slowing of CH4 growth in the atmosphere was due to an approach to a quasi–steady state, reached when global sources and sinks are in balance. Consistent with this view, the study of Schwietzke et al. (2016) found that emissions from oil and gas production have remained stable over the past several decades, implying increasing efficiency in fossil fuel production industries while their production was increasing over time.
Dlugokencky et al. (2003) predicted that CH4 would approach a steady state in the atmosphere of about 1,780 ppb by the 2010s if there were no major changes in its budget. The methane budget did change, however, because the atmospheric growth of CH4 resumed its rise in 2006. The cause of the recent increase in CH4 growth also has been much debated. Based on global observations of the CH4 isotope, 13CH4, the global growth in CH4 appears likely to have been dominated by microbial sources in the tropics (wetlands or agriculture and waste), rather than fossil fuel production (Nisbet et al., 2016; Schaefer et al., 2016), as suggested by some studies (e.g., Rice et al., 2016). Other studies have argued that 13CH4 may not be a very strong constraint on the global methane budget and that changes in the atmospheric CH4 chemical sink are responsible for the global methane changes (Rigby et al., 2017; Turner et al., 2017). However, plausible chemical mechanisms that could explain the changes in the CH4 sink have not been identified. Using space-based retrievals of carbon monoxide, Worden et al. (2017) argued that the isotopic data record also can be consistent with increased fossil fuel emissions if global biomass-burning emissions have decreased twice as much as estimates based on space-based observations of burned areas. If the recent rise of global atmospheric CH4 is indeed due to increases in microbial emissions, then the question becomes whether anthropogenic or natural microbial sources are responsible. Some studies have suggested that anthropogenic microbial sources, such as livestock, are behind the increased atmospheric growth of CH4 (Schaefer et al., 2016; Saunois et al., 2016). If the increase is due to emissions from wetlands, especially in the tropics, then this raises the possibility that changing climate could be changing natural emissions.
Methane Perturbations in the Global Carbon Cycle
The carbon cycle undergoes perturbations caused by a variety of natural processes such as wildfires, droughts, insect infestations, and disease. These processes can themselves be affected by human activities, for example through GHG emissions that change climate, wildfire suppression, and land-use change. During longer periods, variations in the Earth’s orbit also drive significant perturbations to the global carbon cycle. Over the recent several centuries, human activity has resulted in perturbations to the carbon cycle that have no precedent in geological records. Anthropogenic emissions also can directly alter the chemistry of the atmosphere, possibly affecting its ability to remove pollutants.
Atmospheric CH4 also is influenced by diverse human activities, ranging from food production (e.g., ruminants and rice) to waste (e.g., sewage and landfills) to fossil fuel production (e.g., coal, oil, and gas). Future increases in population likely will increase CH4 emissions from agriculture and waste as demand rises for more food production. Furthermore, the current boom in shale oil and gas exploitation has focused attention on leakage from drilling, storage, and transport of fossil fuel (e.g., Peischl et al., 2015; Pétron et al., 2014). Chemical reaction with OH accounts for about 90% of the total CH4 sink (Ehhalt 1974). These OH radicals, produced through the photolysis of ozone (O3) in the presence of water vapor, are destroyed by reactions with CH4 and other compounds. Uncertainty in the sink due to chemical loss by OH is 10% to 20%, because the OH distribution remains uncertain at regional to global scales (Saunois et al., 2016).
Current estimates reported by Saunois et al. (2016) for anthropogenic emissions average 328 Tg CH4 per year (ranging from 259 to 370 Tg CH4 per year). Extraction and processing of fossil fuels account for 32% to 34% of all anthropogenic emissions. Livestock, agriculture, landfills, and sewage together account for another 55% to 57%, with the remainder due to biomass and biofuel burning. A recent study using observations of the isotopic composition of CH4 suggests that emissions from fossil fuel production and geological emissions may be 20% to 60% higher than previously thought. This increase would require a compensating reduction in microbial emissions from natural and anthropogenic sources (Schwietzke et al., 2016) for the atmosphere to be in balance with the observed global average CH4 abundance. Current CH4 levels are unprecedented in over at least 800,000 years (Loulergue et al., 2008). Recent National Oceanic and Atmospheric Administration atmospheric network observations have shown that global CH4 increased rapidly through the late 1990s, leveled off during the early 2000s, and began to increase again in 2007 (Dlugokencky et al., 2009; Rigby et al., 2008). These changes in global CH4 are not well understood and are under debate. Although Dlugokencky et al. (1998, 2003) suggested that the plateau in CH4 growth resulted from an approximate balance between global sources and sinks, some studies suggested that decreases in anthropogenic emissions (Aydin et al., 2011; Kai et al., 2011; Simpson et al., 2012) led to the period of slow CH4 growth. Isotopic evidence points toward increased emissions from microbial sources as an explanation for the recent rise in global CH4 (Nisbet et al., 2016; Schaefer et al., 2016; Schwietzke et al., 2016). However, increases in anthropogenic emissions also have been proposed (Rice et al., 2016), as well as decreases in the chemical loss (Rigby et al., 2017; Turner et al., 2017). Worden et al. (2017) have recently suggested a significant role for fossil fuel emissions in the recent growth of atmospheric CH4 based on decreases in biomass burning that could change the interpretation of methane isotope observations. This result is based on space-based observations of atmospheric CO, which itself may be responding to changes in other sources besides biomass burning. Figure 1.1, p. 45, shows that CH4 contributed just over 0.5 W/m2 in 2017 to global total anthropogenic radiative forcing, an amount which is about one-fourth of that from CO2. Although CH4 is much more effective at absorbing infrared radiation (Hofmann et al., 2006; Myhre et al., 2013),3 it is about a hundred times less abundant in the atmosphere than CO2.
Future Changes in Atmospheric Methane
Coupled carbon cycle–climate models forced with future “business as usual” emissions scenarios suggest that the changing carbon cycle will be a net positive feedback on climate, reinforcing warming, but the size of the projected feedback is highly uncertain (Friedlingstein et al., 2014). Besides the uncertain trajectories of human factors such as fossil fuel emissions, land use, or significant mitigation efforts, various natural processes can lead to the carbon cycle being a positive feedback. For example, a warming climate can lead to increased fires and droughts and less storage of carbon in the terrestrial biosphere. In particular, warming is expected to decrease carbon uptake in the tropics and midlatitudes. In the high latitudes, a warmer climate is expected to lead to a more productive biosphere and more uptake but also may result in increased respiration and release of stored CO2 and CH4 in soils and lakes. Negative feedbacks also are possible, such as increased atmospheric CO2, leading to increased carbon storage in the terrestrial biosphere (e.g., Schimel et al., 2015), although the relative roles of this effect relative to land-use change, nitrogen deposition, and temperature increases on the cumulative land carbon sink over the last century are not fully understood (Huntzinger et al., 2017). Human impacts on land use can directly impact climate. Deforestation and agriculture can affect carbon storage in soil and biomass. Fertilizer use also affects the global nitrogen budget and can increase carbon storage. Large-scale drainage of wetlands and conversion to agricultural land can reduce CH4 emissions from anaerobic respiration while potentially increasing faster soil carbon loss through aerobic respiration.
Frozen Arctic soils compose another potential carbon cycle–climate feedback (see Ch. 11: Arctic and Boreal Carbon, p. 428, and Ch. 19: Future of the North American Carbon Cycle, p. 760). An estimated 1,460 to 1,600 Pg C are frozen in Arctic soils, and warming has proceeded in the Arctic faster than in any other region. Current understanding suggests that approximately 146 to 160 Pg C, primarily as CO2, could be vulnerable to thaw and release to the atmosphere over the next century (Schuur et al., 2015; see Ch. 11: Arctic and Boreal Carbon). This release of carbon from permafrost is likely to be gradual and occur on century timescales (Schuur et al., 2015). If the amount of carbon estimated to enter the atmosphere by Schuur et al. (2015) were released annually at a constant rate, emissions would be far lower than annual fossil fuel emissions (about 9 Pg C per year) but comparable to land-use change (0.9 Pg C per year).
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