Emission of methane from plants Nisbet, R.E.R; Fisher, R; Nimmo, R.H ...
Proceedings of the Royal Society. B, Biological sciences,
04/2009, Letnik:
276, Številka:
1660
Journal Article
Recenzirano
Odprti dostop
It has been proposed that plants are capable of producing methane by a novel and unidentified biochemical pathway. Emission of methane with an apparently biological origin was recorded from both ...whole plants and detached leaves. This was the first report of methanogenesis in an aerobic setting, and was estimated to account for 10-45 per cent of the global methane source. Here, we show that plants do not contain a known biochemical pathway to synthesize methane. However, under high UV stress conditions, there may be spontaneous breakdown of plant material, which releases methane. In addition, plants take up and transpire water containing dissolved methane, leading to the observation that methane is released. Together with a new analysis of global methane levels from satellite retrievals, we conclude that plants are not a major source of the global methane production.
From 2007 to 2013, the globally averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr−1. Simultaneously, δ13CCH4 (a measure of the 13C/12C isotope ratio in methane) has ...shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics, for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short‐term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13C‐depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post‐2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.
Plain Language Summary
Atmospheric methane, which is a powerful greenhouse gas, is increasing rapidly. In the 20th century, methane growth was primarily driven by emissions from fossil fuel sources, such as the natural gas industry and coal mining. Then, in the early years of the 21st century, came a period of stability in methane. However, since 2007, growth has resumed, with especially strong growth in 2014. Evidence from carbon isotopes implies that the primary cause of the new growth is an increase in biogenic emissions, probably from wetlands and also agricultural sources, such as rice fields and cattle. The evidence presented in this research study, from a wide range of measurement sites both in the northern and southern hemispheres, suggests increased tropical emissions, for example from tropical wetlands, may be a principal cause of the global rise in methane. Contributions to the growth may also come from agricultural sources and perhaps some fossil fuel emissions also.
Key Points
Atmospheric methane is growing rapidly
Isotopic evidence implies that the growth is driven by biogenic sources
Growth is dominated by tropical sources
Atmospheric methane grew very rapidly in 2014 (12.7 ± 0.5 ppb/year), 2015 (10.1 ± 0.7 ppb/year), 2016 (7.0 ± 0.7 ppb/year), and 2017 (7.7 ± 0.7 ppb/year), at rates not observed since the 1980s. The ...increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1,775 ppb in 2006 to 1,850 ppb in 2017. Simultaneously the 13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, now trending negative for more than a decade. The causes of methane's recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and subtropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at >5 ppb/year in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained.
Plain Language Summary
The rise in atmospheric methane (CH4), which began in 2007, accelerated in the past 4 years. The growth has been worldwide, especially in the tropics and northern midlatitudes. With the rise has come a shift in the carbon isotope ratio of the methane. The causes of the rise are not fully understood, and may include increased emissions and perhaps a decline in the destruction of methane in the air. Methane's increase since 2007 was not expected in future greenhouse gas scenarios compliant with the targets of the Paris Agreement, and if the increase continues at the same rates it may become very difficult to meet the Paris goals. There is now urgent need to reduce methane emissions, especially from the fossil fuel industry.
Key Points
Atmospheric methane is rising; its carbon isotopic ratio has become more depleted in C‐13
The possible causes of the change include an increase in emissions, with changing relative proportions of source inputs, or a decline in methane destruction, or both
If this rise continues, there are significant consequences for the UN Paris Agreement
The atmospheric methane burden is increasing rapidly, contrary to pathways compatible with the goals of the 2015 United Nations Framework Convention on Climate Change Paris Agreement. Urgent action ...is required to bring methane back to a pathway more in line with the Paris goals. Emission reduction from “tractable” (easier to mitigate) anthropogenic sources such as the fossil fuel industries and landfills is being much facilitated by technical advances in the past decade, which have radically improved our ability to locate, identify, quantify, and reduce emissions. Measures to reduce emissions from “intractable” (harder to mitigate) anthropogenic sources such as agriculture and biomass burning have received less attention and are also becoming more feasible, including removal from elevated‐methane ambient air near to sources. The wider effort to use microbiological and dietary intervention to reduce emissions from cattle (and humans) is not addressed in detail in this essentially geophysical review. Though they cannot replace the need to reach “net‐zero” emissions of CO2, significant reductions in the methane burden will ease the timescales needed to reach required CO2 reduction targets for any particular future temperature limit. There is no single magic bullet, but implementation of a wide array of mitigation and emission reduction strategies could substantially cut the global methane burden, at a cost that is relatively low compared to the parallel and necessary measures to reduce CO2, and thereby reduce the atmospheric methane burden back toward pathways consistent with the goals of the Paris Agreement.
Plain Language Summary
Methane is a powerful climate warmer, and the amount of methane in the air is growing rapidly. Reducing human‐caused methane emissions is urgent if the 2015 United Nations Paris Agreement to limit climate warming is to succeed. There is hope, though the problem of methane mitigation is very wide and complex. Much of the task is in finding, identifying, and quantifying emissions. Rapid technical advances are making it much easier to locate and thus cut emissions from fossil fuel industries (gas, coal, and oil). Assessing emissions from landfill and sewage facilities is also becoming easier. In particular, poorly regulated landfills in fast‐growing tropical megacities need attention. Agricultural emissions are less tractable but may also be reduced to some extent, especially by improving manure management. Many methane mitigation options offer cost‐effective approaches to cut global warming and bring the amount of methane in the air back to a pathway that is consistent with the aims of the Paris Agreement.
Key Points
The atmospheric methane burden is rising fast; this growth is an increasing threat to the Paris Agreement of the UN Framework Convention on Climate Change (UNFCCC)
New gas measurement technologies, on the ground and in the air, have greatly improved our ability to locate and quantify emissions and to identify mitigation targets
Emissions can be cut by ending fossil fuel emissions, cutting biomass burning, improving landfills, especially in the tropics, and changing cattle farming practice
By comparison of the methane mixing ratio and the carbon isotope ratio (δ13CCH4) in Arctic air with regional background, the incremental input of CH4 in an air parcel and the source δ13CCH4 signature ...can be determined. Using this technique the bulk Arctic CH4 source signature of air arriving at Spitsbergen in late summer 2008 and 2009 was found to be −68‰, indicative of the dominance of a biogenic CH4 source. This is close to the source signature of CH4 emissions from boreal wetlands. In spring, when wetland was frozen, the CH4 source signature was more enriched in 13C at −53 ± 6‰ with air mass back trajectories indicating a large influence from gas field emissions in the Ob River region. Emissions of CH4 to the water column from the seabed on the Spitsbergen continental slope are occurring but none has yet been detected reaching the atmosphere. The measurements illustrate the significance of wetland emissions. Potentially, these may respond quickly and powerfully to meteorological variations and to sustained climate warming.
Key Points
Isotopic measurements have been used to identify major sources of Arctic methane
In late summer biogenic methane sources dominate the bulk Arctic source mix
Seabed emissions near Spitsbergen have not been detected reaching the atmosphere
Atmospheric measurements show an increase in CH4 from the 1980s to 1998 followed by a period of near‐zero growth until 2007. However, from 2007, CH4 has increased again. Understanding the variability ...in CH4 is critical for climate prediction and climate change mitigation. We examine the role of CH4 sources and the dominant CH4 sink, oxidation by the hydroxyl radical (OH), in atmospheric CH4 variability over the past three decades using observations of CH4, C2H6, and δ13CCH4 in an inversion. From 2006 to 2014, microbial and fossil fuel emissions increased by 36 ± 12 and 15 ± 8 Tg y−1, respectively. Emission increases were partially offset by a decrease in biomass burning of 3 ± 2 Tg y−1 and increase in soil oxidation of 5 ± 6 Tg y−1. A change in the atmospheric sink did not appear to be a significant factor in the recent growth of CH4.
Plain Language Summary
Methane is the second most important greenhouse gas and is responsible for approximately 17% of the direct radiative forcing from all long‐lived greenhouse gases. Observations of methane in the atmosphere have shown a dramatic increase from 2007 after a period of relative stability between the late 1990s and early 2000s, but the cause of this increase is still under scientific debate. This study uses atmospheric observations of methane and two related tracers, the isotopic ratio of carbon in methane and ethane, to constrain the sources and sinks of methane over the past three decades. The increase in methane between 2007 and 2014 is likely due to an increase in microbial sources, of 24–48 Tg/y (predominantly natural wetlands and agricultural), as well as fossil fuel sources, of 7–23 Tg/y. In contrast to other recent studies, a reduction in the atmospheric sink of methane was found not to be a significant factor in explaining the recent atmospheric increase.
Key Points
Microbial and fossil fuel sources of CH4 have both contributed to the observed increase in atmospheric CH4 from 2007
Fossil fuel sources of CH4 decreased significantly between 1990 and 1996 but increased again from the early 2000s
Variation in the main atmospheric sink of CH4, oxidation by OH radicals, appears not to have significantly contributed to the recent increase in CH4
Earth is over 4,500 million years old. Massive bombardment of the planet took place for the first 500-700 million years, and the largest impacts would have been capable of sterilizing the planet. ...Probably until 4,000 million years ago or later, occasional impacts might have heated the ocean over 100 degrees C. Life on Earth dates from before about 3,800 million years ago, and is likely to have gone through one or more hot-ocean 'bottlenecks'. Only hyperthermophiles (organisms optimally living in water at 80-110 degrees C) would have survived. It is possible that early life diversified near hydrothermal vents, but hypotheses that life first occupied other pre-bottleneck habitats are tenable (including transfer from Mars on ejecta from impacts there). Early hyperthermophile life, probably near hydrothermal systems, may have been non-photosynthetic, and many housekeeping proteins and biochemical processes may have an original hydrothermal heritage. The development of anoxygenic and then oxygenic photosynthesis would have allowed life to escape the hydrothermal setting. By about 3,500 million years ago, most of the principal biochemical pathways that sustain the modern biosphere had evolved, and were global in scope.
A thorough understanding of methane sources is necessary to accomplish methane reduction targets. Urban environments, where a large variety of methane sources coexist, are one of the most complex ...areas to investigate. Methane sources are characterised by specific δ
C-CH
signatures, so high precision stable isotope analysis of atmospheric methane can be used to give a better understanding of urban sources and their partition in a source mix. Diurnal measurements of methane and carbon dioxide mole fraction, and isotopic values at King's College London, enabled assessment of the isotopic signal of the source mix in central London. Surveys with a mobile measurement system in the London region were also carried out for detection of methane plumes at near ground level, in order to evaluate the spatial allocation of sources suggested by the inventories. The measured isotopic signal in central London (-45.7 ±0.5‰) was more than 2‰ higher than the isotopic value calculated using emission inventories and updated δ
C-CH
signatures. Besides, during the mobile surveys, many gas leaks were identified that are not included in the inventories. This suggests that a revision of the source distribution given by the emission inventories is needed.
We present methane (CH4) emissions for East Asia from a Bayesian inversion of CH4 mole fraction and stable isotope (δ13C‐CH4) measurements. Emissions were estimated at monthly resolution from 2000 to ...2011. A posteriori, the total emission for East Asia increased from 43 ± 4 to 59 ± 4 Tg yr−1 between 2000 and 2011, owing largely to the increase in emissions from China, from 39 ± 4 to 54 ± 4 Tg yr−1, while emissions in other East Asian countries remained relatively stable. For China, South Korea, and Japan, the total emissions were smaller than the prior estimates (i.e., Emission Database for Global Atmospheric Research 4.2 FT2010 for anthropogenic emissions) by an average of 29%, 20%, and 23%, respectively. For Mongolia, Taiwan, and North Korea, the total emission was less than 2 Tg yr−1 and was not significantly different from the prior. The largest reductions in emissions, compared to the prior, occurred in summer in regions important for rice agriculture suggesting that this source is overestimated in the prior. Furthermore, an analysis of the isotope data suggests that the prior underestimates emissions from landfills and ruminant animals for winter 2010 to spring 2011 (no data available for other times). The inversion also found a lower average emission trend for China, 1.2 Tg yr−1 compared to 2.8 Tg yr−1 in the prior. This trend was not constant, however, and increased significantly after 2005, up to 2.0 Tg yr−1. Overall, the changes in emissions from China explain up to 40% of the increase in global emissions in the 2000s.
Key Points
Bayesian inversion estimates of methane emissions in East Asia
Methane emissions from China increased by 3% annually from 2000 to 2011
China contributes significantly to the increase in global methane emissions
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