Generated Summary
This research, published as a perspective in Environmental Research Letters, investigates the anthropogenic methane emissions, particularly from agricultural and fossil fuel sources. The study employs a multi-faceted approach, utilizing both top-down (TD) and bottom-up (BU) methods to estimate the global methane budget for the year 2017, with comparisons to the 2000-2006 period. The TD approach relies on atmospheric CH4 concentrations to constrain total possible emissions and distribute them across primary sources, while the BU method uses global inventories and biogeochemical modeling to attribute emissions to specific sources. This dual-method approach allows for a comprehensive analysis of methane sources and sinks, providing insights into regional and sectoral changes in emissions over recent decades.
Key Findings & Statistics
- Global average methane concentrations in the atmosphere reached ~1875 parts per billion (ppb) at the end of 2019, more than two-and-a-half times preindustrial levels.
- Methane (CH4) emissions have contributed almost one quarter of the cumulative radiative forcings for CO2, CH4, and N2O (nitrous oxide) combined since 1750.
- Global warming potential (GWP) ~86 times stronger per unit mass than CO2 on a 20-year timescale and 28-times more powerful on a 100-year time scale.
- The largest methane sources include anthropogenic emissions from agriculture, waste, and the extraction and use of fossil fuels as well as natural emissions from wetlands, freshwater systems, and geological sources.
- Average estimated global methane emissions for 2017 were 596 Tg CH4 yr-1 based on 11 top-down atmospheric inversions, with an ensemble max.-min. range of 572-614 Tg CH4 yr-1. This value is 9% (50 Tg CH4 yr-1) higher than the average for the period 2000–2006 (546 Tg CH4 yr-1, range 538–555).
- Anthropogenic sources also contributed 61% of total TD global methane emissions in 2017.
- The estimate from the BU approach yielded an increase of 51 Tg CH4 yr-1, from 696 (560–834) Tg CH4 yr-1 in 2000-2006 to 747 (602–896) Tg CH4 yr-1 in 2017.
- Anthropogenic sources contributed an estimated 51% of total global BU emissions in 2017.
- Based on TD methods in 2017, tropical sources (<30°N) emitted 64% (383 Tg CH4 yr-1; 351–405) of global methane emissions and northern mid-latitude sources (30°N-60°N) contributed 32% (185 Tg CH4 yr-1; 171–209).
- High-latitude (>60°N) systems yielded only 4% of global methane emissions (24 Tg CH4 yr-1; 21–28).
- Average methane emissions increased by 29 and 32 Tg CH4 yr-1 in the tropics (<30°N) for TD and BU approaches, respectively, and by 15 and 23 Tg CH4 yr-1 in northern mid-latitudes (30°N-60°N).
- The average global atmospheric and soil methane sink estimated for 2017 increased to 571 (540–585) Tg CH4 yr-1 from 546 (531–555) Tg CH4 yr-1 for the 2000–2006 average based on the TD approaches.
- TD estimates of mean anthropogenic emissions in 2017 increased 40 Tg CH4 yr-1 (12%) to 364 (range 340-381) Tg CH4 yr-1. Agriculture and Waste contributed 60% of this increase and Fossil Fuels the remaining 40%.
- Anthropogenic sources are estimated to contribute almost all of the additional methane emitted to the atmosphere for 2017 compared to 2000-2006.
- Based on BU methods, anthropogenic emissions in 2017 rose 52 Tg CH4 yr-1 (16%) to 380 (range 359-407) Tg CH4 yr-1, with 56% of the increase coming from Fossil Fuels and 44% from Agriculture and Waste sources.
- Increased agricultural emissions predominated in South Asia/Oceania, Africa, and South America, with increases of 9-10 Tg CH4 yr-1 in South Asia/Oceania and 7-9 Tg CH4 yr-1 in Africa.
- Europe’s agricultural methane emissions decreased -1.4 to -2.8 Tg CH4 yr-1 for TD and BU methods, respectively.
- The new global BU estimate for natural geological sources (terrestrial and marine) of 45 (range of 18–65) Tg CH4 is 7 Tg CH4 smaller than the value in Saunois et al (2016a).
- Current trajectories in socioeconomic development also suggest the world is likely to follow IPCC Shared Socioeconomic pathways (SSP) leading to relatively higher emission trajectories over the next decade.
- Estimates for 2018 and 2019 show increases in atmospheric methane of 8.5 and 10.7 ppb, respectively, two of the four highest annual growth rates since 2000.
Other Important Findings
- Climate stabilization remains elusive, with increased greenhouse gas concentrations already increasing global average surface temperatures 1.1 °C above pre-industrial levels.
- Methane is far less abundant in the atmosphere than CO2, but it absorbs thermal infrared radiation much more efficiently.
- The difference of ~150 Tg CH4 yr¯¹ in total global emissions between TD and BU methods arises primarily from a divergence in estimates of natural sources, particularly from freshwater and geological ones and from the absence of TD atmospheric constraints for BU approaches.
- The latitudinal attribution of methane emissions highlights the role of tropical and temperate sources relative to boreal and Arctic systems.
- Specific regions contributed the most to greater methane emissions in 2017 compared with 2000-2006: Three regions (Africa and the Middle East; China; and South Asia and Oceania) each increased emissions by ~10–15 Tg CH4 yr¯¹ assessed using both TD and BU methods.
- The next-largest changes occurred in North America, with growth of 6.7 and 5.0 Tg CH4 yr¯¹ for TD and BU approaches, respectively, mostly from the United States.
- Europe was the only region where CH4 emissions appear to have decreased in 2017 relative to 2000-2006, with emissions down -1.6 Tg CH4 yr¯¹ for TD methods and -4.3 Tg CH4 yr¯¹ for BU methods.
- Mean top-down estimates for natural methane sources were 232 (194–267) Tg CH4 yr¯¹ in 2017 compared with 220 (198–243) Tg CH4 yr¯¹ for 2000–2006; mean bottom-up estimates were substantially higher: 367 (243–489) and 368 (245–482) Tg CH4 yr¯¹.
- Wetlands contributed 194 (155–217) Tg CH4 yr¯¹ of the total, about 83% of natural sources based on TD methods.
Limitations Noted in the Document
- Uncertainties in regional and sectoral partitioning vary across models based on transport errors, prior flux ratios, and inversion baselines.
- Uncertainties in the methane chemical sink, particularly on the global burden of OH is about 10%-15% and translates to an uncertainty of approximately ±9% on total global emissions.
- Uncertainties in ‘natural emissions’ for wetlands plus all other inland waters arise from factors that include wetland flux density, seasonal to interannual variability in wetland extent, and some double-counting of wetland and small inland waters, contributing to higher BU estimates for natural sources than in the TD inventory.
- We were unable to include uncertainties in TD total emissions attributable to uncertainties in the methane chemical sink.
- The difference of ~150 Tg CH4 yr¯¹ in total global emissions between TD and BU methods arises primarily from a divergence in estimates of natural sources, particularly from freshwater and geological ones and from the absence of TD atmospheric constraints for BU approaches.
- The uncertainties in isotopic budget studies remain substantial due to the uncertainties in the isotopic signature of the sources.
Conclusion
The study underscores the critical role of anthropogenic activities, particularly in agriculture, waste management, and fossil fuel extraction and use, in driving the increase in global methane emissions. The findings highlight that both agricultural and fossil fuel sources contribute equally to the rising levels of methane in the atmosphere, emphasizing the need for targeted mitigation strategies across these sectors. The analysis reveals that the global average methane emissions for 2017 were significantly higher than the average for the 2000-2006 period. This increase is largely attributed to greater anthropogenic emission sources. The regional analysis indicates that the tropics and northern mid-latitudes are the primary contributors to methane emissions, with a lack of evidence suggesting increasing methane release from the Arctic. The study highlights that anthropogenic sources contribute almost all of the additional methane emitted to the atmosphere. The study highlights the increasing emissions from agriculture, waste, and fossil fuel sectors across various regions, including China, and the fossil fuel sector in the United States, stressing the urgency for stronger mitigation efforts. The difference in emission estimates between top-down and bottom-up methods emphasizes the challenges in accurately quantifying natural sources, such as wetlands. The authors suggest that the timescales for sink responses are too slow to explain most of the increased methane in the atmosphere in recent years. The conclusions from this research reinforce the need for immediate and focused action to reduce methane emissions. Climate policies have yet to substantially alter the global emissions trajectory to date.
IFFS Team Summary
- https://iopscience.iop.org/article/10.1088/1748-9326/ab9ed2/pdf
- https://www.theguardian.com/environment/2020/jul/14/livestock-farming-and-fossil-fuels-could-drive-4c-global-heat-rise
- From 2000-2006 period to 2017 there has been significant increase in methane emissions
- These are from two main sources caused by humans
- 1) Agriculture
- Biggest source is enteric fermentation from ruminant animals
- These include cattle, sheep, goats
- 2) Fossil Fuel industry
- mostly Coal, oil and gas
- Rice paddies and garbage decomposition are another agricultural sources that must be addressed
- There is a natural methane cycle that co exists
- it cannot absorb the excess produced by humans
- The global warming potential (GWP) of methane is 86 times that of CO2 on a 20 year time scale
- 20 year times scale is used by IPCC
- Or calculated at 28 times the GWP of CO2 if calculated on 100 year time scale
- Permafrost thaw, as global temperatures rise, may cause increased methane emissions
- Despite years of awareness, methane emissions are increasing and not decreasing
- Human caused methane emissions are at highest levels ever