Generated Summary
This study, published in Environmental Research Letters, utilizes a combination of top-down (TD) and bottom-up (BU) approaches to assess global methane (CH4) emissions. The TD approach employs atmospheric CH4 concentrations to constrain possible emissions and partition them to primary sources, while the BU method uses global inventories and biogeochemical modeling. The research focuses on the year 2017, comparing these estimates with the reference period of 2000–2006 when CH4 concentrations were relatively stable, to understand the sources and sinks of methane and their geographical distribution. The study aims to provide insights into the regions and economic sectors where emissions have changed most over recent decades, contributing to the understanding of climate change and the role of anthropogenic activities.
Key Findings & Statistics
- Global Methane Emissions in 2017: The average estimated global methane emissions for 2017 were 596 Tg CH4 yr⁻¹ based on 11 top-down atmospheric inversions, with a range of 572-614 Tg CH4 yr⁻¹.
- Increase from 2000-2006: This value is 9% (50 Tg CH4 yr⁻¹) higher than the average for the period 2000–2006 (546 Tg CH4 yr⁻¹).
- Anthropogenic Contribution: Anthropogenic sources contributed 61% of total TD global methane emissions in 2017.
- Top-Down vs. Bottom-Up Discrepancies: 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 and from the absence of TD atmospheric constraints for BU approaches.
- Latitudinal Distribution (TD, 2017): Tropical sources (60°N) systems yielded only 4% (24 Tg CH4 yr⁻¹).
- Regional Emission Increases: 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).
- Arctic Emissions: No evidence of increasing methane release from the Arctic. Methane emissions from northern high-latitude systems (>60°N) were virtually unchanged in 2017 relative to the average for 2000-2006.
- Methane Sink: 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.
- Sink Partitioning (TD, 2017): Average TD atmospheric sink of 531 (502–540) Tg CH4 yr⁻¹ and an average soil sink of 40 (37–47) Tg CH4 yr⁻¹.
- Regional Emission Contributions (2017 vs. 2000-2006): Africa and the Middle East; China; and South Asia and Oceania each increased emissions by ~10-15 Tg CH4 yr⁻¹.
- North America’s Increase: North America showed a growth of 6.7 and 5.0 Tg CH4 yr⁻¹ for TD and BU approaches, respectively, mostly from the United States (5.1 and 4.4 Tg CH4 yr⁻¹, respectively).
- Europe’s Decrease: Europe was the only region where CH4 emissions appeared 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.
- Anthropogenic Emission Increase (2017 vs. 2000-2006): TD estimates of mean anthropogenic emissions in 2017 increased 40 Tg CH4 yr⁻¹ (12%) to 364 Tg CH4 yr⁻¹.
- Sectoral Contribution to Increase (TD, 2017): Agriculture and Waste contributed 60% of this increase, and Fossil Fuels the remaining 40%.
- Anthropogenic Emissions (BU, 2017): Based on BU methods, anthropogenic emissions in 2017 rose 52 Tg CH4 yr⁻¹ (16%) to 380 Tg CH4 yr⁻¹.
- Sectoral Contribution to Increase (BU, 2017): With 56% of the increase coming from Fossil Fuels and 44% from Agriculture and Waste sources.
- Agriculture and Waste in South Asia/Oceania, Africa, and South America: 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.
- Fossil Fuel Emissions in China and North America: Fossil fuel-related methane emissions were the largest in China (5.3 and 12.2 Tg CH4 yr⁻¹ for TD and BU, respectively) and North America, Africa, and South Asia and Oceania (4 to 6 Tg CH4 yr⁻¹ in all three regions and using both approaches).
- Methane and Warming: Methane has a 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.
Other Important Findings
- Methane’s Impact: Methane has contributed almost one quarter of the cumulative radiative forcings for CO2, CH4, and N2O combined since 1750.
- Methane Concentrations: 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.
- Natural Sources: The average global methane emissions estimated from natural sources are relatively unchanged from 2000-2006 to 2017.
- Wetland Emissions: Vegetated wetlands contributed 194 (155–217) Tg CH4 yr⁻¹ of the total natural sources based on TD methods.
- Inland Waters: Inland waters are a significant source of methane, but also the greatest source of uncertainty to the global methane budget.
- Fossil Fuel Contribution: Fossil fuel-related methane emissions in the United States increased 3.4 to 4.0 Tg CH4 yr⁻¹ for TD and BU estimates, respectively, approximately 80% of the total increase for North America from 2000–2006 to 2017.
- Source Attribution: Anthropogenic sources are estimated to contribute almost all of the additional methane emitted to the atmosphere for 2017 compared to 2000-2006.
- Arctic Methane: The study finds no evidence of increasing methane release from the Arctic, despite rapidly warming air temperatures.
Limitations Noted in the Document
- Uncertainties in Methane Sink: The study acknowledges uncertainties in the methane chemical sink, particularly regarding the global burden of OH, which could impact total emissions estimates.
- Transport Errors: Regional and sectoral partitioning of emissions varies across models due to transport errors and prior flux ratios.
- BU vs. TD Discrepancies: The BU approach tends to produce higher estimates for natural sources compared to TD methods.
- Wetland Emission Estimates: Uncertainties in ‘natural emissions’ for wetlands, freshwater systems, and other inland waters arise from factors including wetland flux density, seasonal to interannual variability in wetland extent, and potential double-counting of inland waters in some datasets.
- Limited Temporal Changes for Natural Sources: Estimates for other natural sources such as geological, termites, permafrost, rivers, lakes, and reservoirs available in the literature do not provide any temporal changes in methane emissions, which limits the analysis.
- Geological Sources Uncertainty: Estimates of methane released from natural geological sources, particularly seeps and mud volcanoes, have significant uncertainties.
- Isotopic Budget Studies: Uncertainties remain in the isotopic signature of the sources when analyzing radiocarbon methane (14CH4) in ice cores.
Conclusion
The study underscores the increasing impact of anthropogenic activities, particularly in agriculture and fossil fuels, on global methane emissions. The findings reveal that anthropogenic methane emissions have increased significantly, with agriculture, waste, and fossil fuel sectors being the primary drivers. The study highlights that emissions from these sectors in regions like South Asia, Oceania, and China have seen substantial increases. The contrasting trends in Europe, where emissions have decreased, provide a point of interest for further investigation into mitigation strategies. The study also emphasizes the need for more robust monitoring and mitigation strategies, particularly in agriculture and fossil fuel sectors. The analysis shows that the overall global emissions trajectory is still increasing and is consistent with a scenario of significant warming. In this context, the insights gained from this research emphasize the urgency of strong mitigation actions in key sectors to alter the current emissions trajectory. While the atmospheric and soil sinks of methane are also considered, the analysis suggests that their response times are not sufficient to offset the recent increases in atmospheric methane. The findings, therefore, call for immediate and concerted efforts to reduce methane emissions from anthropogenic sources to mitigate climate change. “Increased emissions from both the agriculture and waste sector and the fossil fuel sector are likely the dominant cause of this global increase.” The role of Europe, which appears to be decreasing emissions, also provides important information for future studies. Overall, this study reinforces the need for urgent and widespread efforts to reduce methane emissions from human activities to address climate change effectively. “Climate policies overall, where present for methane mitigation, have yet to alter substantially 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