The release of methane to the atmosphere is a major focus of efforts to reduce greenhouse gas emissions. Methane is a potent greenhouse gas, accounting for about 20% of anthropogenic global warming since 1750.1 Since 2007, globally averaged atmospheric methane has been increasing at an accelerating rate.2 In addition to its impact on climate, methane contributes to the formation of tropospheric ozone (O3), a major air pollutant with significant adverse impacts on human health, photosynthesis by crops, and the ability of terrestrial ecosystems to absorb carbon. Recent research suggests that methane emissions from the fossil fuel energy system have been underestimated.3
Methane is produced during the coal formation process and gets trapped on the surface of the coal (adsorption) and in small pores and fractures. It is released during the extraction of coal, the collapse of surrounding strata in underground mines, and when it is purposely vented from mines for safety reasons. Abandoned coal mines can continue to release methane for long periods after closure. Coal mining accounts for about one-third of methane emissions from all fossil fuel activities, and about 12% of all anthropogenic sources of methane.4
Data from the Global Energy Monitor (GEM)5 reveals a distinct geographic pattern to methane emissions from coal mines Not surprisingly, the largest coal producing nations emit the most methane from their coal mines. China alone accounts for three-quarters of methane emissions followed by the United States (5.2%), Russia (4.2%) and Australia (3.6%). But there is not a one to one alignment with coal production and methane emissions from coal mines. For example, China has a 75% share of emissions but a 50% share of coal production. Conversely, Indonesia’s share of production is five times its share of methane emissions. Such differences stem from differences in mining technology, type of coal mined, and other factors.
Mine depth is an important determinant of the methane content of coal (m3 per tonne). Because pressure increases with the depth of the coal seam and the adsorption capacity of coal increases with pressure, deeper coal seams generally contain more methane than shallow seams.6 As a result, underground coal mining releases more methane than surface or open-pit mining because of the higher gas content of deeper seams. Rank of coal (lignite, bituminous, anthracite) also affects methane content. At a given temperature and pressure, higher rank coals contain larger volumes of methane than lower rank coals.7 A country’s distribution of coal by depth and rank therefore influences the quantity of methane released in mining.
The term “superemitter” applied to anthropogenic sources of methane refers to the observation that a relatively small number of point sources often account for a large percentage of total emissions.8 Methane emissions from coal mines exhibit this phenomenon at a global scale. Of the approximately 2800 operating mines in the GEM database, just 25 mines release about 10% of all emissions (all but two of those mines are in China). The top 309 mines release half of total emissions, while less than half of the world’s coal mines account for 90% of methane emissions. The 1000 smallest emitters account for less than 2% of emissions. The superemitter phenomenon occurs at smaller geographic scales. Six coal mines in Queensland, Australia account for 7% of national coal production but emit about 55% of total national emissions from coal mining.9
Much of the focus on reducing coal’s contribution to greenhouse gas emissions is focused on reducing its use in electricity generation. This should remain a cornerstone of sound climate policy. But methane emissions from coal mines require greater attention. Emissions estimates vary in large part due to methods used to estimate emissions, requiring more dialogue on how to reconcile the approaches.10 The high concentration of emissions in a relatively small number of mines suggest that “cherry picking” these mines for mitigation is a sound starting point. In practice, of course, there are numerous technological, political, and economic challenges to this approach. Effective mitigation measures must also address the vexing challenge of methane emissions that can occur long after a mine is closed.
This next visualization is a self-directed exploration of the coal mine methane information. Use the controls to combine and visualize the data in different ways.
1 P. Forster, T. Storelvmo, K. Armour, W. Collins, J.-L. Dufresne, D. Frame, D.J. Lunt, et al. The Earth’s energy budget, climate feedbacks, and climate sensitivity, Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press (2021),https://www.ipcc.ch/report/ar6/wg1/
2 WMO Greenhouse Gas Bulletin (GHG Bulletin) – No.18: The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2021, Link.
3 Hmiel, Benjamin, V. V. Petrenko, M. N. Dyonisius, C. Buizert, A. M. Smith, P. F. Place, C. Harth, et al. “Preindustrial 14CH4 Indicates Greater Anthropogenic Fossil CH4 Emissions.” Nature 578, no. 7795 (February 2020): 409–12. https://doi.org/10.1038/s41586-020-1991-8; Tate, Ryan Driskell. “Bigger than Oil or Gas? Sizing Up Coal Mine Methane.” Global Energy Monitor, March 2022. https://globalenergymonitor.org/wp-content/uploads/2022/03/GEM_CCM2022_final.pdf.
4 Saunois, Marielle, Ann R. Stavert, Ben Poulter, Philippe Bousquet, Josep G. Canadell, Robert B. Jackson, Peter A. Raymond, et al. “The Global Methane Budget 2000–2017.” Earth System Science Data 12, no. 3 (July 15, 2020): 1561–1623. https://doi.org/10.5194/essd-12-1561-2020. The data cited here is from their “bottom-up” method.
5 Global Coal Mine Tracker, Global Energy Monitor, July 2022 release, Link.
6 Intergovernmental Panel on Climate Change (IPCC), Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (2000), https://www.ipcc-nggip.iges.or.jp/public/gp/english/
7 Cheng, Yuanping, Haina Jiang, Xiaolei Zhang, Jiaqing Cui, Cheng Song, and Xuanliang Li. “Effects of Coal Rank on Physicochemical Properties of Coal and on Methane Adsorption.” International Journal of Coal Science & Technology 4, no. 2 (June 1, 2017): 129–46. https://doi.org/10.1007/s40789-017-0161-6
8 Zavala-Araiza, Daniel, David Lyon, Ramón A. Alvarez, Virginia Palacios, Robert Harriss, Xin Lan, Robert Talbot, and Steven P. Hamburg. “Toward a Functional Definition of Methane Super-Emitters: Application to Natural Gas Production Sites.” Environmental Science & Technology 49, no. 13 (July 7, 2015): 8167–74. https://doi.org/10.1021/acs.est.5b00133. As applied to methane from coal mines, see Tate (2022) in [3] above.
9 Sadavarte, Pankaj, Sudhanshu Pandey, Joannes D. Maasakkers, Alba Lorente, Tobias Borsdorff, Hugo Denier van der Gon, Sander Houweling, and Ilse Aben. “Methane Emissions from Superemitting Coal Mines in Australia Quantified Using TROPOMI Satellite Observations.” Environmental Science & Technology 55, no. 24 (December 21, 2021): 16573–80. https://doi.org/10.1021/acs.est.1c03976
10 IEA (2023), Methane Tracker, IEA, Paris https://www.iea.org/data-and-statistics/data-tools/methane-tracker, accessed February 25, 2023; see also Tate (2022) in [3} above.