The amount of methane released to the atmosphere from the Nord Stream subsea pipeline leaks remains uncertain, as reflected in a wide range of estimates^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18}. A lack of information regarding the temporal variation in atmospheric emissions has made it challenging to reconcile pipeline volumetric (bottom-up) estimates^{1,2,3,4,5,6,7,8} with measurement-based (top-down) estimates^{8,9,10,11,12,13,14,15,16,17,18}. Here we simulate pipeline rupture emission rates and integrate these with methane dissolution and sea-surface outgassing estimates^9,10 to model the evolution of atmospheric emissions from the leaks. We verify our modelled atmospheric emissions by comparing them with top-down point-in-time emission-rate estimates and cumulative emission estimates derived from airborne¹¹, satellite^8,12,13,14 and tall tower data. We obtain consistency between our modelled atmospheric emissions and top-down estimates and find that 465 ± 20 thousand metric tons of methane were emitted to the atmosphere. Although, to our knowledge, this represents the largest recorded amount of methane released from a single transient event, it is equivalent to 0.1% of anthropogenic methane emissions for 2022. The impact of the leaks on the global atmospheric methane budget brings into focus the numerous other anthropogenic methane sources that require mitigation globally. Our analysis demonstrates that diverse, complementary measurement approaches are needed to quantify methane emissions in support of the Global Methane Pledge¹⁹.

The Nord Stream subsea pipeline leaks in September 2022 resulted in an unprecedented amount of methane to be released to the atmosphere. However, the total amount emitted remains ambiguous, reflected in a wide range of pipeline volumetric estimates (bottom-up) and measurement-based (top-down) emissions estimates. To derive a conclusive estimate of emissions, the United Nations Environment Programme’s International Methane Emissions Observatory (UNEP’s IMEO) has brought together a multitude of research groups spanning several disciplines to synthesise, contextualise and, where possible, reanalyse modelled emissions estimates. In this presentation, we present new pipeline rupture emission rate simulations and compare them with various top-down quantification approaches applied to the Nord Stream pipeline leaks. We show that our simulated bottom-up emissions are reconciled with airborne, satellite and tall tower estimates over various points throughout the emission event, indicating our cumulative total is a robust estimate of emissions. UNEP’s IMEO’s approach of synthesising and reanalysing existing data from all available sources assists in overcoming the methodological limitations of the individual approaches and provides a more holistic quantification of methane emissions from the Nord Stream pipeline leaks. This approach demonstrates how sharing data generated from various disciplines and quantification platforms can be used to overcome key assumptions when modelling and quantifying emissions. More generally, this study highlights the benefits of applying diverse measurement approaches to quantifying methane emissions in support of reduction commitments such as the Global Methane Pledge. Affiliations 1-20 listed in Authors' Affiliations section 21. Deutscher Wetterdienst, Frankfurter Strasse 135, 63067 Offenbach, Germany 22. National Centre for Earth Observation, STFC Rutherford Appleton Laboratory (RAL), Chilton, UK 23. Remote Sensing Group, STFC Rutherford Appleton Laboratory, Chilton, UK 24. Technische Universität Braunschweig, Institute of Flight Guidance, Braunschweig, Germany 25. Enagás Transporte SAU, Madrid, Spain 26. GHGSat Inc., Montreal, Canada 27. Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany 28. National Centre for Earth Observation, University of Leicester, Leicester, UK 29. School of Physics and Astronomy, University of Leicester, Leicester, UK 30. School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, United States 31. Key Laboratory of Coastal Environment and Resources of Zhejiang Province, School of Engineering, Westlake University, Hangzhou, Zhejiang Province, 310030, China

This study presents the results from a comparison of measurement quantification methods of methane emissions from two onshore liquefied natural gas (LNG) export terminals, comparing site-level measurements, made using an in situ airborne technique, and estimates based on emission factors (EFs) derived from measurements using a remote sensing, ground-based, differential absorption LIDAR (DIAL) technique. The methane emissions from each site were quantified at an approximately one-year interval for each of the two techniques. DIAL was used to measure emissions at the sub-site, functional element (FE) level and calculate EFs for each FE using the specific FE activity data (AD). The total site methane emissions during the airborne measurements were estimated for each site using these EFs and the AD at the time. The results show the estimated methane emissions and the airborne measurements are close to agreement when considering the average of all the flight curtains (down to a 7% difference between uncertainty limits), whilst individual curtains were potentially significantly different. These results highlight the importance of fully characterising the methodology and uncertainty of both approaches. Using up-to-date, site-specific EFs or comparing over a statistically large sample size should improve agreement by reducing unknown emission uncertainties associated with site changes affecting the emission profile. Understanding each FE emission profile across a range of AD is critical to address potential differences due to non-linearity. It is important that accurate, specific and up-to-date AD is obtained to give a reliable estimate of emissions. The potential of the concept to estimate methane emissions from the FE EFs is demonstrated.

In 2022, Australia produced over 450 million tonnes of coal, accounting for approximately 7% of global production. About 75% of this output came from coal mines located in the Bowen Basin (Queensland) and the Hunter Coalfields (New South Wales). According to Australia’s 2022 UNFCCC submissions, open-cut and underground coal mining contributes around one-fifth of the nation’s methane emissions, positioning it as a key focus for methane mitigation efforts. These emissions are calculated using a combination of IPCC Tier 2 methodologies, which rely on average basin-specific coal gas contents, and IPCC Tier 3 methodologies, which involve mine-specific coal core gas distribution modelling. Despite employing higher Tier IPCC reporting methods, top-down studies using the TROpospheric MONitoring Instrument (TROPOMI) have suggested that fugitive methane emissions may be underestimated at some Australian coal mining facilities (Sadavarte et al., 2021; Palmer et al., 2021). However, the spatial resolution of TROPOMI (~7 km by 5.5 km) limits its capability for identifying emissions from individual facilities within an observation footprint cell, which constrains its effectiveness for bottom-up emission verification for individual mines.Here, we present an overview of findings from a series of mine-scale atmospheric surveys conducted across coal mines in the Bowen Basin between 2022 and 2023, and in the Hunter Coalfields in 2024. These studies utilized aircraft-based in-situ and remote sensing instruments, along with ground-based EM27/SUN Solar Absorption Spectrometers, all capable of isolating methane emission rates from individual coal mine facilities. We discuss the broader implications of these results within the context of Australia’s national and international greenhouse gas reporting framework.References:Palmer, P. I., Feng, L., Lunt, M. F., Parker, R. J., Bösch, H., Lan, X., Lorente, A., and Borsdorff, T.: The added value of satellite observations of methane for understanding the contemporary methane budget, Phil. Trans. R. Soc. A., 379, 20210106, https://doi.org/10.1098/rsta.2021.0106, 2021. Sadavarte, P., Pandey, S., Maasakkers, J. D., Lorente, A., Borsdorff, T., van der Gon, H. D., Houweling, S., and Aben, I.: Methane emissions from superemitting coal mines in Australia quantified using TROPOMI satellite observations, Environmental Science & Technology, 55, 16573–16580, https://doi.org/10.1021/acs.est.1c03976, 2021.

Chlorine-36 (36Cl) has a half-life of 301 kyr making it a commonly used tracer for constraining old groundwater residence times (~0.1 - 1 Myr). However, 36Cl ability to resolve subtle variations in environmental systems remains underutilised as a tracer of surface water processes, especially when used in conjunction with other stable halogen measurements. We highlight the utility of 36Cl to trace both regional natural and anthropogenic processes in three catchments in eastern Australia, the Nogoa, Namoi and Murrumbidgee. These catchments have headwater dams, regulated flow, urban centres and extensive areas of irrigated agriculture.Between 2017-2020, river water was sampled approximately bi-monthly at monitoring stations located between the headwaters and irrigation districts. Rainwater was also collected and analysed. The respective R36Cl (36Cl/Cl x 1015) were measured using Accelerator Mass Spectrometry (AMS).Surface waters across all catchments showed a general reduction in R36Cl downstream, with each catchment driven by different processes. Groundwater discharge during drought reduced R36Cl in the Nogoa and Namoi, while anthropogenic factors decreased R36Cl in a Murrumbidgee diverted sub-catchment. Agricultural irrigation returns, often associated with increased salinisation, had little influence in the studied catchments under drought conditions. Flooding, groundwater discharge, recycled sewage (groundwater input) and the wider catchment water management had a clear effect on halogen distributions. When paired with halogen concentrations, 36Cl measurements reduce ambiguity in source attribution and process interpretation in surface waters. As a complementary tracer, 36Cl can play an important role in improving our understanding of surface of water salinisation processes worldwide.