Satellites will enable effective global methane monitoring

Spaceborne methane measurements are revolutionising global understanding of methane emissions and will form the backbone of an emerging global methane observing system.

Why measure methane?

Methane is a powerful greenhouse gas with a far greater global warming potential than carbon dioxide, being over 80 times more potent per tonne over 20 years and around 30 times more potent over 100 years. Methane concentrations have increased rapidly since preindustrial times and are responsible for causing half a degree (0.5°C) of present-day global warming (1.1°C).  A large part of this increase is driven by emissions from a few economic sectors, namely fossil fuels (35%), waste (20%), and agriculture (40%). 

To keep the Earth’s temperature below the 1.5°C aspiration of the Paris Agreement, rapid reductions in methane are urgently needed from all the above-mentioned sectors. For this reason 159 countries have signed the Global Methane Pledge, a voluntary commitment to reduce global methane emissions by at least 30% by 2030 compared to 2020 levels. Reliably monitoring methane emissions worldwide is critical to help guide mitigation efforts, establish transparency and accountability, and monitor progress towards the stated goals.

Until recently, most of our understanding of global emissions came from either scientific studies carried out in a limited geographic area or bottom-up inventories, i.e. accounting of methane emitting activities and generic emission factors. While immensely useful, these cannot account for the variability of methane emissions and the diversity of methane-emitting infrastructures in different parts of the world, limiting mitigation efforts.

The role of satellites in a global observing system

An increasing number of methane-sensing instruments are orbiting the earth, bringing a broader, more global view of methane emissions and providing the fine spatial and temporal coverage missing from our understanding. For example, observations from TROPOMI, a methane-sensing instrument onboard EU’s Sentinel-5P satellite, have allowed detailed studies of regional emissions around the world, from continent to city scale. Point source imagers, including GHGSat, EMIT, Sentinel-2, PRISMA, EnMAP, and Gaofen 5 have changed our understanding of methane emissions, showing the prevalence of large emissions from “abnormal” operation conditions in oil and gas operations and the persistence of such emissions from other sectors, including coal and waste. These instruments are increasingly spotting sites with persistent large emissions that can be cost-effective targets for mitigation. Impressively, such observations have even been used to drive rapid-mitigation of substantial previously-undetected but easy-to-fix emissions from the oil and gas industry. 

By combining their strengths, these satellites are improving our ability to tackle methane emissions. Satellite observations of large area emission fluxes are highlighting deficiencies and knowledge gaps in existing emission inventories and are guiding efforts to improve our understanding of specific high-emission areas. By comparing regional emissions, these observations are also helping measure the impact of different fossil fuel production practices and regulations on methane emissions and can underpin regulation development and trade agreements. Satellite observations at facility scale can help spot large emission events and drive quick mitigation. They can also attribute emissions to different operators and thus highlight how different operating practices can affect emissions. If enough observations are available, such facility-level observations can be used to check the validity of reported facility emissions, increasing transparency and helping enforce compliance to regulations.

Designing an effective methane observation strategy

No single technology can capture all methane emissions effectively. The choice of technology depends on the specific context. Often, a multi-tiered observing system—a combination of different measurement methods—is required to gain a comprehensive understanding of emissions and to drive effective action. Satellites, with their global reach and relative cost-effectiveness, are well suited to serve as the backbone of this type of observing system.

Here are a few aspects that should be considered when designing such a system.

Purpose of methane monitoring: No observing system can capture all aspects of methane emissions, so designers should aim to collect data fit for their intended application. If the goal is to provide independent, top-down observations to validate and refine reported emissions, frequent satellite coverage over large regions is needed. For detecting large abnormal emissions from a few sites to support methane accounting, facility-scale satellite monitoring may be sufficient. However, if the aim is to support emission mitigation, satellite observations must be complemented by local monitoring tools capable of pinpointing the exact source of emissions within a facility.

Methane source characteristics:  An observing system needs to be adapted to the sources it aims to monitor. For example, if emissions are expected to be intermittent, the system should foresee regular observations to capture rare events. On the contrary, if relatively constant emissions are expected, less frequent observations might be enough. 

Environmental conditions:  As this report demonstrates, environmental factors, including cloud cover and strong winds, affect the efficacy of methane detection by satellite. The extent of these impacts varies by location and time of the year. All measurement techniques will be affected (in different ways) by environmental conditions, so the observing system should be adapted to such local constraints. 

Availability and cost of measurement techniques: Every observing system should be optimised to give the maximum possible benefit given the constraint of the available resources. The availability and cost of various observation technologies varies greatly between regions; moreover, the financial resources and capacity of stakeholders will vary greatly around the world. The design of each observing system should take this into account.

Environmental constraints on satellite methane monitoring 

This report aims to show that, despite the huge value they offer, satellites will be less effective in monitoring  methane in some locations, as environmental conditions will not allow them to reach every part of the world with the consistency and frequency needed to fully inform global mitigation efforts.  For example: 

• persistent cloud cover will reduce satellite data coverage in tropical regions; 
• during winter months, with the sun remaining low above the horizon, high latitudes will remain relatively dimly lit, depriving sensors of the light needed to effectively observe methane, increasing their already large detection threshold; 
• high winds and dark surfaces, like forests, can  make even large emission events hard to observe;
• rough terrain in mountain regions will make the interpretation of some satellite measurements harder or impossible, creating a year-round obstacle for reliable methane observations in these areas. 

Of course, the diversity of space-borne instruments means that these factors will affect their capabilities differently, and, to some degree, an observing system combining several spaceborne sensors will increase the observability of methane emissions in many parts of the world. Still, the combined environmental challenges may result in areas with reduced data coverage in certain regions.

In this report we map the regional effects of the various environmental parameters that impact methane sensing from space. To do that, we examine the combined effect of five parameters: cloud cover, sun elevation, ground reflectivity, uneven terrain, and wind speed. This report shows where one or more of these parameters should be expected to hinder some satellite observations during significant portions of the year. In many cases, these locations can be observed, but data in those locations will be less available – it will only be available from a subset of methane-observing satellites, and/or only available on an infrequent basis. By mapping these effects, we aim to help governments, academia, and other stakeholders build realistic expectations of satellite data availability in their region and ultimately plan for integrated methane observing systems.