# **1. Introduction**

The World Meteorological Organization forecasts that the current greenhouse gas (GHG) emissions trend will increase global temperature 3-5 degrees C by 2100 (Reuters 2018). This would far overshoot the 2-degree limit pledged by the 2015 Paris Climate Accords (COP21) and might have a catastrophic impact (Steffen et al. 2018; World Bank 2012). In response, several industrial nations pledged very steep emissions reductions at the Leaders’ Summit on Climate in April 2021. The response deepened at COP26 in Glasgow with the introduction of the Global Methane Pledge, whose 122 current participating nations have joined a collective effort to reduce global methane emissions at least 30 percent from 2020 levels by 2030. The Pledge highlights the particular importance of methane reduction: “Methane is a powerful but shortlived climate pollutant that accounts for about half of the net rise in global average temperature since the pre-industrial era. Rapidly reducing methane emissions from energy, agriculture and waste ... is regarded as the single most effective strategy to keep the goal of limiting warming to 1.5 ̊C within reach while yielding co-benefits including improving public health and agricultural productivity.”<sup>1</sup> Unfortunately, pledges to reduce methane and other GHGs confront a striking information problem at the outset: National reporting has relied almost entirely on voluntary disclosure of emissions calculated by “bottom-up” methods that apply engineering parameters to estimated activity levels in emissions-generating sectors. Recently, advances in satellite-based measurements have shown the potential to supplement voluntary national reporting with directly-observed, transparent measures of GHG emissions. High-resolution observations of atmospheric GHG concentrations are now available from several platforms, including NASA’s OCO-2 and OCO-3 instruments, the European Space Agency’s METOP-A and TROPOMI (Sentinel5P) platforms, China’s TANSAT and the Japan Space Exploration Agency’s GOSAT and GOSAT-2. Detailed technical assessments of measures from these platforms have verified that they provide useful and comprehensive information for global carbon emissions analysis (Sha et al. 2021; Weir et al. 2021; Nassar et al. 2021; Pan et al. 2021; Wu et al. 2020; Hakkarainen et al. 2019; Labzovskii et al. 2019).

This paper focuses on satellite measurement to support implementation of the Global Methane Pledge. The first requirement is an easily-updated template for tracking atmospheric CH4 (methane) concentrations at local and regional scales. Using observations from the Sentinel-5P (S5P) platform of the European Space Agency (ESA), we develop the template from data filtering techniques pioneered by Hakkarainen et al. (2019) and applied to satellite-based CO2 measures by Dasgupta, Lall and Wheeler (2022a,b).

The second requirement is support for prioritizing actions to reduce methane emissions from energy, agricultural and waste subsectors whose spatial distributions are highly non-uniform across and within countries. At present, priority-setting is guided by spatially-formatted “bottom-up” estimates from national or global emissions inventories such as the Emissions Database for Global Atmospheric Research (EDGAR). These inventories combine sectoral activity

> 1 https://www.globalmethanepledge.org/#about

1