Observation-Based Emission Estimates of the Potent Greenhouse Gas HFC-23 for Article 5 and Non-Article 5 Country Groups under the Montreal Protocol

Abstract The Kigali Amendment to the Montreal Protocol requires parties to destroy, to the extent practicable, HFC-23 generated as a byproduct during the production of controlled hydrochlorofluorocarbons and hydrofluorocarbons. Global total HFC-23 emissions estimated from reported abatement and known sources (bottom-up) agreed with those inferred from atmospheric observations (top-down) before 2015, but have been substantially lower since then. Here, we show that during 2011–2013, bottom-up estimates of global total HFC-23 emissions were right for the wrong reasons: bottom-up estimates for the non-Article 5 (mainly developed) country group were lower than top-down estimates (1.7 ± 0.2 vs 3.1 ± 0.5 Gg yr–1 on average), while those for the Article 5 (mainly developing) country group were higher (11.8 ± 0.4 vs 9.7 ± 1.0 Gg yr–1), resulting in a cancellation at the global scale. We further show that during 2015–2023, while the underestimation of global bottom-up emissions was dominated by the Article 5 country group, the non-Article 5 country group also contributed in 2023 [bottom-up (byproduct emissions reported to UNEP plus other known sources) vs top-down: 0.5 ± 0.2 vs 1.3 ± 0.6 Gg yr–1]. Our results underscore improvement of emission accounting and mitigation for both country groups under the Kigali Amendment.