Montreal Protocol and its Kigali Amendment

The Montreal Protocol on Substances that Deplete the Ozone Layer phases out the production and consumption of the chemicals that deplete the ozone layer.¹ It was adopted in Montreal on 16 September 1987 and entered into force on 1 January 1989.² The UN Treaty Collection listed 198 Parties on 24 October 2026. It was the first treaty in UN history to be ratified by every country.^2,3 The Parties meet once a year and have amended and adjusted the Protocol since, most recently with the Kigali Amendment in 2016 and an adjustment in 2018.^1,3 A Multilateral Fund helps developing countries whose consumption and production of these chemicals is below 0.3 kg per person a year to comply; the Protocol calls them Article 5 Parties.³

Hydrofluorocarbons, HFCs, replaced many of the substances the original Protocol eliminated. They do not deplete ozone, but they are powerful greenhouse gases.¹ The Kigali Amendment, agreed at the Twenty-Eighth Meeting of the Parties in Kigali in October 2016, lists them as Annex F and phases them down.⁴ Each Party caps its HFC consumption and production, counted in CO2 equivalents, as a falling share of a baseline. For Parties not operating under Article 5 the baseline is the 2011 to 2013 average plus part of their HCFC baseline, and the cap steps down from 90 per cent in 2019 to 15 per cent from 2036.⁴ Article 5 Parties use a later baseline, from 2020 to 2022 or for some from 2024 to 2026, and reach 20 per cent of it from 2045 or 15 per cent from 2047.⁴ Parties that make HCFCs or HFCs must destroy the HFC-23 they give off as a by-product, as far as practicable, from 2020.⁴ The amendment entered into force on 1 January 2019.³ It states that it does not take HFCs out of the commitments under the UN climate convention and its Kyoto Protocol.⁴

The Protocol rests on two kinds of measurement: what Parties report, and the state of the atmosphere. Under Article 7 each Party reports data on controlled substances to the Secretariat, and Kigali extended this to HFCs, with baseline years and each facility's annual HFC-23 emissions.⁴ Under Article 6, the Scientific Assessment Panel assesses the depletion of the ozone layer and related atmospheric science, in a report every three or four years written by hundreds of scientists.⁵ The reports are published as the WMO/UNEP Scientific Assessment of Ozone Depletion.^5,6 The 2022 assessment updated trends in ozone-depleting substances, HFCs, global and polar ozone and the link between stratospheric ozone and climate. It added a chapter on stratospheric aerosol injection.⁶ NOAA, NASA, the University of Cambridge and the National University of Rwanda each supplied one of its four co-chairs.⁶ The 2022 assessment expected the ozone layer to return to 1980 levels around 2040 over most of the world, 2045 over the Arctic and 2066 over Antarctica.³ The panel also answers specific questions from the Parties, with reports on unexpected emissions of CFC-11 and on emissions of HFC-23.⁵

The modelling work is global and atmospheric. It tracks the controlled gases in the air and projects how the ozone layer recovers as they decline. For HFCs the same accounting feeds climate work, since the amendment counts HFCs and HCFCs in CO2 equivalents through global warming potentials listed in its annexes.⁴ The ozone layer as a case of a model and a measurement changing policy is told in Questions people ask.