In terms of the environment, there are now indications that the ‘hole’ in the ozone layer is healing. As a result of the phase-out so far of 99 per cent of the historic baseline levels of production and consumption of controlled ozone-depleting substances globally in the atmosphere, the Antarctic Ozone Hole is no longer getting larger and is expected to return to the 1980 levels by about 2070. The global ozone layer is expected to return to the pre-1980 levels — the time before the occurrence of a significant ozone layer depletion — by around 2050. Without the Montreal Protocol, it is estimated that the Antarctic Ozone Hole would have been about 40% larger in size by 2013.

In terms of the global climate, ozone protection efforts have contributed significantly to the mitigation of climate change by averting an estimated net amount of more than 135 billion tonnes of carbon dioxide equivalent emissions going into the atmosphere from 1990 to 2010, rendering it one of the most effective tools to date in climate change mitigation. The Montreal Protocol's contribution towards reducing global greenhouse gas emissions has been estimated for 2010 to be about five times larger than the annual emissions reduction target for the first commitment period (2008–2012) of the Kyoto Protocol on climate change.

In terms of the “human face” of the Protocol's achievements, up to 2 million cases of skin cancer may be prevented each year by 2030; 283 million cases of skin cancer avoided for those born between 1890 and 2100 in the US, 8.3 million being melanoma; 1.6 million deaths from skin cancer prevented; and 46 million cases of cataract prevented according to the United States Environmental Protection Agency.

In terms of green economy, the phase-out of the ozone-depleting substances has brought investment in greener alternatives. It has changed the production and consumption patterns by stimulating more efficient production processes including energy efficiency and by innovative redesigning of products and equipment to use greener chemicals. The phase-out has greened global value chains in the process.

The Montreal Protocol has also led to technology transfer, with the Multilateral Fund contributing more than US$ 3.8 billion from 1991 till today to all developing countries. This has helped in the transition worldwide to greener chemicals, both in production and consumption. The parties to the Montreal Protocol have further agreed in 2017 to replenish the Fund with US$ 540 million for the triennium 2018–2020.

In terms of contribution to the Gross Domestic Product, the global phase-out of CFCs under the Montreal Protocol will result in an estimated US$ 1.8 trillion in global health benefits, US$ 1.109 billion for skin cancer only, and almost US$ 460 billion in avoided damages to agriculture, fisheries, and materials, which would have been caused by the depletion of the ozone layer (cumulative estimates from 1987 to 2060).

The story of the Montreal Protocol's effort to rid the world of the chemicals that are destroying the earth's protective ozone layer starts in 1974, when Mario Molina and F. Sherwood Rowland published their ground-breaking research warning that chlorofluorocarbons (CFCs) would destroy the ozone layer. Then, in 1985, Joseph Farman, Brian Gardiner, and Jonathan Shanklin (Farman et al., 1985) published the evidence of a recurring springtime Antarctic Ozone Hole. The Montreal Protocol was adopted in 1987 with the goal to reduce production and consumption of ozone-depleting substances and their abundance in the atmosphere, thereby protecting the ozone layer.

CFCs and HCFCs (hydrochlorofluorocarbons) were quite pervasive in houses and workplaces. Air conditioning to keep people comfortable, food kept in fridges, usually transported from afar in refrigerated trucks and ships, computers and other electronic equipment and appliances contained parts cleaned with ozone-depleting solvents.

Car dash boards, insulation foams in our houses and office buildings, water boilers and even shoe soles were made using CFCs or HCFCs. Offices, businesses and computer facilities, military bases, airplanes and ships extensively used halons for fire protection. All kinds of sprays such as hair sprays and inhalers used by asthma patients were propelled by CFCs. A lot of the vegetables we eat were fumigated by methyl bromide to kill pests. Ozone-depleting substances have been used in thousands of products across 240 sectors.

They have been so extensively used that phasing them out required a lot of work in developing the substitute chemicals and alternative technologies, without any compromise in quality and without disruptions to the functioning of the society. Back in the 1980s, technological alternatives to CFCs were either non-existent or considered too costly or unfeasible. The arguments for controlling CFCs rested on scientific theories on the effects of ozone layer depletion to human health and some of the main industries were opposed to any controls.

Convinced of the seriousness of the threat, the United Nations Environment Programme, then led by the Egyptian scientist Dr. Mustafa Tolba, contributed to a diplomatic effort to negotiate a treaty to protect the stratospheric ozone layer. The result was the adoption of the Vienna Convention for the Protection of the Ozone Layer in 1985, and its Montreal Protocol two years later. The Montreal Protocol gave the signal to industry to invest in research and technology to pursue alternatives for chemicals that were thought to be indispensable.

The Montreal Protocol started modestly, and it moved quickly in the following years. The parties learned by doing, became more confident, and increased their ambition gradually. With increasing scientific knowledge about ozone depletion, manmade ozone-depleting gases were added through amendments, and phase-out schedules were accelerated through adjustments brought to the Protocol, eventually reaching the phase-out of nearly 100 substances. The result is that 99% of controlled ozone-depleting substances have been phased out to date (Figs. 1 and 2).

The Protocol is an example of how science and diplomacy can interact effectively to address a global threat. Its governance structure has succeeded in delivering tangible results.

• • Scientific assessment panels were established to review scientific evidence and technical, economic, and environmental issues on an ongoing basis. Based on this information, nations were able to strengthen the treaty's provisions.
• • Public–private partnerships from big industries to small medium enterprises together with the United Nations Environment Programme, governments and non-governmental organizations, worked together to spread new technologies to eliminate the ozone-depleting substances.
• • Clearly defined targets and schedules were established to phase out specific controlled substances.
• • A dedicated financial mechanism, the Multilateral Fund, was created to assist developing countries in acquiring new technologies.
• • Parties created a mechanism to monitor compliance with their reporting requirements and their phase-out targets, in an approach that facilitated compliance rather than punishing non-compliance.
• • Trade restrictions on trade with non-parties to the Protocol in controlled substances were implemented and parties have all established licensing systems to monitor their imports and exports.

The Montreal Protocol's success in phasing out ozone-depleting substances (ODSs) has inspired a new effort to avoid global warming hydrofluorocarbons (HFCs). HFCs are non-ozone-depleting, but are powerful greenhouse gases. They have been used mainly as replacements for ODSs under the phase-out schedules of the Protocol.

After six years of discussions and one year of formal negotiations, in 2016 the parties agreed to amend the Protocol and include under its remit, a list of 19 HFCs as controlled substances to be phased down¹. Phasing down HFCs may avoid up to 0.5 degrees Celsius of temperature increase by the end of the century (Fig. 3).

Table 1 and Fig. 4 show the phase-down schedules of the HFCs under the Kigali Amendment.

In addition to adopting the Amendment, the parties to the Protocol took a separate but related decision dealing with solutions to the challenges of managing HFCs. The challenges that have been discussed for years were related to issues like flexibility to implement national policies for the HFC phase-down, funding and safety issues of the new refrigerants, exemptions, Intellectual Property Rights, synergies with the United Nations Framework Convention on Climate Change (UNFCCC). The main messages from Table 1 are the following:

• • HFCs production and consumption are to be phased down gradually by between 80–85 per cent from 2019 to the late 2040s;
• • non-Article 5 parties will go first, with most of them starting to phase down from January 2019. A small number of Article 2 countries will start the phasedown from 2020;
• • article 5 parties will phase down in two groups. Group 1 has a freeze date of 2024, with the starting point of their phase down to be the average HFC consumption (or production) for 2020 to 2022. Their first reduction will be 10% of their starting point and will start from 2029 onwards;
• • group 2 of Article 5 countries have a freeze date of 2028, a starting point for the reduction the average consumption (or production) for 2024 to 2026, and an initial reduction of 10% in 2032.

The Amendment will make use of the tested mechanisms of the Montreal Protocol, including the agreed phasedown targets and schedules and the financial mechanism to assist developing countries to meet their phase-down obligations. Countries are now working on arrangements to ratify and implement the Amendment, which will enter into force on 1 January 2019, provided 20 parties have ratified it by that date. So far 23 parties have ratified it, and therefore the condition for its entry into force has been met.

Governments, experts, scientists, industries, non-governmental organizations, all brought together under the United Nations had their own role to play in negotiating the Kigali Amendment.

It is amazing to see that the governance structure of the Protocol has remained unchanged in its 30 years of life so far. Despite the rather confined mandate, the exposure to the various stakeholders is bigger than in any other convention. The recent incorporation of the HFCs has definitely broadened the Protocol's mandate, but it is confined in the same sectors of the ozone-depleting substances without challenging its institutions and its governance structure. The lessons drawn from the recent negotiations that led to amending the Montreal Protocol also provide with a response as to what has kept this Protocol alive, successful, and relevant after thirty-one years.

5.1 Science had a critical role to play

5.2 Setting realistic and tangible objectives led up, like stepping stones, to the overall objective of amending the Protocol

5.3 Reframing the discussion to first resolve the technical issues before the beginning the negotiations on the language of the amendment assisted the parties to reach an acceptable outcome for all

5.4 Informal discussions alongside the formal ones offered a forum for different approaches, ideas, and positions to be clearly and freely expressed, unhindered by country positions

5.5 Engaging all stakeholders through parallel discussions–governments with governments and industries with industries from the developed and the developing world–assisted identifying problems and finding solutions

5.6 Perhaps the most important parameter in reaching consensus was the re-injection of a sense of optimism, pride, ownership of the process, and building trust among the parties

5.7 New skills in a digital era

5.8 Finally, the Montreal Protocol itself enabled the parties to reach a fair outcome

Scientific discoveries continue to be made and will continue to be made in perpetuity. As far as the environment is concerned, precaution is better than cure. No matter how imperfect the scientific knowledge of today may be, it will keep improving as long as nations, stakeholders, organizations are guided by a sincere desire to protect the environment, as long as they adhere to the principle of precaution, and as long as the collective consciousness urges them to continue to act. When improvements in scientific knowledge are brought to the attention of global decision-makers through fora such as the United Nations, effective global action can be taken to protect the environment, the people, and the planet.

The Montreal Protocol is firmly built on this understanding. The parties, in 1987, adopted an approach based on the knowledge available at the time recognizing the ongoing developments in the underlying science. Rather than locking themselves into a rigid framework, they built in flexibility so that as scientific knowledge expanded, the Protocol could be adjusted and amended accordingly. The Montreal Protocol is one agreement where the reliance on constantly updated scientific information has been a key driver of success. The results speak for themselves.

The Montreal Protocol is also firmly built on the commitment and engagement of the parties in resolving a significant global problem. Their commitment and engagement are a function of how they engage both with the issues and with each other. Allowing space for parties to have frank and open informal discussions within the framework of established ground rules enhances the parties’ engagement with each other and with the technical issues and fosters better understanding. Understanding is a key step to building trust towards other parties, towards the process, and ultimately towards the outcome.

The Montreal Protocol experience proves that the United Nations can successfully take on the role of the honest broker that facilitates the process by which decision-makers can be informed, get to grips with the problem, identify policy options and their implications and then come to an agreement on an appropriate course of action. The Kigali Amendment proves that this can be done in an inclusive manner that addresses not only the underlying science, but also the realities, concerns and capacities of different countries and stakeholders. When that is achieved, the outcome will reflect the complexity and urgency of the real-world problems that science seeks to explain and will ensure that no-one is left behind.

The next step is perhaps to provide the scientific evidence that will be needed at the national level, to make science accessible and relevant to the realities and concerns of all people around the globe. Public opinion can generate pressure for action, but it has to be well-informed to generate effective pressure for environmental action.