Since 23 June 2015, all countries of the United Nations, the Cook Islands, the Holy See, Niue and the European Union have ratified the original Montreal Protocol (see external link below) , with South Sudan being the last country to ratify the agreement, bringing the total to 197. These countries also ratified the london, Copenhagen, Montreal and Beijing amendments.  Initially signed in Montreal by 24 countries, including Canada, the Montreal Protocol is considered one of the most successful multilateral agreements. It has obtained the elimination of the vast majority of the ODS and, as a result, the ozone layer is recovering. Since many of the substances controlled by the protocol are also greenhouse gases, their elimination has also contributed significantly to the fight against climate change. The fund is replenished at three years by donors. Commitments amounted to $3.1 billion between 1991 and 2005. These funds will be used, for example, to finance the conversion of existing production processes, to train staff, to pay licence fees and patent fees for new technologies, and to create national ozone boards. In December 2019, the fund amounted to just over $4.1 billion in revenue and $3.8 billion in disbursements.  The parties to the protocol meet once a year to make decisions to ensure the proper implementation of the agreement. These include adapting or modifying the protocol, which has been implemented six times since its inception. The most recent amendment, the Kigali amendment, called for the gradual reduction of hydrofluorocarbons (HFCs) in 2016.
These CFCs were used to replace a batch of ozone-depleting substances that were eliminated by the original Montreal protocol. Although they do not deplete the ozone layer, they are known to be powerful greenhouse gases and thus contribute to climate change. In 1973, chemists Frank Sherwood Rowland and Mario Molina, then at the University of California, Irvine, began studying the effects of CFCs in the Earth`s atmosphere. They found that CFC molecules were stable enough to remain in the atmosphere until they rose in the middle of the stratosphere, where they would eventually be degraded (after an average of 50-100 years for two CFCs) by ultraviolet radiation releasing a chlorine atom. Rowland and Molina then suggested that these chlorine atoms could cause large amounts of ozone (O3) to be depleted in the stratosphere. Their argument was based on an analogy with the contemporary works of Paul J. Crutzen and Harold Johnston, who had shown that nitrogen monoxide (NO) could catalyze the destruction of ozone. (Several other scientists, including Ralph Cicerone, Richard Stolarski, Michael McElroy and Steven Wofsy, had independently suggested that chlorine could catalyze ozone loss, but none had realized that CFCs were potentially a significant source of chlorine.) Crutzen, Molina and Rowland were awarded the Nobel Prize in Chemistry in 1995 for their work on this problem. In the 1970s and 1980s, there was growing international concern that SO was harming the ozone layer. In 1985, international cooperation in this area was formalized by the Vienna Convention on the Protection of the Ozone Layer.
This cooperation led to the signing in 1987 of the Montreal Protocol on ozone-depleting substances. Another group of substances, hydrofluorocarbons (HFCs), has been introduced as non-ozone depletion alternatives to support the timely abandonment of CFCs and HCFCs. CFCs are now widely used in air conditioning systems, refrigerators, aerosols, foams and other products.