The only UN treaty related to environmental issues which has been ratified by…

Concept

A multilateral treaty achieves "universal ratification" only when every single member state of the United Nations becomes a party to it. This is extremely rare in international environmental law, because treaties usually impose binding obligations (emission cuts, phase-out timelines, financial duties) that some states refuse or delay to accept.

Application

Among the four environmental instruments listed, only the Montreal Protocol on Substances that Deplete the Ozone Layer (1987) reached this milestone, becoming the first treaty in UN history to be ratified by all member states. Its success rested on a clear, scientifically agreed target — phasing out ozone-depleting substances such as CFCs — backed by a financial mechanism (the Multilateral Fund) that helped developing countries comply.

Note on the count: the United Nations currently has 193 member states; the figure of 197 refers to the states that are parties to the Montreal Protocol (197 states, together with the European Union, making 198 parties in all).

Cross-check

Contrast the listed instruments by their actual standing:

• Montreal Protocol — ozone-layer protection; the listed treaty that achieved participation by every UN member state.

• Paris Agreement — climate change; widely joined but not ratified by every state, with notable withdrawals and non-parties.

• Kyoto Protocol — earlier climate treaty; several major emitters never ratified or later renounced it.

• Basel Convention — transboundary movement of hazardous waste; broad membership but not universal, with key non-parties.

Hence, among the options listed, the environmental treaty ratified by every UN member state is the Montreal Protocol, which is widely cited as the first treaty in UN history to achieve such universal participation.