The coverage of the Paris Agreement in light of Trump’s withdrawal decision has been quite remarkable. It seems as if the world just got a crash-course on the treaty. Every news article is now able to point out that the Agreement is essentially non-binding, that nothing will change for at least three years (since the Agreement does not allow withdrawal during the first three years), and that there are just a couple of countries that have not signed the Agreement, etc. etc. (Nicaragua and Syria, see explanation by Washington Post.)
There are only a few environmental treaties where membership is truly universal, such as the Montreal Protocol on Ozone Depleting Substances and the underlying Vienna Convention on the Protection of the Ozone Layer, as well as the UN Framework Convention on Climate Change (the underlying framework treaty for the Paris Agreement), all of which have 197 member countries. That includes the 193 UN countries, plus the European Union, the Holy See/Vatican, and a couple of other small non-UN member states.
But there are also a number of other international environmental treaties where the US part of a small group of countries that are not members. For example, the Basel Convention on the Transboundary Movement of Hazardous Wastes has 185 member countries. But apart from the U.S., the only other states that are not members are small island nations or poor developing countries – Angola, Fiji, Grenada, Haiti, San Marino, Sierra Leone, Solomon Islands, South Sudan, Timor-Leste, Tuvalu, and Vanuatu.
However, the US is in an even more lonely place with the 1992 Convention on Biological Diversity (CBD). That treaty is the other important environmental treaty from the 1992 Rio Earth Summit (the first one being the UN Framework Convention on Climate Change). The CBD has 196 member states (including the EU and other small non-UN countries). The Holy See is not a member – of course, that seems to make sense since it has little territory and thus little in natural area to control. The more notable exception however is . . . the U.S.