protocol, term referring to rules governing diplomatic conduct or to a variety of written instruments. Examples of the latter are authenticated minutes of international conferences; preliminary agreements, or statements of principle, which eventuate in a formal treaty; and agreements that do not require ratification. Sometimes the term protocol is applied to an agreement that in all essentials of form or content is similar to a treaty; an example of this was the Geneva Protocol approved by the Assembly of the League of Nations in 1924, which branded aggressive war an international crime. It provided that no signatory would engage in war with other signatories who observed their international obligations. Signatories were to participate in an international disarmament conference. The protocol was supported by most nations, but British refusal to support it in the League Council prevented it from coming into force. The Locarno Pact and the Kellogg-Briand Pact were later agreements having the general tenor of the Geneva Protocol. Diplomatic protocol is the code of international courtesy governing the conduct of those in the diplomatic service or otherwise engaged in international relations. It is basically concerned with procedural matters and precedence among diplomats. Each office of foreign affairs (or equivalent body) has an official in charge of protocol.

Montreal Protocol, officially the Protocol on Substances That Deplete the Ozone Layer, treaty signed on Sept. 16, 1987, at Montreal by 25 nations; 168 nations are now parties to the accord. The protocol set limits on the production of chlorofluorocarbons (CFCs), halons, and related substances that release chlorine or bromine to the ozone layer of the atmosphere. On the basis of increasing scientific knowledge about the effects of CFCs and halons on the ozone layer, the original protocol has been amended several times. At meetings in London (1990), Copenhagen (1992), Vienna (1995), and Montreal (1997) amendments were adopted that were designed to speed up the phasing out of ozone-depleting substances; not all parties to the main protocol are parties to these amendments. The production and consumption of halons was phased out by Jan. 1, 1994, and of CFCs, carbon tetrachloride, methyl chloroform, and hydrobromofluorocarbons by Jan. 1, 1996, subject to an exception for agreed essential users. Methyl bromide was to be phased out by 2005 but a number of users of the chemical have won temporary exceptions from the ban, and hydrochlorofluorocarbons are to be phased out by 2020. (Phaseout dates are later for developing countries.)

Under the protocol, the ozone-depleting potential, or ODP, of any substance is measured with respect to an equal mass of CCl₃F, or CFC-11, which is assigned a value of 1.0. Most other CFCs have ODPs that range from about 0.5 to about 1.3. Hydrochlorofluorocarbons, which are being used as transitional replacements (until 2020) for CFCs in refrigeration, have ODPs that are generally less than 0.5. Hydrofluorocarbons, which are also replacing CFCs as refrigerants, have ODPs of zero. Ozone-depleting potentials are based on existing scientific knowledge and are to be reviewed and revised periodically.

Kyoto Protocol: see global warming.

Geneva Protocol: see protocol.