There are many uncertainties that face Parties implementing the Montreal Protocol, and Parties are not always able to agree on the specific measures required to implement the phase-out of ozone depleting substances (ODS). What are the target ODS? What sorts of alternatives are available, and are they feasible? What should be the timeframe for phasing out the various ODS?
In light of these uncertainties, the Montreal Protocol works through amendments and adjustments. An amendment is a change in the Protocol itself, while an adjustment refers to changes in timetables for phasing out a particular ODS. These amendments and adjustments are based on scientific review of the Vienna Convention and Montreal Protocol, identifying necessary changes to the commitments.
For more information, see http://www.unep.org/ozone/