The provision of Article 62(1) of the Vienna Convention on the Law of Treaties (VCLT) allowing states to terminate or withdraw from a treaty due to the unforeseen “fundamental changes of circumstances” is a well-established rule under customary international law but may be invoked only in exceptional circumstances, as comes from the treaty text and further interpretation by the International Court of Justice (ICJ). 

Under Article 62, a state needs to meet the following conditions to terminate or withdraw from a treaty due to the unforeseen “fundamental changes of circumstances”: (a) the changed circumstances were “essential” for the decision to enter into the treaty, and (b) the changes of circumstances “radically” transform the obligations under the treaty so that further implementation of the treaty becomes unduly burdensome.

Thus, a Contracting Party to the Energy Charter Treaty (ECT) wishing to invoke Article 62 of the VCLT with respect to the ECT, including the “sunset clause” of Article 47, may need to show that the changed circumstances were essential at the time of the conclusion of the ECT, their change was unforeseen, and that such change radically transforms the extent of obligations under the ECT.

For example, on the requirement of foreseeability, in Gabčíkovo-Nagymaros Project (Hungary/Slovakia), where the agreement in dispute was entered into in 1977 and sought to be terminated in 1992, the ICJ observed the following: “The Court does not consider that new developments in the state of environmental knowledge and of environmental law can be said to have been completely unforeseen.”