Customary international law is not reliable because it is ambiguous and lacks certainty needed for investment
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Furthermore, experts often disagree on the existing norms of international law.122 The ambiguity exists because the international customary law that applies to ocean activities is derived from numerous conventions, judicial decisions, state practice, and interpretations by international organizations. The customary law is not universally accepted, and it changes over time based on state practice.123 To obtain financing and insurance and avoid litigation risk, “U.S. companies want the legal certainty that would be secured through the Convention’s procedures in order to engage in oil, gas, and mineral extraction on our extended continental shelf.”124 Also, American companies may not use customary law to claim the right to seabed mining. There is no customary practice for dealing with seabed mining, and such practice is necessary for the formation of customary law.125
Moreover, because it is so difficult to prove the extent of customary law, according to some experts, “[a]bsent express agreement, mandatory obedience to the decisions of international organizations or tribunals is for all practical purposes out of the question.”126 The weaker the sense of legitimacy, the less restrained state practice is likely to be. There is a tendency among nations “to take treaty obligations more seriously than customary law obligations,” which leads to increased self-restraint.127 As Admiral Mullen testified when he was Vice Chief of Naval Operations, “[i]t is too risky to continue relying upon unwritten customary international law as the primary legal basis to support U.S. military operations.”128
Opponents of UNCLOS claim that the United States should not become a party because the United States already enjoys the benefits of UNCLOS through customary law and, therefore, should not unnecessarily incur the treaty's burdens. However, this ignores the fact that customary law can change and can also be influenced by how parties to UNCLOS decide to interpret its provisions.