December 2015 was a landmark month for treaty-based developments in international environmental law, after the successful conclusion of the Paris Agreement (see Jorge Vinuales’ three-part analysis here, here, and here, and subsequent reactions from Annalisa Savaresi here and Po-Hsiang Ou here). However, one should not also overlook more modest jurisprudential developments arising from the International Court of Justice’s 16 December 2015 Judgment on the Merits in Certain Activities Carried Out by Nicaragua in the Border Area (Costa Rica v. Nicaragua) and Construction of a Road in Costa Rica along the San Juan River (Nicaragua v. Costa Rica). While the Court in this case continued to affirm as settled law that States have to conduct environmental impact assessments (EIAs) for projects that could result transboundary harm – even innovatively introducing provisional measures in 2011 that required parties to cooperate on environmental monitoring – the Court ultimately remained opaque on the method and criteria it used to assess the degree of “risk of transboundary harm” that would be sufficient to trigger a State’s obligation to conduct an EIA. It was a regrettably lost opportunity for the Court to provide practical and conceptual guidance to States on how to assess “significant risk of transboundary harm” which triggers the international legal duty of a State to conduct an EIA before starting the proposed activity. The question of transboundary harm risk assessment has become increasingly urgent in recent years, particularly as more cross-border public-private partnership projects proliferate and States assume the international legal burden of conducting proper EIAs at the outset of any such cross-border PPP project.