ELC 2025: International Precedent as a Common Law Tool by Anthony Moffa

In 2025, the International Court of Justice issued its long-awaited advisory opinion answering the question of whether governments have a legal obligation to deal with climate change. The short, bold answer was “YES.”  

In pretty much every country that’s not the United States, the public reaction was intense and even celebratory. Here, the reaction in popular media was pretty much: 🤷‍♂️ [shrug].   The New York Times ran an article asking does it even matter. After much deliberation on the matter, in the end, I think so. And here’s why.

The majority of U.S. jurists and attorneys have been outright dismissive of international law for decades. The roots of that conspiracy go all the way to the top. The United States Supreme Court has been clear that it need not follow its international brethren, repeatedly minimizing the legal significance of the opinions of international tribunals. According to our highest court, such opinions on the meaning of treaties and international law obligations deserve only “respectful consideration”—words worth about as much as the paper they were written on in 1998 and even less in today’s hyper-polarized domestic environment. In the early 2000s, the Supreme Court gave this “respect” to the International Court of Justice, permitting states to disregard the ICJ’s holdings.  

In the most straightforward sense, then, it remains practically impossible to wield the ICJ opinion against any domestic government in the United States (federal, state, or municipal) to compel them to act on climate change. They can just go about their business and ignore the ICJ’s determination that they have an obligation to combat climate change.  

Before just ending the analysis in that hopeless pit, I spent some time thinking about what use the ICJ opinion serves if it’s not binding on U.S. courts and the federal government continues to take policy and legal positions in direct opposition to what it commands. The answer I arrived at calls back to the origins of our system of law. 

The common law has always been a vehicle for courts to acknowledge the obligations citizens owe to one another. It does not depend on federal policy or even state policy or some legislative guidance. This is particularly true in the context of civil wrongs. In tort law, courts, aided by the public consciousness as embodied in the jury, serve as the ultimate arbiters of what behavior is reasonable. In determining what is reasonable, courts and juries can and do rely on their experiences (both shared and unique), their values (again, shared and unique), and, inevitably, the popular consensus of right and wrong. The standard of liability can, and does, change with the times. It is in this context that the ICJ’s opinion has the potential to most significantly shape American law. It contains text, and logic, that can be wielded by skillful, creative litigators. Their arguments could persuade juries and jurists to adopt some of the ICJ’s reasoning and incorporate it into the common law of one, or several, states.  

What follows are a few examples of specific holdings in the ICJ opinion and how they might relate to the determinations juries are asked to make in the common law tort cases.

Duty

The modern negligence action requires the plaintiff to establish that the defendant owed them a duty of care. Duty thus forms the foundation of common law responsibility; we do not hold people financial responsible to others unless we determine that some moral, and therefore legal, obligation to treat them with care exists. Duty is a question of law, and common law judges have the final say on its reach.

The ICJ spent considerable real estate laying out the duties owed by nations with respect to environmental harm, and climate change more specifically. In paragraph 272 of its advisory opinion, the ICJ held that the “duty to prevent transboundary environmental harm” is a well-established principle of international law. In so holding, the ICJ quoted the Trail Smelter arbitration between the United States and Canada, which found that “no State has the right to use or permit the use of its territory in such a manner as to cause injury by fumes in or to the territory of another or the properties or persons therein, when the case is of serious consequence and the injury is established by clear and convincing evidence.”  The Court went on to definitively state: “the duty to prevent significant harm to the environment also applies to the climate system.”

When considering the duty owed by one private actor to another, this language, and the logic it reflects, could prove powerful.  Just as any state has a duty to avoid contributing to climate change in such a way that injury occurs across its borders, so too might private actors when climate injury occurs outside their own property and sphere of operations.

Standard of Care

The scope of any duty owed defines the boundaries of liability. In other words, tort law asks the jury to determine whether the obligation the duty imposes has been satisfied. In making that determination, common law courts across our country have instructed juries using relatively generic and infinitely malleable terms—“reasonableness” chief among them. 

In the climate context, the ICJ’s opinion provides some guidance as to what behavior meets the obligations owed. In the first instance, the ICJ simply says that fulfilling the duty requires acting with “due diligence.” This phrase essentially takes the place of “reasonable care” in a common law torts analysis, and is almost as malleable and unhelpful. In Paragraph 280, the ICJ essentially admits as much, saying due diligence depends on circumstances and can evolve over time.

The ICJ, thankfully, goes on, writing “the general duty to prevent significant harm to the environment consists of substantive elements (e.g. the obligation to take appropriate measures) and procedural elements (e.g. the obligation to notify and consult).”  Appropriate measures, in the ICJ’s view, must be “designed to achieve the deep, rapid, and sustained reductions of GHG emissions that are necessary for the prevention of significant harm to the climate system.”

The standard of care with respect to the climate system might be applied by a jury to the decisions of private, corporate actors. Like the states the ICJ directed its opinion at, large fossil fuel producers and greenhouse gas emitters have the power to drastically reduce output. The failure to reduce output, in the face of the same facts the ICJ considered, could be considered culpable by common law jury.  

The most famous, and concrete, articulation of the reasonableness standard is Learned Hand’s eponymous formula.  In it, the burden (B) of a particular precaution (i.e. its cost) is compared to the expected value of the loss the precaution protects against (i.e. the product of the probability of harm and its magnitude (PxL)).  If B is less than PxL, reasonableness requires the precaution be taken.  This formula is dreaded by math-averse law students and rarely applied with precision, if at all, in the courtroom.  A common error occurs when the harm is construed overly narrowly, focusing only on the potential economic loss to the entity forgoing (or not) the precaution.  For the formula to be correctly applied, the relevant loss should not be confined to the loss felt by the entity forgoing the precaution (i.e. the one charged with negligence) but should reflect the total loss felt by society that foreseeably flows from the decision to forgo precaution.  With respect to climate change, those foreseeable looses are so significant that the ICJ has found obligations to engage in precaution at the nation state, rather than corporate entity, scale.  

Negligence Per Se-ish

The ICJ refers specifically to private actors on a few occasions in its opinion. In Paragraph 428, the ICJ is willing to attribute private conduct to states when the governments of those states fail to take regulatory measures to control them.  

Negligence per se is a doctrine that allows a jury to conclude that behavior is unreasonable because it violates codified law designed to protect people like the plaintiffs.  

If governments are obligated to control emissions, one might perceive that control as an implied protection regime. Emitters who significantly contribute to climate change would violate that regime.  

As Learned Hand famously held in the classic case of the TJ Hooper, “a whole calling may have unduly lagged in the adoption” of measures or policies.  In other words, one cannot simply point to collective inaction as conclusive evidence that to do nothing is reasonable.  Here, the ICJ has effectively held that the whole world, not just a particular calling, has unduly lagged.  The industries most responsible for climate change are arguably the worst laggards of the bunch.  

While this argument would certainly not be afforded the legal presumption, and strong jury instructions, that accompany traditional negligence per se situations, it might very well resonate with a jury.  

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These are just my first cuts at finding ways to incorporate ICJ precedent into the common law and thereby make international climate law more meaningful domestically in the United States. My hope is that some reader(s) will take my suggestions, expand on them, and go for it. It is worth noting in closing that this strategy is not limited by subject matter or particular international tribunal; other areas where bold pronouncements have been made (e.g. human rights) also deserve common law inroads.

Anthony Moffa is a Professor of Law & Associate Dean for Innovation at the University of Maine School of Law