An international tribunal said on Tuesday that countries have binding legal obligations to prevent greenhouse gasses from harming the world’s oceans in a landmark opinion that experts say could strengthen climate-vulnerable countries’ efforts to hold major polluters accountable and drive action to fight climate change.

The advisory opinion, which was sought by an international organization called the Commission of Small Island States (COSIS), said that human caused greenhouse gas emissions are a form of marine pollution as defined by the U.N. Convention on the Law of the Sea (UNCLOS), and that governments must take “all necessary measures” aimed at staunching greenhouse gasses. 

The International Tribunal on the Law of the Sea said that its opinion does not require the immediate cessation of emissions largely related to burning fossil fuels, but, in stark language, listed a series of “stringent” legal obligations that governments must meet to “prevent, reduce and control” emissions and to “protect and preserve” oceans from climate change impacts.

Those obligations include adopting laws and regulations aimed at reducing greenhouse gas emissions; preventing emissions under their jurisdiction or control from causing damage to other states; protecting fragile ecosystems and threatened species; cooperating with other governments and sharing scientific information; adopting measures aimed at controlling invasive species; and restoring degraded ecosystems. 

Warning that there are “high risks of serious and irreversible harm” to oceans from a warming planet, the tribunal said governments must take into account the best available science and the standards presented in climate change treaties, including the Paris Agreement’s goal to hold global warming to 1.5 degrees Celsius over pre industrial levels. What constitutes a “necessary measure” will vary according to each country’s capabilities and developing countries are entitled to special assistance measures including financial assistance, the tribunal said. 

Payam Akhavan, a lawyer for COSIS, said the tribunal answered a question that has long divided countries:

“Does a state which makes a nationally determined contribution, which is grossly inadequate to ensure that temperature rise will be limited within 1.5 degrees, satisfy international law?” he said. “Or do states have to go beyond what the Paris Agreement obligates them to do?”  

Akhavan said the opinion made clear that for governments to meet their obligations under international law, it is insufficient for them to only submit periodic emission reduction goals under the auspices of the Paris Agreement, which some governments argue is their only current legal requirement. 

Tuesday’s advisory opinion is the first time an international tribunal has specified the legal obligations governments have to reduce greenhouse gas emissions under the U.N. Convention on the Law of the Sea. The opinion was issued unanimously from 21 judges elected by the 169 member countries of the convention. 

The United States is not a party to the convention and did not participate in the proceedings. But, because parts of UNCLOS are considered customary international law, which is the general practice of states accepted as law, some experts say the opinion may have legal application to countries that are not parties to the convention. 

Advisory opinions can be relied upon by litigants in national and sub-national courts, which issue legally binding decisions. Tuesday’s opinion is also expected to play a role in upcoming climate negotiations under the U.N. Framework Convention on Climate Change (UNFCCC), beginning with talks taking place in Azerbaijan later this year, giving developing countries that are disproportionately harmed by climate change greater negotiating power. 

“That will have far reaching consequences,” Akhavan said. 

Gaston Browne, prime minister of Antigua and Barbuda, hailed the ITLOS opinion as a “historic milestone in our journey to climate justice,” and said he cannot fathom what will happen to his country and other small island states if the world does not change course by more aggressively reducing greenhouse gas emissions. 

COSIS’s nine member countries, including Vanuatu, Saint Kitts, and Antigua and Barbuda, have been heavily impacted by climate change in the form of sea level rise, more intense hurricanes, drought and the loss of marine species. 

For Tuvalu, the Bahamas and some other countries, whether governments hold global warming at or near 1.5 degrees Celsius compared to preindustrial levels will determine whether the countries remain habitable. The world’s current warming trajectory, under which global temperatures are projected to rise by 2.8 degrees Celsius or more by 2100, means that Tuvalu will be subsumed by sea level rise by the end of the century. 

Such existential threats to the continued existence of some small island states is why those nations have long been referred to as the “conscience” of the UNFCCC negotiations—they have contributed very little to the greenhouse gasses warming the planet but are on the front line of impacts that are becoming more intense. 

Until now, the UNFCCC, which spawned the Paris Agreement, has dominated international climate governance. Under that framework, fossil-fuel producing and wealthy countries have largely resisted signing onto legally binding obligations that would require them to reduce their greenhouse gas emissions. The Paris Agreement requires countries to periodically submit more stringent emission reduction goals, but some governments maintain that they are not legally bound to meet those targets. 

It was out of frustration with the slow pace of those negotiations and progress on the issue of losses and damages that COSIS was formed in 2021 on the sidelines of the COP26 climate summit in Glasgow, according to Akhavan.

In December 2022, COSIS filed a request with the International Tribunal on the Law of the Sea, asking the court to advise on governments’ legal duties to address climate change under the U.N. Convention on the Law of the Sea. That treaty governs maritime law, including the rights and obligations of countries with regard to fisheries and the conservation of marine species and ecosystems. It also requires countries that ratify the treaty to prevent, reduce and control pollution of the marine environment.

Experts say Tuesday’s advisory opinion marks a turning point where global climate governance has been pushed from the UNFCCC’s politics-dominated realm of voluntary commitments to concrete legal obligations. 

“When it comes to climate governance, the Paris Agreement is not the only game in town,” said Joie Chowdhury, a senior attorney at the Center for International Environmental Law in Washington and Geneva. 

Over 40 governments and international organizations participated in the advisory opinion proceedings, part of which took place via written submissions as well as in-person presentations at the tribunal’s headquarters in Hamburg, Germany, last September. 

The questions before the tribunal were two-fold: first, whether greenhouse gas emissions constitute a form of marine pollution under the U.N. Convention on the Law of the Sea and what governments’ legal obligations are to “prevent, reduce and control” that pollution; and second, what are governments’ obligations to protect and preserve the marine environment from climate change impacts.

Nearly all countries that participated in the proceedings agreed on the scientific consensus that greenhouse gas emissions deleteriously affect oceans and their ecosystems. Oceans have absorbed about a quarter of human-caused carbon dioxide emissions and about 90 percent of global warming, causing coral bleaching, species extinction, ocean acidification, more intense storms and sea level rise from the expansion of warming water and the melting of land-based ice. 

The past decade was the ocean’s warmest since at least the 1800s, and since March of last year, global ocean surface temperatures have broken records daily. Compared to pre-industrial levels, sea levels have risen over half a foot, enough that some low-lying coastal communities have already been displaced. 

But countries that participated at the ITLOS proceedings split on whether the tribunal had the authority to advise on countries’ legal obligations related to climate change and whether the U.N. Convention on the Law of the Sea, which was negotiated in 1982 before climate change was on the public agenda, could be applied to the problem of global warming. The United Kingdom, Australia and, to a lesser degree, the European Union, argued that climate agreements like the UNFCCC and Paris Agreement should solely determine countries’ climate change obligations, including what governments must do to control marine pollution in the form of greenhouse gasses. 

“… and what science requires and justice demands to prevent further harm to oceans and the life that they sustain.”

The opposing view, espoused by many climate-vulnerable countries, is that overlapping international laws, from human rights treaties to the U.N. Convention on the Law of the Sea, define governments’ legal responsibilities when it comes to causing damage to other countries and the global commons. 

“The UNFCCC and Paris Agreement were negotiated against the backdrop of long-standing legal rules, like the transboundary harm principle, which prohibits activities in one state from causing environmental harm outside that state’s boundaries,” said Nikki Reisch, director of the climate and energy program at the CIEL.  

Reisch said that the ITLOS opinion is unlikely to set off an explosion of state-to-state litigation over climate obligations, and that the more likely effect will be that it “provides more impetus for states to do the right thing and rectify the massive gap between their conduct, and what science requires and justice demands to prevent further harm to oceans and the life that they sustain.”

The International Tribunal for the Law of the Sea’s advisory opinion sets the stage for two more climate change advisory opinions pending before different international courts. Proceedings before the International Court of Justice, the highest U.N. judicial authority, and the Inter-American Court of Human Rights are underway and experts say Tuesday’s advisory opinion will likely have an impact on the outcome of those proceedings, though the questions before the ICJ and Inter-American Court are slightly different than what was asked of the ITLOS. 

The U.N. General Assembly asked the ICJ to answer specific legal questions about governments’ obligations to protect the “climate system” for “present and future generations,” and what governments’ responsibilities are for “acts and omissions” that have caused harm to the climate, with an emphasis on harm to “particularly vulnerable” small island developing countries. 

The governments of Chile and Colombia requested an advisory opinion from the San José, Costa Rica-based Inter-American Court of Human Rights about governments’ obligations for responding to climate change under human rights laws. 

One commonality among all three climate change advisory opinion requests is that the international courts overseeing them are being asked to interpret laws drafted before the era of climate change and apply those laws to the context of a warming planet. 

Whether those laws are “living documents” that ought to be adapted to new contexts, or whether only climate-centered treaties like the Paris Agreement should govern countries’ obligations to reduce greenhouse gasses, is likely to remain a flashpoint among governments as the other advisory opinion proceedings progress. 

As Akhavan, counsel for COSIS put it, those other international tribunals will, like the ITLOS, need to decide whether international law governing climate change goes beyond the Paris Agreement.

Countries’ second round of written arguments are due to the ICJ on June 24th, and the Inter-American Court is holding a second round of hearings on the advisory opinion request before it in Brazil this week. A fourth request for an advisory opinion on climate change is expected to be lodged with the African Court of Human and People’s Rights in the coming months. 

The ITLOS advisory opinion comes just weeks after another international court, the European Court of Human Rights, ruled that Switzerland violated its citizens’ human rights by not doing enough to reduce its greenhouse gas emissions. In recent years, litigation over climate change has boomed, with lawsuits filed to force polluters to reduce emissions and compensate local jurisdictions for climate-related damages, and others brought to delay such action. The majority of that litigation has taken place in U.S. courtrooms.

The United States, the world’s biggest historical emitter of greenhouse gasses, is a participant in the advisory opinion proceedings pending before the International Court of Justice. One of the laws under consideration by the ICJ is the U.N. Convention on the Law of the Sea. As such, Tuesday’s advisory opinion is expected to influence the outcome of the ICJ’s advisory opinion—international courts aim to maintain consistency among themselves and tend not to break with another court’s jurisprudence absent a compelling reason. 

COSIS member countries include Antigua and Barbuda (co-chair), Tuvalu (co-chair), Vanuatu, St. Kitts and Nevis, St. Vincent and the Grenadines, St. Lucia, Niue, Palau, and the Bahamas.