On 13 March, a group of teenagers filed a climate-change case before the constitutional court of South Korea. The country ranks as the sixth-largest emitter of carbon dioxide, per capita, in the Organisation for Economic Co-operation and Development. The plaintiffs claimed that their rights had been violated by their government. The case is one of around a dozen brought by members of the general public, in countries including France, Canada and Colombia, who seek protection from climate impacts and demand greater emissions reductions from their nations.

In 2019, the Urgenda Foundation, a Dutch non-profit group that aims to mitigate climate change, together with 886 Dutch citizens, successfully sued the government of the Netherlands for doing too little to reduce emissions. As legal counsel to Urgenda, I know that science is instrumental in arguing these cases. Specifically, courts need researchers to translate benchmarks for global emissions into fair and equitable targets for individual countries.

Courts throughout the world now readily accept the scientific evidence that society and the planet face vast, systemic dangers if global emissions are not curbed rapidly. Increasingly, they seem willing to attach legal consequences to governments that fail to step up. In its judgment last December, the Dutch Supreme Court concluded that climate change poses a “real and immediate” threat to people living in the Netherlands, and ordered the government to increase its emissions-reduction efforts.

Emissions: world has four times the work or one-third of the time

Similar cases elsewhere have had different outcomes. Courts in Germany, Ireland and New Zealand recognized governments’ responsibility to mitigate the risks of climate change but stopped short of finding that their climate policies were unlawful. Many cases are still on appeal, so the outcomes might change. Meanwhile, the judgments so far hold lessons on how the scientific community can supply criteria that will be useful.

An upper limit for emissions is readily available at the global level. The reports of the Intergovernmental Panel on Climate Change (IPCC) and the United Nations Environment Programme place it at 25 gigatonnes of CO 2 equivalent — a cut of 55% from 2018 levels — by 2030 to limit global warming to 1.5 °C. Much less is published, however, on how the global reductions needed to reach this target should be divided fairly for individual countries. Current contributions leave us on course for 3 °C of warming.

Scientists have worked to determine a country’s fair share of reductions through ‘effort-sharing models’. These take into account principles such as responsibility, capability and equality to translate the necessary global reduction to the national or regional level. An evaluation of effort-sharing models that the IPCC included in its report in 2007 was instrumental to Urgenda’s successful arguments in court. In the Paris agreement of 2015, countries created a framework in which each would determine its own contributions rather than negotiate reductions up front, and so the scientific community has largely disengaged from effort-sharing models, and many are outdated.

Not slashing emissions? See you in court

Assessing a ‘fair share’ of reductions in the context of litigation is as much a legal issue as a scientific one, so legal scholars and scientists should cooperate. Certain effort-sharing methods — such as ‘grandfathering’, in which outsized emitters are entitled to emit more in the future — have no basis in international law and are best avoided.

Courts also need help in determining the ramifications a country should experience if it does not follow the emissions trajectory that corresponds to its ‘fair’ share. Governments commonly assert in court proceedings that they are committed to reducing emissions and will achieve the right level in future. Researchers could model the costs, consequences and feasibility of delays for individual countries. More insight is also needed on the consequences for a nation’s land use and economy when emissions larger than the country’s fair share are offset on its own territory.

One argument that governments make for delay is the prospect of deploying ‘negative emissions’ systems and technologies to remove carbon from the atmosphere; these include reforestation, extracting energy from biomass while capturing carbon, or pulling CO 2 directly from the air. These approaches have limitations, however. For instance, reforestation would make land unavailable for crops or development, and other techniques remain untested, especially at regional and national scales.

Finally, the scientific community must continue to update the contrast in impacts between different global temperature targets, as was done in the 2018 IPCC report; this showed that risks of deaths from extreme heat, flooding and malnutrition are all substantially higher under the scenario of 2 °C of temperature rise than that of 1.5 °C. Such differential impacts should also be forecast for distinct regions and countries.

If nations do not take the drastic action now required to avert widespread climate damage, members of the public, such as the plucky South Korean teenagers, will continue to turn to the courts. Scientific evidence, based on robust research and modelling, will lie at the core of these cases. Funding agencies and national scientific associations should help researchers to work independently on these issues. The scientific community has a crucial part to play in analysing what constitutes a fair share of emissions reduction, and what will happen to states that don’t play fair.