Climate change is an unusually hard ethical problem, because its causes and effects are spread across the whole world and across centuries.

What the science says

The Intergovernmental Panel on Climate Change (IPCC) was set up in 1988 by the World Meteorological Organization and the United Nations Environment Programme. It does not carry out its own research; it assesses and summarizes published research, and governments approve its summaries line by line. In its sixth assessment cycle (2021 to 2023), the IPCC concluded that human activities, chiefly greenhouse gas emissions from burning fossil fuels and from land use, have unequivocally caused global warming. Global surface temperature in 2011 to 2020 was about 1.1 degrees Celsius above the 1850 to 1900 average. The IPCC also reported that each additional increment of warming raises risks such as heat extremes, heavy rainfall, droughts and sea level rise, and that how much more warming occurs depends mainly on future emissions.

Real uncertainties remain, for example about exactly how sensitive the climate is, how quickly ice sheets will respond and how impacts will play out in particular regions. Uncertainty cuts both ways: outcomes may be milder or worse than central estimates.

A collective action problem

In 1968 the ecologist Garrett Hardin described the tragedy of the commons: when a resource is open to all, each user gains the full benefit of taking more but bears only a small share of the damage, so the resource can be ruined even though everyone would prefer it preserved. The atmosphere is a global commons. Each country, company or person contributes only a small part of total emissions, which tempts each to free ride on the efforts of others. The political scientist Elinor Ostrom later showed that communities often manage shared resources well through rules, trust and monitoring, so the tragedy is not inevitable.

Who should pay?

• The polluter pays principle: those who caused the emissions should bear the costs. Countries that industrialized early have emitted much of the carbon dioxide now in the air.
• Critics reply that people in the past did not know emissions were harmful, and today's citizens did not do the past emitting. In answer, some propose the beneficiary pays principle: those who enjoy the wealth built on past emissions should share the costs, whoever emitted them.
• The ability to pay principle says those with the most resources should contribute most, whatever the history.

Rich and poor

Poorer countries need more energy to reduce poverty, and many will suffer serious climate impacts despite having emitted little. The 1992 UN Framework Convention on Climate Change adopted the principle of common but differentiated responsibilities: every country shares the duty to act, but not equally. The 2015 Paris Agreement asked all countries to set their own national targets, aiming to hold warming well below 2 degrees and to pursue efforts to limit it to 1.5 degrees.

Honest policy debates

Accepting the science does not settle policy. Reasonable people disagree about:

• Pace and cost: how fast to cut emissions, weighing the costs of change against the costs of damage.
• Tools: many economists favor carbon pricing, through a tax or a cap-and-trade system, because it lets people find the cheapest ways to cut emissions. Others prefer regulations, public investment or support for innovation, and some worry that higher energy prices fall hardest on poorer households.
• Mitigation and adaptation: how to balance cutting emissions with preparing for changes already under way.
• Technologies: disagreement over nuclear power, and over proposals to deliberately cool the planet, which critics say create a moral hazard by weakening the will to cut emissions.

Worked example: three countries at the table

Imagine three invented countries negotiating how to share the cost of cutting emissions.

• Northland industrialized 150 years ago. Its people are wealthy, and its emissions per person are high, though falling. It produced a large share of the carbon dioxide already in the atmosphere.
• Midland is industrializing fast. Its total emissions now exceed Northland's, but emissions per person are lower, and many of its people have only recently escaped poverty.
• Southland is poor, emits very little, and faces rising seas and longer droughts.

Apply the principles. Polluter pays points mostly at Northland, because of its history, and increasingly at Midland. Northland replies that its great-grandparents did not know the harm. Beneficiary pays answers that today's Northlanders still enjoy the roads, schools and wealth those emissions built. Ability to pay also points at Northland, and partly at Midland's richer regions.

Midland raises a fairness point of its own: Northland grew rich on cheap fossil energy, so it seems unfair to deny Midland the same path. Northland answers that if Midland follows that path, the warming will harm everyone, Southland most of all. A possible bargain: Midland commits to cleaner growth, and Northland helps pay for it with finance and technology.

Southland asks for help adapting to changes it did little to cause. Northland and Midland each want the other to move first, the free-rider temptation in miniature.

Real negotiations, from the 1992 Convention to the Paris Agreement, have turned on exactly these arguments. No principle settles everything, but naming them makes clearer what each side is really claiming.