PRINCETON – When President Donald Trump announced that the US was withdrawing from the Paris climate agreement, he justified the move by saying "the bottom line is that the Paris Accord is very unfair, at the highest level, to the United States.” Is it?

To assess Trump’s claim, it is important to understand that when we ask how much countries should cut their greenhouse-gas emissions, we are essentially discussing how to distribute a limited resource. It’s as if we were discussing how to divide an apple pie when more hungry people want a big slice than there are big slices.

In the case of climate change, the pie is the atmosphere’s capacity to absorb our emissions without triggering catastrophic change to our planet’s climate. The people wanting big slices are the countries that would like to emit large quantities of greenhouse gases.

We all know one way to divide a pie: give everyone an equal slice. For the atmosphere, that would mean calculating what quantity of greenhouse gases the world as a whole can safely emit up to a given date, and dividing that by the current population of the world. That yields everyone’s per capita share of the atmosphere’s capacity to absorb our greenhouse gases, up to the selected date.

But the world is divided into sovereign states, not individuals, and there is no way to assess each individual’s greenhouse-gas emissions. So we need to move to allocations for each country. To do this consistently with "equal shares,” we need to multiply the per capita share by the country’s population to reach its emissions quota.

By this standard, was the Paris accord unfair to the US? Hardly. The US currently has less than 5% of the world’s population, but emits nearly 15% of the world’s greenhouse gases. If fairness means that everyone’s slice of pie should be the same size, it is the US that is being unfair, by grabbing a slice that is three times bigger than it should have.

India, by contrast, has 17% of the world’s population and emits less than 6% of its greenhouse gases, so it would be entitled to almost three times its current emissions. Many other developing countries use an even smaller fraction of their per capitashare of the atmosphere.

Perhaps equal slices are not the fairest way to divide a pie. One obvious objection is that equal division takes no account of how much the people seeking slices really need them. Are the pie-seekers genuinely hungry, or are they already well fed and just looking for a treat?

But taking need into account does nothing to assist Trump’s case that the US was unfairly treated by the Paris accord, because Americans could easily cut back on luxuries like vacation travel, air conditioning, and meat consumption, whereas less affluent countries need to industrialize to lift their populations out of depths of poverty unknown in the US.

A different principle of fairness arises if we consider greenhouse gases as pollution, and apply the principle that whoever caused the pollution should pay to clean it up. The reason that climate change is a problem now is that over the past two centuries, some countries have been putting large amounts of carbon dioxide and other greenhouse gases into the atmosphere.

No country has emitted more greenhouse gases over this period than the US. That is a reason to require the US to make deeper cuts now than other countries must make, especially given that the US is continuing to emit greenhouse gases at a much higher per capita rate than other large emitters, such as China and India. If the older industrialized countries caused the problem, it seems reasonable to ask them to do the most to fix it.

We could also view countries’ historical contributions to climate change in terms of a per capita share over time. Other countries can claim that the US has already used up its historical per capita share of the atmosphere’s capacity to absorb greenhouse gases, and they should be entitled to emit more in the future so that we will at least come closer to equal per capita shares over time. (Other countries cannot use as much as the US and Europe have already used, because of global warming then exceeding 2^oC, the point at which, in the view of most scientists, climate change would become unpredictable and quite possibly catastrophic.)

So on the three most plausible principles of fairness that can be applied to climate change – equal shares, need, and historical responsibility – the US should make drastic cuts to its greenhouse-gas emissions. On the equal shares principle, US emissions should be no more than one-third of what they are today, and on the other principles, even less. Instead, President Barack Obama committed the US to cut its emissions by just 27%, relative to 2005, by 2025. Trump’s claim that the Paris climate agreement was unfair to the US does not withstand scrutiny. In fact, the opposite is the case: the US got off very lightly.

If the US now fails to achieve even the very modest target it set itself in Paris, and thus fails to carry out its fair share of the reductions necessary to stabilize our planet’s climate, what should the rest of the world do? China and the European Union have already indicated that they will abide by their commitments. But we should not simply allow the US to free-ride on other countries’ reductions, while burning unlimited quantities of fossil fuel to provide cheap energy for its industries. Instead, the world’s citizens should take matters into their own hands, and boycott products manufactured in a country that so manifestly refuses to do its part to save the planet.

Peter Singer is Professor of Bioethics at Princeton University and Laureate Professor at the University of Melbourne. In 2013, he was named the world’s third "most influential contemporary thinker” by the Gottlieb Duttweiler Institute.

Copyright: Project Syndicate.