With the notable exception of Donald Trump, leading politicians at the recent G20 summit in Hamburg accepted the necessity of a worldwide policy to combat climate change. Trump demurred, not because he finds flaws in the policy contained in the Paris agreement to which his 19 colleagues re-committed themselves, but because he is not concerned by the threat at all. In this he appears to be entirely mistaken: there is an accumulation of evidence collected over the last 30 years or so which should convince the doubters that indefinite further growth in greenhouse gas emissions is dangerous.

The consensus among climate scientists (many of the most influential are Americans) is that emissions should not just be stabilised at current levels but should ideally be reduced. If this is not done, steadily and consistently over several decades, the build-up in the atmosphere will eventually cause, they believe, widespread economic and social dislocation. These dislocation costs would far exceed the inconvenience of reducing emissions.

There is really only a small minority who doubt that emissions reduction is the prudent policy to pursue

There are always dissenting voices from the scientific consensus and there is a range of views about the imminence of the climate change threat. Some are alarmed that atmospheric concentrations of carbon and other greenhouse gases have already passed safe levels while others believe that there is ample time for corrective action. But there is really only a small minority who doubt that emissions reduction is the prudent policy to pursue.

The UN-sponsored panel on climate change has assembled a truly worldwide body of evidence which is difficult for any lay person to ignore. But President Trump has chosen to side with the small minority, to the intense exasperation of the numerous American experts who have pioneered the scientific study of climate change.

While the Paris agreement accepts the scientific consensus about the need for emissions reduction, it is out of tune with another important consensus. This is the recommendation from economists who have studied the policy options that a system of quantitative national ceilings on emissions is not the best way to go.

The world as a whole needs to discourage the consumption of goods and services which generate high emissions: where production takes place is immaterial

They point out that the Earth has just one atmosphere and that national ceilings are irrelevant. The world as a whole needs to discourage the consumption of goods and services which generate high emissions: where production takes place is immaterial. Since the world does not have a single government, this recommendation lacks an implementation mechanism. Were it to be agreed, it would require something along the lines of a worldwide minimum tax on the consumption of carbon-intensive items.

Instead, the 200 or so countries in the world have squabbled about the allocation of national emission quotas and numerous countries have either withdrawn from, or have simply failed to deliver on, previous accords. Some parts of the world agonise over their performance while others, including it now appears the US, are unwilling to contribute at all to the necessary global effort.

China largest emitter

Almost 60% of global energy-related emissions are generated by China, the US and the EU, in that order. In per-capita terms, the EU countries are the lowest emitters in the developed world: the US emits more than double the European average per head. Other high emitters include Saudi Arabia and Australia, but these countries have small populations.

Chinese emissions remain well below those of the developed countries in per-capita terms (two-thirds of European countries and less than half the US figure) but have been rising rapidly. With its huge population, China is the biggest aggregate emitter and Chinese policy has tilted towards acceptance of the need for emissions reduction in recent years.

Of the three major emitters, China, the US and the EU, it is anomalous that Europe, which has been doing most, should be most committed to further action. China and the US between them emit four times the entire EU total and continued growth in those two countries will swamp whatever effort is made in Europe.

So the apparent departure of the US from the Paris accord, which many climate campaigners thought was disappointing anyway, is potentially very serious. Every country has an incentive to leave emissions reduction, which is costly, to others.

Everyone bears the cost of their own efforts but the benefit is shared, since the Earth has just one atmosphere

Everyone bears the cost of their own efforts but the benefit is shared, since the Earth has just one atmosphere. The danger is that China, and other fast-growing middle-income countries, will prove less willing to bear the burden if the US is seen to shirk its obligations.

The argument about climate policy in the US is not over. President Trump’s actions at federal level are limited in scope, since congressional approval is required for some of his preferred changes. Moreover, several states and cities have already announced that they plan to persist with local initiatives.

More importantly, there is, after all, a four-year term limit for US presidents.

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