California scientists have worked out how to reduce global warming so as to make the world 20 trillion US dollars better off. It’s simple. Just stick to the spirit of an international agreement that the American President Donald Trump has already broken.

The researchers arrived at their forecasts of climate profit and loss to calculate that if the 195 nations who agreed in Paris in 2015 to contain global warming to “well below” 2°C by the end of the century kept their promise – and global temperatures have already crept up 1°C in the last century as a consequence of the profligate use of fossil fuels – then there would be a 60% chance that the benefits would exceed $20 trillion.

That represents the savings made by avoiding the calamitous economic damage that would accompany higher temperatures. The same scientists also argue that 71% of the world’s nations – including China, Japan and the US – with 90% of the world’s population have a 75% chance of experiencing reduced economic damage, if global warming is limited to 1.5°C: that is, to just an extra half of a degree this century.

And although conjectures about wealth that has yet to be generated and disasters that have yet to happen are subject to enormous uncertainties, the scientists stand by their argument: if the world fails to meet the 2°C limit, the economic damage could add up to 15% of the world’s entire economic output.

Calculations like these are difficult enough, but at least one of the authors has been making the case for concerted global action for years. Noah Diffenbaugh, a climate scientist at Stanford University, has already warned that global extremes of heat and drought are an inevitable consequence of continued warming.

He says warming that has happened so far has already increased California’s vulnerability to devastating drought and may now be influencing the south Asian monsoon, on which a billion people depend.

“It is clear from our analysis that achieving the more ambitious Paris goal is highly likely to benefit most countries – and the global economy overall – by avoiding more severe economic damages,” Professor Diffenbaugh said.

And Marshall Burke, his Stanford colleague who led the study in the journal Nature, said: “Over the past century we have already experienced a 1°C increase in global temperature, so achieving the ambitious targets laid out in the Paris Agreement will not be easy or cheap. We need a clear understanding of how much economic benefit we’re going to get from meeting these different targets.”

Worse outcome possible

The researchers think they may even have underestimated the costs of a dangerously hotter world: they cite, for instance, the rapid rise in sea levels if the Greenland and Antarctic ice caps melt, or heat waves and floods intensify more dramatically than anything seen so far in human history.

Although the richest nations stand to benefit most from sticking strictly to the Paris ambitions, some of the world’s poorest regions will also feel the benefit, with a noticeable increase in gross domestic product per head.

“The countries likely to benefit the most are already relatively hot today,” Dr Burke said. “The historical record tells us that additional warming will be very harmful to these countries’ economies, and so even small reductions in future warming could have large benefits for most countries.”