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HERE鈥橲 a trivia question for you. What do Bhutan and Suriname have in common? If you said that they are the only countries that, in effect, add no greenhouse gases to the air, a state of grace known as net zero, then well done 鈥� though you might want to get out more.

Even if you aren鈥檛 a climate obsessive, net zero is a phrase you have probably heard. Greta Thunberg talked about it last month at the World Economic Forum meeting in Davos, Switzerland. Last year, the UK and France became the first major economies to legally commit to achieving it; both chose 2050 as year zero. Sweden and New Zealand have also enacted similar net zero targets. Eleven other countries have legislation in the pipeline, with Finland aiming for 2035 and Norway and Uruguay an even more ambitious 2030.

Smaller political entities such as states and cities are increasingly declaring net zero targets, as are big companies. Net zero doesn鈥檛 change what we have to do, but seems to be focusing minds in a way that vague global temperature goals have failed to do.

On the surface, net zero seems a good idea and a straightforward one. Wherever possible, stop emitting greenhouse gases, and have enough carbon dioxide-absorbing trees to counter any remaining emissions, or capture those emissions and bury them.

Unlike temperature goals, net zero sounds unequivocal. It draws on very solid science 鈥� the concept of a carbon budget: the amount of extra CO₂ (or other greenhouse gases) we can emit without cooking Earth. It is clearly a useful way of framing the challenge and aiming for solutions.

Simple. But, as with all things climate-related, there is devil in the detail 鈥� and a good deal of wiggle room.

Net zero has loopholes galore, especially aviation and shipping. To achieve national net zero, a country must include all emissions from planes and ships within its airspace and territorial waters. But what about emissions once the craft have flown and sailed into international ones? Historically, these emissions have been exempt from climate change talks because of the difficulty of allocating them to specific countries. But without a mechanism to do so, net zero will be an illusion.

鈥淎ction by the big emitters seems to be moving things on: 18 nations are on a downward net emissions trajectory鈥�

To its credit, the UK government has committed to accounting for these emissions, though has yet to specify how. But other countries may not be so strict.

There鈥檚 more. Countries could claim to be net zero, but consume products made abroad whose manufacture creates emissions. A requirement to gauge a nation鈥檚 consumption globally will close this loophole, but in the meantime nations can shift some emissions off their books this way.

Even with a meaningful target, you then have the problem of actually hitting net zero, which is where the hard yards really start. Bhutan and Suriname succeeded largely because they have small populations, lots of hydroelectric power and intact forests. The rest of the world isn鈥檛 so blessed.

But there is positive evidence that concerted action by the big emitters is moving things in the right direction. Eighteen advanced economies (including the UK) are currently on a downward net emissions trajectory, largely as a result of their climate and energy policies.

At the moment, even the best of them are only achieving an annual reduction of about 2 per cent, when what is needed is more like 7 or 8 per cent. To hit that target, every sector of the economy has to put its shoulder to the wheel and tough policies will have to be implemented, such as completely refurbishing the housing stock to make it low energy and putting a price on carbon emissions.

Even that won鈥檛 be enough; we will also require widespread deployment of carbon capture and storage technology (CCS), which is unproven on a large scale, and which opens up another loophole. Instead of pouring money into CCS, advanced economies could pay to plant trees in other countries 鈥� usually developing economies 鈥� and count this towards their domestic target.

While it is easy to be cynical about governments committing to something they won鈥檛 be judged on for three decades, there are optimistic noises emerging from climate science. The UK鈥檚 declaration of net zero has 鈥渟hifted the dial鈥�, says Jim Skea, professor of sustainable energy at Imperial College London, with even North Sea oil and gas companies accepting the need for net zero targets. And a national commitment will be useful for peer shaming at the critical UN climate talks due in the UK in November, says Joeri Rogelj of the Grantham Institute in London.

It still won鈥檛 be easy. It will still take unprecedented transformation of every aspect of society. But the optimism is rising: we can be zeroes.