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ECSTATIC celebrations broke out in Paris just before Christmas. World leaders acclaimed their freshly forged deal to fight climate change. There were hugs and tears. But hold on a second. Were the celebrations justified? The Paris agreement says governments will find a way to limit global warming to 鈥渨ell below 2 掳C鈥� and do their darndest to have no more than 1.5 掳C. Is that even possible?

Aiming for 1.5 掳C is definitely a good idea, since two degrees of warming will almost certainly wipe low-lying islands off the map (see 鈥�1.5 掳C versus 2 掳C鈥�). According to some climate scientists, it is also an impossible goal. Others are more optimistic, and think 1.5 掳C might just be within reach. Just. One thing is clear: if we are going to do it, we will have to create a whole new industry to suck vast quantities of carbon dioxide directly from the air. Without these 鈥渘egative emissions鈥� we can bid farewell to 1.5 掳C.

So what would this carbon-sucking industry look like? From planting the Sahara to farming the oceans, there are a number of solutions we could deploy. The question is how realistic are they, and could they ever be sufficient?

The essential task is to control the build-up of CO₂ in the atmosphere. Humans have pushed levels from 280 parts per million before the industrial revolution to 400 ppm now by burning fossil fuels and trashing nature. As a result, in 2015, meteorological stations around the world recorded a 1 掳C rise in global temperatures above pre-industrial times. The key number here is 430 ppm 鈥� that鈥檚 the level that the UN Intergovernmental Panel on Climate Change (IPCC) says corresponds to 1.5 掳C of warming. At this stage, it is effectively impossible to stop us overshooting that number. But we could, theoretically, get back down to 430 ppm by the end of the century 鈥� assuming we are willing to create a whole new mega-industry of negative emissions.

鈥淲e will have to create a whole new industry to suck carbon from the air鈥�

Joeri Rogelj of International Institute for Applied Systems Analysis (IIASA), a think tank based in Austria, is one of the first to analyse how we could do this. From his calculations, it鈥檚 possible to draw up a hypothetical road map to 1.5 掳C. is to bring net global emissions to zero by 2050, emitting no more than 800 gigatonnes cumulatively between now and then (see illustration). Keeping emissions at zero after 2050 should be enough to limit warming to below 2 掳C. To bring warming back down to 1.5 掳C, we would then have to remove some 500 Gt from the air.

Net zero emissions by 2050 is no small feat. Some fossil fuels could be burned, but only as much as can be sucked out of the atmosphere. Still, there is a glimmer of hope. According to a preliminary assessment by the Global Carbon Project, last year, CO₂ emissions from power stations and industrial sources fell back to 2013 levels, despite a global economy that was 6 per cent larger 鈥� the first time this has happened outside a global recession.

, who led the analysis and is head of the Tyndall Centre for Climate Change Research at the University of East Anglia, UK, cites three causes. First, we are burning less coal. China is getting out of coal fast, motivated by its smog problem as much as anything else. And coal鈥檚 contribution to US electricity generation has shrunk from 53 per cent to 35 per cent in five years. Second, investment in renewable energy now exceeds that going into fossil-fuel power stations globally. Optimists hope the Paris deal will boost this trend. Finally, energy efficiency continues to improve in most places.

Le Qu茅r茅 says emissions will probably still rise again, but we may be surprisingly close to their peak. China, the US and the European Union 鈥� the world鈥檚 biggest emitters 鈥� are on the right track.

Killing oil and natural gas may prove harder than the assassination of King Coal. We have to hope that technology can deliver breakthroughs for transportation systems. The most likely solution is electric vehicles, which in turn requires better batteries. Planes can鈥檛 be plugged in, so may have to run on biofuels. 鈥淭he transport sector is particularly hard to decarbonise,鈥� says Rogelj.

Let鈥檚 assume technology and investments do manage to all-but-eliminate emissions by mid-century. That is the easy half of the road to 1.5 掳C. We still need to suck large amounts of CO₂ out of the air to bring temperatures back down below 2 掳C.

鈥淲e need a solution that also boosts food production. It鈥檚 not as crazy as it seems鈥�

The whole idea of negative emissions is controversial. Kevin Anderson of the Tyndall Centre has ridiculed the notion as akin to a pantomime-style faith in a carbon-sucking fairy godmother. on the table for how to take greenhouse gases out of the atmosphere and lock them away 鈥� the question is whether they can achieve enough without competing with other things we need, like food. Taking the optimist鈥檚 stance then, what wand does Earth鈥檚 fairy godmother need to wave?

There are several ways to chemically absorb CO₂ directly from the air 鈥� you may have heard of them as 鈥渁rtificial trees鈥�. All are prohibitively expensive, both in cash and energy demand, says Pete Smith of the University of Aberdeen, UK, who headed a global study of negative emissions options published during the Paris conference. to meet Rogelj鈥檚 500 Gt target would cost about $270 trillion and consume a quarter of the world鈥檚 energy supply, he says.

Equally unlikely on cost grounds is the idea of augmenting the natural erosion of carbonate rocks, a process that removes CO₂ from the atmosphere. Which pretty much rules out those ideas.

If chemicals and rocks can鈥檛 do the job, what about trees themselves? Deforestation is currently responsible for more than 10 per cent of our CO₂ emissions. If we are to stop temperatures rising, the target agreed in New York in 2014 for zero global deforestation by 2030 is a must. After that, replanting could make forests into negative emitters. Where land is available, this is a bargain option. Smith estimates that the cost is an order of magnitude lower than the chemical or geological options. That said, it still adds up to a total bill of $14 trillion over 50 years.

Plus, trees take a lot of land. Planting enough of them to soak up 500 Gt of CO₂ over 50 years would call for 10 million square kilometres 鈥� an expanse as big as the United States, which isn鈥檛 available. The Sahara is also big enough, but plans to cover it in forest aren鈥檛 viable.

Don鈥檛 give up on trees, though 鈥� they could still be a part of the solution. Why not, asks Smith, plant some trees, cut them when they reach maturity 鈥� at which point they stop soaking up as much CO₂ anyway 鈥� and burn them in power stations? If we then planted the cleared land with new trees or fast-growing energy crops, they could eventually soak up the same amount of CO₂ again, resulting in carbon-neutral energy over the long term. Now add an extra bit of tech. Capture the concentrated CO₂ before it leaves the power station and bury it. The net effect would be negative emissions that also generate energy.

Bioenergy with carbon capture and storage (BECCS), as this is known, effectively reverses the way we generate energy today. Instead of digging up concentrated carbon in the shape of coal, oil and gas, burning it and releasing the waste into the atmosphere, we could grab diffuse CO₂ from the air with plants, burn them, and bury the carbon in concentrated form underground. The more electricity we generate, the bigger the negative emissions. Could this be the fairy godmother that can save the climate?

Smith鈥檚 study found that BECCS could capture 500 Gt of CO₂ over the second half of the century, while contributing a large fraction of the world鈥檚 electricity. 鈥淥f all the ways of achieving negative emissions, BECCS seems to be the most promising,鈥� says Florian Kraxner of IIASA.

All the elements have been tried and tested, on a small scale. Carbon capture and storage systems, for instance, currently put some 20 million tonnes of CO₂ underground every year 鈥� often as a means of flushing the last oil and gas from old seams. Some worry about the safety and reliability of such burial systems. But the world probably has the capacity to store at least 20 times the 500 Gt target in former gas and oil formations or in saline aquifers, says Stuart Haszeldine of the University of Edinburgh, UK. on a larger scale in the UK 鈥� by burying emissions from the biomass-burning Drax power station beneath the North Sea 鈥� was abandoned last year after the government withdrew support.

Harness the oceans

The big issue with BECCS is land. It requires less than simply planting trees, but at its height, delivering 500 Gt of negative emissions would still take anywhere between 3.8 and 7 million square kilometres, says Smith. And ideally most of that would be in the tropics, where plants grow faster 鈥� but where the need for land to grow food is also greatest. 鈥淲e have to feed a population of 9 to 10 billion people in 2050,鈥� Smith says, 鈥渨hich is a challenge of similar proportions to that posed by limiting climate change to 1.5 掳C.鈥�

The danger is that people will find new land to grow their food by clearing forests or grasslands. An analysis last year by Andrew Wiltshire of the UK Met Office and Taraka Davies-Barnard of the University of Exeter argued that this could reduce the effectiveness of .

What is really needed is a solution that sucks CO₂ out of the atmosphere while boosting food production. It鈥檚 not as crazy as it seems. Farmers already use tricks to keep carbon in the ground. For instance, ploughing encourages the breakdown of dead plant matter and release of CO₂, so many now practice 鈥渘o-till farming鈥�, which allows plant matter to build up in the soil instead. A more intense solution, which could match the scale of the climate challenge, would be to bury biochar: agricultural waste like straw, manure and unused food that has been pyrolysed to charcoal.

Burying biochar is an industrialised version of traditional methods practised by tribes in the Amazon and elsewhere to enrich their soils. Because biochar contains tightly bound carbon atoms, it is stable for hundreds of years. Smith calculates that biochar could provide up to 125 Gt of negative CO₂ emissions .

And there鈥檚 another option that could help. Consider the fact that most of the planet is ocean 鈥� Earth鈥檚 largest carbon sink. Tricking the oceans into taking in more CO₂ by fertilising plankton with iron or nitrogen, something that has been widely discussed in recent years, is stymied by our ignorance of how carbon cycles work below the waves, says Smith, who dismisses this idea as unrealistic. But farming the oceans might work.

Last year, Brian Walsh at the IIASA made the case for cultivating microalgae on a massive scale. The microalgae could be used as fuel for BECCS power stations or as animal feed, replacing soy and other land-hungry crops. Prototype microalgae farms are at work in Australia. By mid-century, there could be up to 50 million hectares of them growing billions of tonnes of biomass.

Walsh estimates that such an expanse of microalgae could absorb up to 25 Gt of CO₂ a year. It might take a while to get going at scale. But even at a quarter of Walsh鈥檚 rate, just 20 years could deliver the remaining 125 Gt needed to reach Rogelj鈥檚 500 Gt target, while feeding up to 10 per cent of the world鈥檚 livestock. Forests could be saved for nature; fields for growing food; and global warming could be capped at 1.5 掳C.

Whether this will work depends on how well we know the climate. If we are unlucky, and greenhouse gases have a greater impact on temperatures than we think, then the chance of hitting 1.5 掳C is zero. If current sensitivity estimates are right, then we stand a chance. We can dream. Or perhaps the world should get on with it and see if it works.

1.5 掳C versus 2 掳C

All plausible futures with just 1.5 掳C of global warming require massive negative emissions, says Joeri Rogelj of International Institute for Applied Systems Analysis in Austria (see main story). On the other hand, we could have a 2 掳C world simply by kicking our carbon habit. No negative emissions required. Is the extra effort worth it?

Yes, says Johan Rockstr枚m, director of the Stockholm Resilience Centre: 鈥淭wo degrees contains significant risks for societies everywhere.鈥� By contrast, 1.5 掳C could prevent the disappearance of Arctic sea ice, save forests, dramatically reduce the risk of crop failures in sub-Saharan Africa, and curb sea level rise. Michiel Schaeffer of Wageningen University in the Netherlands calculates that 1.5 掳C would limit sea level rise over the next two centuries to 1.5 metres, whereas 2 掳C would lock in 2.7 metres. for low-lying island states such as Kiribati, an archipelago of Pacific atolls none of which rises more than 2 metres above the waves. Kiribati led the push in Paris for the conference to adopt a 1.5 掳C target. Anything more, it argued, would consign their nation to oblivion.

But peeking barely 50 centimetres above the waves will not save many island states. There will be no high ground to run to when seasonal hurricanes throw storm surges at their shores. Pacific atoll nations 鈥� those most fixed on a 1.5 掳C target 鈥� may be doomed already.

This article appeared in print under the headline 鈥淗ello, cool world鈥�