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HUMANS have now pumped so much carbon dioxide into the atmosphere that the only way to meet our climate goals is to extract a stupendous amount of it. Last month, a headline-grabbing study suggested that we can do this just by planting a vast number of trees. But is it really that easy?

We know that we need to employ some form of carbon negative technology for two reasons. First, with emissions still rising, planetary warming is on course to exceed 2°C above pre-industrial temperatures not long after 2050. If we can reduce CO₂ levels, we stand a chance of cooling the planet back to safer temperatures.

Second, there are some activities – farming, flying, cement production, steel-making – that are really hard to do without emitting lots of carbon. Even if the world got serious about reducing emissions from these processes, the technology to do it doesn’t necessarily exist. Carbon removal might allow us to keep on flying, making steel and so on, and still get to zero overall emissions.

Tree planting for this purpose is a beguiling idea: trees are beautiful, cheap and effective at sucking in carbon. A longer-standing idea is to grow plants, burn them to make electricity and then filter out the carbon released and somehow store it. This is known as bioenergy carbon capture and storage (BECCS).

Almost all studies that claim we can limit warming to 2°C assume a massive deployment of BECCS. Yet we don’t know if it would work. No one has shown it can be done affordably on a large scale.

“Some are exploring exotic ideas, like grinding rocks that react with CO₂ and spreading them on farms”

Even if it is doable, it would require enormous swathes of land. Around a tenth of all land is already used for growing crops, and another three-quarters is exploited, for instance for grazing. There isn’t space to feed a growing, meat-hungry population, build more cities, conserve wildlife, save forests and also grow crops for bioenergy on a massive scale. Ecologists have been saying this all along, and it is expected to be acknowledged by the UN’s Intergovernmental Panel on Climate Change in an 8 August report on land use.

Is there a plan B? Ostensibly, yes. The recent tree study suggested that there is enough suitable land to plant a trillion trees, and that these would soak up two-thirds of the extra CO₂ in the atmosphere (Science, ).

Trees definitely can help. But many say the study greatly exaggerates their potential. The main reason is that half the CO₂ we have pumped out has been absorbed by oceans and plants. If CO₂ is removed from the air, much of this will be released – as would be the case for any other method of carbon capture. This means that even if we did plant enough trees to remove two-thirds of the extra CO₂, levels would only fall by roughly a third. What’s more, it would take up to a century for these trees to do their work.

Afforestation on this scale would also harm biodiversity. For instance, it would require forests to be planted on natural grasslands. “In savannahs and grasslands that are ancient ecosystems with a unique diversity, tree planting is a process of degradation,” says ecologist at the University of Edinburgh, UK.

We would also have to plant trees in the Arctic on land otherwise covered in sunlight-reflecting snow. This would mean more of the sun’s heat being absorbed. Plus, if monocultures of trees are planted, as often happens, that will do wildlife no favours. There needs to be scrutiny around large-scale tree planting, says Lehmann. “What is it really for? Carbon removal, biodiversity or job creation? These do not always align,” she says.

Fortunately, there are other ways to remove CO₂ from the atmosphere. One little-known method is wetland restoration. When vegetation sinks to the bottom of still water, the carbon it contains is locked up as peat. This dries out and decomposes when marshes are drained to create farmland. “They emit phenomenal amounts of carbon,” says Pete Smith at the University of Aberdeen, UK.

Simply re-wetting these lands would stem the loss of carbon, says Smith. Restore them, and they would start taking it up again. The downside is that the land couldn’t be used for growing food.

Another approach is to manage soils so they retain more carbon. So-called soil carbon sequestration can involve a range of measures for maintaining soil’s structure, from tilling less often to growing crops with deeper roots.

Then there is heating plant matter in the absence of air to make biochar, which is used to improve soils by trapping nutrients and water. It can take centuries for this charcoal to break down and release its carbon. But there is a limit to how much plant material is available for making it, and growing plants solely to make it would require more land.

There are huge uncertainties about the potential of all these methods. A study last year by Sabine Fuss at the Mercator Research Institute on Global Commons and Climate Change in Germany and her team if done on a sustainable basis. That exceeds what afforestation could achieve (see Graph). To put this into context, global CO₂ emissions are currently about 40 Gt a year.

This is why some people are exploring more exotic methods, such as grinding up rocks that react with CO₂ and spreading the powder on farmland – known as enhanced weathering because it speeds up this natural process. However, grinding up so much rock would itself require vast amounts of energy.

Others want to remove carbon dioxide directly from the air, then pump it underground or convert it into other materials, like plastics. At first sight, this plan has big advantages. “You don’t need to use large tracts of land and you don’t need lots of water,” says Ajay Gambhir at the Grantham Institute for Climate Change in the UK, who has .

But no one has got it to work at anything like the required scale. If anyone did, it would still require lots of energy. This strategy makes most sense if that energy comes from renewable sources – but we don’t have plenty of that to spare. According to Fuss’s study, direct air carbon capture and storage could remove 5 Gt of CO₂ a year at most by 2050 if done sustainably.

Adding up her team’s estimates of the impacts of carbon negative strategies suggests that, if we used them all, we could remove between about 4 and 25 Gt of CO₂ a year without damaging the environment. Yet this is still an overestimate, says Fuss, because land used for growing bioenergy crops, say, can’t be turned into forests as well. However, her study didn’t include every known carbon-removing strategy.

If you hoped that carbon negative technology would allow us to carry on pumping out CO₂ recklessly, these estimates are bad news. On the flip side, they show we really could achieve net zero emissions if we also cut CO₂ emissions hard and fast. Just don’t think that planting trees will be enough on its own.