Tens of millions of dollars have been invested in growing seaweed to absorb carbon dioxide and slow climate change. But due to unwanted side effects, this technique could fail to significantly decrease the CO2 in the atmosphere, and it might even increase it.

Carbon dioxide removal (CDR) will be needed to meet the Paris Agreement goal of limiting global warming to 2°C, to the UN, and many have hoped seaweed could be a cheap way to do that. The US start-up Running Tide raised $70 million to grow seaweed on pucks of wood that would eventually become sodden and sink to the deep sea, sequestering the carbon, but it ran out of financing and last year.

Dutch company Kelp Blue has raised at least $2 million to expand the seaweed that it is currently growing to produce sustainable agricultural fertiliser in Namibia. Because small particles of this seaweed may break off and drift into the depths, it it could eventually “sequester and offset” up to 500 million tonnes of CO2 per year.

But a global seaweed-cultivation programme could in many places rob nutrients from phytoplankton, which also sequester carbon when they die and sink to the depths, two studies have found.

“It could backfire locally,” says at the University of Bern, Switzerland, who worked on one of the studies. “In some places, you’d actually reduce how much carbon the ocean takes up. The potential is extremely limited, with large ecological consequences.”

Except for sargassum, macroalgae species live near the coast, where nutrients are plentiful. During photosynthesis, they consume carbon dissolved in seawater, allowing the ocean to absorb more CO2 from the atmosphere.

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Marine organisms and microbes eventually most of that seaweed, emitting an estimated nine-tenths of its carbon. To sequester more carbon, seaweed would have to be grown or transported further offshore, where it could be baled or otherwise sunk to the deep sea.

But nutrients are scarce in the open ocean, and most research before now hasn’t examined how the lack of iron could limit seaweed growth. Berger and her colleagues modelled the cultivation of 20 billion tonnes of seaweed per year across waters up to 200 nautical miles from coastlines.

They found the seaweed would quickly start depleting nitrogen, phosphorus and iron in the water, and after 25 years, its growth would have declined 95 per cent. Moreover, this would diminish global phytoplankton growth by as much as 8 per cent.

In some scenarios, seaweed cultivation could still remove billions of tonnes of CO2. But depending on what species of seaweed are grown and how much nutrients they consume, it could also increase the amount of carbon in the atmosphere by half a tonne for every tonne of seaweed carbon grown.

Patches off Senegal and southern Australia, about 0.05 per cent of the ocean, are the only places seaweed could flourish without significantly decreasing phytoplankton, the model suggests.

“If you have only a few very specific locations, you can’t grow enough seaweed to have a gigatonne of removal,” says Berger.

In another study, at the UK National Oceanography Centre and his colleagues modelled what would happen if seaweed-cultivation areas were fertilised with iron, finding that up to 40 billion tonnes of CO2 could be removed each year. But that would also halve the plankton in the ocean, with dire consequences for the fish that eat them.

“You’re robbing the surface ocean of nutrients… and transferring those to depth,” says Yool. “Essentially, you’re curtailing or slowly strangling the natural ecosystem.”

Furthermore, such seaweed cultivation and sinking would require setting up cages or other frameworks across 14 per cent of the ocean surface, largely in the nutrient-rich but stormy seas of the Southern Ocean and northern Pacific and Atlantic.

And if all this ocean wasn’t fertilised with iron, the seaweed carbon removal wouldn’t fully compensate for the plankton loss, increasing CO2 in the atmosphere by up to 700 million tonnes per year.

“You can’t just grow macroalgae and assume that you’re going to be undertaking CDR if you’re not accounting for offsetting phytoplankton growth,” says at the UK National Oceanography Centre, another member of the team.

A new type of all-carbon molecule has been studied under normal room-temperature conditions. This marks only the second time this has ever been done, after spherical buckyballs were synthesised 35 years ago. The breakthrough could lead to extremely efficient materials for new electronic and quantum technologies.

Cyclic carbons, molecules made up of a ring of carbon atoms, could display bizarre chemical behaviour or conduct electricity in unusual ways – much like their all-carbon molecular cousins, buckyballs and nanotubes. But these rings are so delicate they usually fall apart, or in some cases even explode, before researchers have a chance to study them.

“Cyclic carbons are intriguing molecules, and we’ve been trying to make them for a long time,” says at the University of Oxford. Doing so has traditionally required extremely harsh conditions in order to keep the molecules around long enough to be studied. But Anderson and his colleagues found a way to stabilise cyclic carbons at room temperature.

The technique involves modifying a cyclic carbon. The researchers demonstrated this on a never-before-studied molecule: a ring of 48 carbon atoms, called cyclo[48]carbon, or C₄₈. Anderson and his colleagues added “bumpers” to the C₄₈, threading it through three smaller rings, to protect the 48 atoms from colliding with each other – or with other molecules.

“There’s no unnecessary decoration,” says at the University of Ulm in Germany. “There’s an absolute beauty in the simplicity.”

The new structure, called cyclo[48]carbon [4]catenane, remained stable enough to study for about two days, enabling researchers to examine cyclo[48]carbon in detail for the first time. Intriguingly, the molecule’s 48 carbons acted like they were arranged in an infinite chain, a structure theoretically capable of transferring electric charge from one atom to the next indefinitely.

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This possible electricity-conducting potential hints cyclic carbons could be used in a range of next-generation technologies, including transistors, solar cells, semiconductors and quantum devices. However, further research is needed to confirm this.

The new technique for stabilising cyclic carbons may also inspire other researchers to study their own exotic carbon molecules. “I think maybe there will be a race now,” says von Delius. “Think of this long ring as a stepping stone to making the infinite chain.”

A chain of single carbon molecules, von Delius explains, would make an even better conductor than a ring like C₄₈. “This will be truly, truly amazing – and truly the next step,” he says.

Many forests are losing their winter snowpack as global temperatures rise, and that could substantially slow their growth – and reduce the amount of carbon they remove from the atmosphere.

Current projections “are not incorporating that complexity of winter climate change, so they are likely overestimating what the future carbon storage will be”, says at Boston University in Massachusetts.

Warming temperatures are generally expected to boost growth in temperate forests, mainly by spurring decomposition and making more nutrients available during the warm growing season. However, models largely don’t account for changes during winter – especially the loss of snow.

“The loss of deep, insulating snowpack cannot be understated,” says at the University of New Hampshire. Her research has shown deep snow days will disappear across most of the US by the end of the century, with consequences for water storage and ecosystem health.

To get a better handle on these cold-weather changes, Conrad-Rooney and their colleagues simulated how a global temperature increase of 5°C would affect the growth of red maple trees (Acer rubrum) in an experimental forest in New Hampshire. In some plots, they used buried cables to warm the soil during the growing season. In others, they also removed snow during winter and warmed the soil to induce cycles of freeze and thaw.

Measured over 10 years, the trees in both plots grew more than trees that were left alone. However, the plots where the snow was removed grew much more slowly, adding about half as much growth. The researchers attribute this difference to root damage caused by the snowless soil being more exposed to changing temperatures.

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“The snow typically acts as an insulating blanket to keep soils from freezing,” says Conrad-Rooney. “With less snow, there are more freeze-thaw cycles.”

Extrapolating to similar forests across the Northeast US, the researchers estimate the loss of snowpack expected by the end of the century would reduce carbon storage by a little over one million tonnes per year, compared with models that don’t account for disappearing snow.

“Snowpacks that come and go throughout the winter diminish the stable soil conditions our north-east ecosystems require for long-term storage of carbon,” says Burakowski.

However, not all snow-covered forests will respond to snowpack loss in the same way as the deciduous forests of the Northeast, says at the University of Utah. He points out accurately modelling various ecological responses remains a big challenge. “There’s so many things that are changing,” he says.