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掳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 鈥渟equester and offset鈥� 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.

鈥淚t could backfire locally,鈥� says at the University of Bern, Switzerland, who worked on one of the studies. 鈥淚n some places, you鈥檇 actually reduce how much carbon the ocean takes up. The potential is extremely limited, with large ecological consequences.鈥�

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鈥檛 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.

鈥淚f you have only a few very specific locations, you can鈥檛 grow enough seaweed to have a gigatonne of removal,鈥� 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.

鈥淵ou鈥檙e robbing the surface ocean of nutrients鈥� and transferring those to depth,鈥� says Yool. 鈥淓ssentially, you鈥檙e curtailing or slowly strangling the natural ecosystem.鈥�

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鈥檛 fertilised with iron, the seaweed carbon removal wouldn鈥檛 fully compensate for the plankton loss, increasing CO2 in the atmosphere by up to 700 million tonnes per year.

鈥淵ou can鈥檛 just grow macroalgae and assume that you鈥檙e going to be undertaking CDR if you鈥檙e not accounting for offsetting phytoplankton growth,鈥� says at the UK National Oceanography Centre, another member of the team.

The average size of trees in the Amazon rainforest has been steadily increasing as carbon dioxide levels have risen, meaning these larger trees play a more important role in determining whether the forest can remain a carbon sink.

How forests will react to a changing climate is an open question. For example, one hypothesis is that larger trees will decrease in abundance because they are more susceptible to climate-linked phenomena such as drought or high winds. Understanding how it will play out is crucial for models of the future climate because forests take up huge amounts of CO₂ from the atmosphere, locking it away to slow global warming.

at the University of Cambridge and her colleagues at the have been measuring the diameter of trees in 188 plots with an average area of 12,000 square metres across the Amazon basin. The monitoring periods varied, but some were as long as 30 years.鈥疍uring that time, CO₂ concentrations in the atmosphere have risen by nearly a fifth.

鈥淲hat we鈥檙e following is some space in the forest and in that space the average tree size is bigger, meaning that the trees can pack more carbon in that space than they could in the past,鈥� says Esquivel-Muelbert. The researchers have found that, on average, trees have increased in diameter by about 3.3 per cent each decade.

鈥淭he structure of the Amazon Forest is changing quite consistently across the whole basin,鈥� says team member at the University of Bristol, UK. 鈥淲e have more bigger trees and fewer smaller trees, so the average size has shifted up towards those bigger trees.鈥�

Normally the average diameter of trees in an area of undisturbed old-growth forest would stay roughly the same, she says, as saplings take the place of fallen big trees and grow. The researchers think the Amazon trees are responding to the increase in atmospheric CO₂ levels by growing more, and accumulating more biomass. 鈥淭he winners are the big trees that compete better for light and for water,鈥� says Esquivel-Muelbert.

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This means the big trees are disproportionately important to the amount of carbon the forest can hold, and the consequences of losing them would be disproportionately big, she says.

鈥淭he important finding is that CO₂ has been acting as a fertiliser, increasing tree growth, and in many ways that is reassuring, because wood is a globally significant carbon sink,鈥� says at Durham University, UK. 鈥淗owever, will this continue to be the case as the climate continues to change, potentially shifting the balance between growth, nutrients, temperature and CO₂?鈥�

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 鈥� 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.

鈥淐yclic carbons are intriguing molecules, and we鈥檝e been trying to make them for a long time,鈥� 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 鈥渂umpers鈥� to the C₄₈, threading it through three smaller rings, to protect the 48 atoms from colliding with each other 鈥� or with other molecules.

鈥淭here鈥檚 no unnecessary decoration,鈥� says at the University of Ulm in Germany. 鈥淭here鈥檚 an absolute beauty in the simplicity.鈥�

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鈥檚 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. 鈥淚 think maybe there will be a race now,鈥� says von Delius. 鈥淭hink of this long ring as a stepping stone to making the infinite chain.鈥�

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

Tropical forests populated with a diversity of seed-dispersing animals can accumulate carbon up to four times as fast as fragmented forests where these animals are absent or their movement is restricted.

鈥淭his shows a linkage between animal biodiversity loss and a process that exacerbates climate change,鈥� says at the Massachusetts Institute of Technology. 鈥淲e鈥檙e losing the regrowth potential of tropical forests.鈥�

Animals contain just a tiny fraction of the carbon stored in the environments where they live. But there is increasing recognition their activities can have outsized impacts on their ecosystems鈥� carbon. One important contribution comes from animals like monkeys, birds and rodents, whose behaviour disperses a great diversity of seeds across a wide area.

Still, 鈥渋t鈥檚 been really hard to translate that to the long-scale processes like the carbon recovery of entire landscapes鈥�, says Fricke.

Fricke and his colleagues analysed more than 3000 plots in tropical forests where trees were growing back 鈥� and accumulating carbon 鈥� after a disturbance. They then estimated the amount of disruption to the movement and diversity of seed-dispersing animals in each plot. The estimates relied on factors like the amount of forest fragmentation and data from tracked animals.

They found more disruption to the movement of seed dispersers was clearly linked with a lower rate of carbon accumulation. Forests that had the least disruption to their animals鈥� habits grew four times as fast as the most disrupted ones.

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On average, disruptions to seed-dispersing animals鈥� diversity and movement reduced the amount of carbon the plots could accumulate by half. This means the disruptions had an even larger negative effect than other factors limiting tree regrowth, such as fires or livestock grazing.

Conversely, forests with the least disruption accumulated carbon even faster than monoculture tree plantations. 鈥淣atural growth amplified by animals offers a low cost and biodiversity-positive restoration strategy,鈥� says Fricke.

Previously, ecological models suggested seed dispersers could have a substantial effect on carbon. But this study 鈥渋mproves our understanding of how important these animals could be鈥�, says at Yale University. 鈥淎nd it shows that they鈥檙e going to be important.鈥�