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.

African forests are now emitting more carbon dioxide than they absorb, a fundamental shift that will make it more difficult for the world to cut its net emissions to zero.

Forests and shrubby woodlands on the continent have previously been one of the world鈥檚 biggest carbon sinks, accounting for 20 per cent of all the CO2 taken up by plants. The lion鈥檚 share of this is in the Congo rainforest, the second largest in the world after the Amazon. Sometimes called the 鈥渓ungs of Africa鈥�, it absorbs an estimated 600 million tonnes of CO2 per year. However, that number has been falling as the rainforest is destroyed by logging and mining.

Now, researchers have found that after gaining biomass from 2007 to 2010, African forests lost 106 million tonnes of biomass per year from 2011 to 2017. That is equivalent to roughly 200 million tonnes of CO2 emissions per year. This was driven by deforestation in the Congo rainforest, says at the University of Leicester, UK.

鈥淚f we are losing the tropical forests as one of the means of mitigating climate change, then we basically have to reduce our emissions of greenhouse gasses from fossil fuel burning even faster to get to near-zero emissions,鈥� he says.

Balzter and his colleagues estimated the amount of biomass with satellite measurements of the colour and moisture content of the forest canopy, as well as its height at certain points. They compared this with measurements taken on the ground, although these are sparse in Africa.

But at University College London says satellite data can鈥檛 detect the type of trees in a forest and isn鈥檛 reliable for estimating the carbon absorbed by high-biomass intact forests or emitted in forests degraded by selective logging. A dense hardwood like mahogany might hold more carbon than a light balsa wood of the same size, for instance.

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鈥淒eforestation in Democratic Republic of Congo鈥� is higher than it was in the 2000s. And we all know that,鈥� he says. 鈥淏ut whether that is enough to tip the whole carbon balance of the entire continent is unknown.鈥�

The study also didn鈥檛 include the wet peatlands found underneath much of the Congo rainforest, which absorb a small amount of CO2 each year and hold about 30 billion tonnes of ancient carbon.

The Amazon rainforest, which was also once a major carbon sink, emitted more CO2 than it absorbed in several recent years. But whereas deforestation in the Amazon has fallen under a government crackdown, it has been growing in the Congo.

In the Democratic Republic of the Congo, impoverished farmers often destroy rainforest for slash-and-burn agriculture. Companies, many of them foreign-owned, illegally log colourful hardwoods like African teak and coralwood.

At the COP30 climate summit in the Amazon this month, Brazil announced the Tropical Forests Forever Facility, a fund that will pay its investment returns to tropical countries at a rate of $4 per hectare of forest left standing. But so far countries have donated only $6.6 billion to the fund, far short of the $25 billion goal.

Balzter says this mechanism could be more effective than carbon credits, which reward 鈥渁voided鈥� emissions and have in many cases been found to be worthless.

鈥淚t鈥檚 really important to make this Tropical Forest Forever Facility work, and make it work quite quickly, to try and reverse this trend of the African tree biomass actually releasing carbon into the atmosphere,鈥� he says.

Atmospheric carbon dioxide levels jumped by a record amount in 2024 to push concentrations to their highest point since measurements began, the UN鈥檚 World Meteorological Organization (WMO) has reported.

Between 2023 and 2024, the global average concentration of CO2 surged by 3.5 parts per million (ppm) to reach 423.9 ppm, the WMO has said. This is the largest increase since modern measurements started in 1957 and is well in excess of the 2022 to 2023 increase of 2.3 ppm.

It marks the latest in a trend of accelerating annual increases, with growth rates tripling since the 1960s. The last time Earth experienced a comparable concentration of CO2 was 3 million to 5 million years ago.

Excess CO2 in the atmosphere will have a warming effect on the planet for centuries to come, the WMO warns. 鈥淭he heat trapped by CO2 and other greenhouse gases is turbocharging our climate and leading to more extreme weather,鈥� at the WMO said in a statement.

Ongoing emissions from fossil fuels, alongside a surge in emissions from wildfires and a slump in the carbon uptake by the world鈥檚 lands and oceans, were the key drivers of last year鈥檚 record surge, according to the WMO.

Researchers expected a slump in the uptake of carbon by oceans, forests and other ecosystems in 2024 due to the recent El Ni帽o weather pattern, which pushed up global temperatures and dulled carbon absorption by driving heat, fires and drought in key regions like the Amazon. The amount of tropical forest lost in 2024 was double that of 2023, scientists noted earlier this year. 鈥淚t is normal for some tropical lands to be drier and store less carbon during warm El Ni帽o years such as 2024,鈥� says at the University of Reading, UK.

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But there is concern that this dip in carbon uptake by the planet 鈥� particularly by the land 鈥� is part of a longer-term trend that could mean climate change is weakening the planet鈥檚 ability to soak up excess carbon.

鈥淭here has been some suggestions that the land sink was particularly low in 2023 and 2024, even for El Ni帽o years, and that there has been a worrying reduction over time, particularly in the northern hemisphere outside the tropics,鈥� says r, a climate scientist at the Breakthrough Institute. 鈥淚n short, there are worrying signs that the land sink in particular is declining, but it鈥檚 too early to know with confidence without another few years of data.鈥�

In the meantime, it is more urgent than ever for humanity to curb ongoing fossil fuel emissions, says at the University of Leeds, UK. 鈥淭he biggest reason for the ongoing increase [in CO2 concentrations] is fossil fuel emissions being at a persistent all-time high and not yet coming down.鈥�