Growing seaweed has long been as a way to slow climate change. But this approach is mistaken, a new study argues, as the loss of seaweed emits far more carbon than what is being absorbed by attempts to grow it.

Seaweeds absorb an estimated 3.7 billion tonnes of carbon dioxide through photosynthesis every year, drawing it from seawater, which then draws more CO2 from the air above. They mostly grow near the coast, where they are eventually digested or decomposed, emitting carbon. But fragments of fronds, as well as from the seaweeds that dissolve in seawater, can sink into the deep sea, sequestering a of the CO2 as “” for thousands of years.

Scientists and start-ups have suggested that seaweed growth, especially kelp, could be expanded to take up more CO2, something the UN has said we will need to do on a massive scale to eventually reverse dangerous levels of global warming.

But there are concerns that cultivating huge amounts of seaweed could backfire and diminish growth of phytoplankton, another important carbon sink. And even the kelp forests we currently have, which are estimated to cover around 640,000 square kilometres worldwide – an area slightly larger than France – are dying off as ocean temperatures reach record highs.

Now, at the University of Western Australia and her colleagues have concluded that, with some 60 per cent of kelp forests in decline, these ecosystems could eventually begin emitting more CO2 than they absorb. Researchers, as well as governments and companies trying to offset their emissions, need to focus on preserving rather than growing seaweed, they suggest.

“Instead of asking how much carbon we can remove with seaweeds, we should be asking how much carbon we can stop from being released as these ecosystems are damaged by climate change,” says Filbee-Dexter.

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Some studies have the possibility of removing a billion tonnes of carbon per year with seaweed, but there are already signs that this may not be feasible. For instance, the US carbon removal start-up Running Tide, which raised $70 million to cultivate kelp on wooden buoys that would gradually sink into the deep sea, in the open ocean and shut down last year.

Other projects have tried to bring back lost kelp forests. An experiment in northern Norway of kelp by poisoning seaweed-eating sea urchins, which can multiply when overfishing and hunting remove their predators.

While kelp restoration like this is a win for biodiversity, it only draws down about 50,000 tonnes of CO2 per year. Meanwhile, global kelp loss is emitting an estimated 14 million tonnes of CO2 per year.

The study by Filbee-Dexter and her colleagues also notes that preserving kelp ecosystems is at least eight times cheaper per hectare than restoring them, yet only 2 per cent of kelp forests lie in protected areas.

Firms are also cultivating seaweed to make nori for sushi, animal feed and thickeners for products like ice cream and toothpaste. Fragments of seaweed still drift away and sink into the deep sea, and aquaculture companies like Kelp Blue have to eventually sequester and offset hundreds of millions of tonnes of CO2.

Seaweed farming now takes up 4000 square kilometres of ocean. But that’s only about a third of the kelp forest cover lost each year. And after factoring in emissions associated with farming operations – like boat fuel and plastic lines – nine out of 11 seaweed farms were found to emit more CO2 than they sequestered in a .

Microplastics could be hampering the ocean’s ability to capture carbon

A global survey of microplastics in oceans reveals that tiny particles of plastic are prevalent throughout the water column, which could harm marine ecosystems and affect carbon storage in the deep sea

“Seaweed restoration and farming is unlikely to be a major climate change solution,” Filbee-Dexter says. “But that doesn’t mean it isn’t worthwhile.”

Restoration can improve biodiversity, and farming can make low-carbon alternatives to fossil-fuel products like fertiliser, she notes.

at Queen’s University Belfast in the UK, however, says scientists don’t have a solid enough understanding of how much carbon seaweed sequesters to write it off yet. Macroalgae restoration, conservation and farming can all still be climate solutions, even if their impact has often been exaggerated in the past, she argues.

“Blue carbon is part of the puzzle,” she says. “It can make a contribution. The question is how that is determined and validated.”

Floating Futures
Alessandro Gandolfi

Imagine a city where suburbs float on water. Floating homes already exist in coastal communities in places like the Philippines and Vietnam. Now, high-income nations facing dangerous sea-level rise are looking to those examples.

Among them is the Netherlands, where over a quarter of the land is below sea level and around two-thirds of it is at risk of flooding.

This photograph shows floating houses in the IJburg district at the edge of Amsterdam, the largest Dutch city. The image comes from , a new series by photojournalist Alessandro Gandolfi.

The affluent, relatively high-density IJburg community comprises 158 waterside homes, 55 of which are floating. Residents are connected by bridges and roads, and most houses have a boat outside instead of a car.

Simply raising the dykes is no longer enough against the rising waters, Gandolfi writes in his photo essay. “Experts warn that mastering water’s whims is an illusion. The answer, they say, lies in a shift of mindset – the philosophy of meebewegen: not fighting against water, but learning to live with it.”

The homes at IJburg aren’t houseboats or retrofitted vessels like the ones lining Amsterdam’s canals, but architect-designed homes intended to encourage community living. They are built to be sustainable, with rooftop solar panels and careful management of sewage and other waste.

But their biggest difference from other houses will always lie below, in their watery foundations.

The world is likely to overshoot 1.5°C of warming within the next few years, a by the UN Environment Programme (UNEP) acknowledges. However, it says it is still possible to cool the planet back down to this level by the end of the century. Can it really be done? It isn’t impossible, but it certainly isn’t the most likely outcome.

The goal of limiting the increase in the average global surface temperature to 1.5°C above pre-industrial levels looked implausible even back in 2015, when it was adopted as part of the Paris Agreement. The UNEP report can be seen as an “official” acknowledgment of what many have been saying for years: that it is now certain that we are going to blast right past this limit.

But some aren’t giving up on the target. There are no good outcomes if we remain above 1.5°C, says at UNEP. “Even if we exceed 1.5, let’s be very clear: that 1.5 target is still the goal. But now we need to approach it differently, from above.”

Much of the report looks at the consequences of overshooting 1.5°C and the need for adaptation to cope with the severe impacts this will bring. “We will see increased food insecurity, increased water insecurity, impacts on human health – physical and mental – danger to our homes and our infrastructure,” says at the University of KwaZulu-Natal in South Africa, one of the authors of the report. “This will lead to the loss of livelihoods and certainly lives.”

The world isn’t prepared for overshooting 1.5°C, she says. “Therefore we have to advance our adaptation and development strategies to deal with those higher levels of warming, the greater range of risks and impacts… Adaptation needs to be transformational to allow us to plan for a changing world.”

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There will be no going back to how things were, says Roberts. “Some losses will be irreversible, which means returning to 1.5 doesn’t mean we return back to the same world we left.”

But what exactly should we be preparing for? The report focuses on a scenario in which rapid emissions cuts and massive-scale removal of carbon dioxide from the atmosphere result in a return to 1.5°C of warming by the end of the century.

But achieving this would require limiting temperature rises to no more than 1.8°C, says Roberts. Beyond this, there will be cascading effects from all the irreversible changes that make it much harder.

However, restricting warming to this level requires a halving of global emissions by 2035, says report author at Imperial College London. That seems highly unlikely given that emissions are still rising worldwide.

In addition to emissions cuts, the scenario envisaged in the report also requires carbon removal on a truly gigantic scale – enough not just to balance out hard-to-tackle emission sources, such as farming, and achieve net zero, but to go well beyond that to remove around 10 gigatonnes of CO2 each year. That compares with current emissions of around 40 gigatonnes.

This means scaling up carbon removal by a factor of around 100,000, says at ETH Zurich in Switzerland, who wasn’t involved in the report. Cheap forms of carbon removal, such as planting forests, can’t be scaled up this much because there isn’t enough land. Direct air capture could be scaled up, but it is really expensive, he says.

Knutti thinks achieving this is technically and financially possible, but not politically plausible. “We should be honest when we talk to policy-makers and the public that this is not the thing to plan for,” he says. “I think the plan should be to adapt for at least 2 degrees, if not more.”

at the University of Exeter in the UK is even less optimistic. “There are still no credible plans for large-scale carbon dioxide removal,” he says. “Human societies must adapt and prepare for a warmer and more dangerous world that will persist potentially for centuries.”

, also at ETH Zurich, points out that among , only two imply a return to 1.5°C before 2150. The scenario based on current policies suggests warming of around 3°C by 2100.

Until recently, overshooting and returning to 1.5°C was no more than a scientific thought experiment, says at the German Institute for International and Security Affairs. Now it could become the “official” plan B for 1.5°C. “The overshoot concept may turn out to be no more than a way to mask ongoing failure,” he says.

But even if getting the temperature back down to 1.5°C isn’t likely, we have to try, says Weber. “We have to give it a shot. There’s no real alternative.”

“Now is the time to double down on climate action, not to give up on it,” says Andersen. “Every fraction of a degree avoided, every year by which overshoot is shortened and every adaptation action taken will save lives, protect ecosystems and reduce economic losses.”

The UK’s dry, hot and sunny summer has provisionally entered the record books as the hottest season for the country since records began in 1884.

The average temperature for the months of June, July and August was 16.5°C (61.7°F), the national weather service, the Met Office, said on 1 September, blowing through the previous summer record of 16.1°C (61°F)  set just last year.

“The conditions we’ve seen this summer have broken UK climate records once again,” said at the Met Office in a statement.

Summer in the UK began with the second warmest June on record, followed by the second-warmest July. But the heat remained almost constant throughout the three-month period, with 38°C (100.4°F) recorded for the first time in June in Norfolk, and 38.1°C (100.6°F) recorded at Kew Gardens in London on 13 August.

Much of England is now in the grip of a severe drought after rainfall vanished from many parts of the country during July. The hot, dry conditions have damaged crops, strained healthcare systems and triggered severe wildfires.

Fossil fuel emissions fuelled the high temperatures, with climate change making a summer like 2026 around 130 times more likely to occur, the Met Office said.

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“In a climate unaffected by human emissions of greenhouse gases, a summer like this would be exceptionally unlikely, i.e. 1 in over 1,000 years,” at the Met Office said in a statement. “In our current climate, which is changing because of the effect of burning of fossil fuels, we should now expect this to be roughly a 1 in 9 year event.”

The top five warmest summers for the UK have all occurred since 2003. This trend will only intensify in the future as rising global temperatures cause summers in the UK to be increasingly hot and dry. Under 4°C of warming, the summer just gone would be considered cool or below average for the UK, the Met Office said.

In response to the announcement, at the World Wildlife Fund said in a statement: “These stark figures prove what we have been experiencing first hand, we are getting hotter and hotter. And the climate will continue to heat up until we cut our emissions to net zero.”