Plastic bottles and smokestacks are easy to blame for climate change. Carbon credits are more complicated. They exist largely out of sight, traded between companies, investors and certification bodies, all built on the promise that a measurable amount of carbon has been removed from the atmosphere. But what happens when that carbon exists mostly on paper?
That question now hangs over one of the world's largest mangrove restoration schemes in Senegal. Launched with enormous ambition and hailed as a model for blue-carbon finance, the project mobilised more than 100,000 villagers, planted millions of mangrove propagules and generated carbon credits that were sold to companies seeking to offset their emissions. Yet a recent scientific investigation suggests that a substantial share of those credits may have been based on carbon that was never actually stored.
Researchers have described the discrepancy as "ghost carbon", carbon removals claimed by the market but unsupported by ecological reality. The findings have reignited debate about the credibility of voluntary carbon markets and the scientific rigour underpinning nature-based climate solutions.
What was the Senegal mangrove restoration project?
The restoration programme began in 2008 and expanded rapidly across the estuaries of Casamance and Sine-Saloum. Led by the Senegalese environmental organisation Océanium with support from private-sector partners and the Livelihoods Carbon Fund, the initiative aimed to restore more than 10,000 hectares of degraded mangrove habitat. The project quickly gained international attention. According to project documentation from Livelihoods, approximately 80 million mangrove propagules were planted over three years with the participation of more than 100,000 residents. The scheme was promoted as a model capable of delivering environmental restoration, coastal protection, fisheries recovery and climate mitigation simultaneously.
Mangroves occupy a unique position in climate policy because they are among the most efficient natural carbon sinks on Earth. Unlike many terrestrial forests, mangroves store substantial quantities of carbon not only in their biomass but also in deep, waterlogged soils where organic matter decomposes very slowly. Studies have repeatedly shown that these coastal ecosystems can hold exceptionally large carbon stocks, making them attractive candidates for carbon credit projects. The Senegal project, therefore, appeared to offer a rare opportunity: restore damaged ecosystems while generating revenue through the sale of carbon credits.
Why researchers found that much of the claimed carbon storage never existed
The controversy emerged after researchers used satellite imagery, field observations and ecological analysis to assess what had happened to the restored mangroves more than a decade after planting. Their findings, published in 2026 in the journal Environmental Challenges by researchers from the University of Côte d'Azur, France, painted a far more complex picture than the project's original success narrative. The study concluded that only around 31.7 per cent of the project area appeared as mangrove cover in remote-sensing data, while only about 19 per cent of the total area could be directly attributed to successful replanting efforts. The researchers estimated that less than 30 per cent of the carbon represented by issued credits had actually been stored.
A major reason was ecological site selection. Many planting areas were established on salt flats and mudflats that lacked the tidal conditions required for mangrove survival. Some plots were located kilometres away from existing mangrove ecosystems, where hydrological conditions differed significantly from natural mangrove habitats. According to the researchers, restoration efforts expanded so aggressively that many unsuitable sites were included simply to meet ambitious planting targets. The study reported that 36 per cent of restoration plots experienced complete failure, meaning no mangroves survived. Across the wider project area, overall survival rates were estimated at roughly 18–20 per cent. Despite these losses, carbon calculations used in credit generation appear to have assumed far greater restoration success.
Professor Julien Andrieu, lead author of the study Remote Sensing Assessment of the World's Largest Mangrove Reforestation Program (Senegal, 2009–2012): Implications for Carbon Crediting and Future Restoration Efforts Dedicated to Climate Change Offsetting, explained that the problem was not the measurement of surviving trees themselves but the extrapolation of those measurements across the entire restoration area. The result was an inflated estimate of total carbon storage.
How 'ghost carbon' exposed weaknesses in voluntary carbon markets
The phrase "ghost carbon" refers to carbon reductions or removals that are credited and sold despite not existing in the real world. In Senegal's case, researchers calculated a discrepancy of approximately 168,000 tonnes of carbon dioxide between the amount of carbon claimed and the amount likely stored by the restored ecosystem. Depending on carbon market prices, the difference could represent several million US dollars in carbon credit value. The issue carries implications far beyond Senegal. Carbon credits are often purchased by companies seeking to compensate for emissions that they cannot immediately eliminate. The credibility of the entire system depends on one fundamental principle: each credit must correspond to a real, measurable and additional climate benefit.
Offsetting vs Real Action
The sources distinguish between a carbon credit (a measurable reduction in emissions) and its use as an offset. The "ghost" element arises when corporations purchase cheap and questionable credits to justify continued fossil fuel use, and companies claim to be "carbon neutral" based on these purchases while avoiding genuine emissions reductions at the source. This practice has been criticised for allowing polluters to "buy their way out" of meaningful climate action.
"If the amount of carbon captured by a project is overestimated, companies may believe they are offsetting emissions that, in reality, remain in the atmosphere," Andrieu wrote. He added that certifiers and monitoring organisations must properly account for project failure rates because otherwise they are effectively selling "ghost carbon".
Importantly, the researchers do not argue against mangrove restoration itself. In fact, recent studies continue to show that strategically planned mangrove restoration can deliver substantial carbon sequestration, biodiversity gains and coastal resilience benefits. Their criticism is directed at weak monitoring systems and carbon-accounting methodologies that fail to capture ecological realities on the ground. The Senegal case therefore serves as a warning rather than a rejection of nature-based climate solutions. Ambitious restoration targets may attract headlines and investment, but long-term ecological success depends on choosing the right locations, monitoring outcomes rigorously and ensuring that every carbon credit sold reflects carbon genuinely removed from the atmosphere. Without that scientific discipline, climate markets risk trading promises instead of measurable climate action.
