How did a major mangrove restoration project in Senegal end up selling ‘ghost carbon’?

Nature-based solutions have become central to global climate strategies. From tropical forests to wetlands, restoring nature means trees and plants can capture carbon dioxide, reduce global warming and regenerate biodiversity.

Among these solutions, mangroves (coastal forests) occupy a special place. They're among the most efficient natural carbon sinks on Earth. A carbon sink is a place where carbon is stored, like within the trunk of a tree or in waterlogged soil. A mangrove forest can also make money by selling carbon credits and paying a fair share to local communities.

Carbon credits are tradeable units that represent one tonne of carbon dioxide removed from the atmosphere or avoided through activities such as reforestation, renewable energy projects or ecosystem restoration. Companies buy these credits to compensate for, or offset, some of the emissions they produce.

But, in practice, projects can go wrong. A mangrove restoration project in Senegal is one example.

I am a geographer who studies mangrove forests. By analysing satellite data, I track changes in mangrove cover over time. On the ground observations help provide a more detailed understanding of the forests. I also interview local communities and other stakeholders to better understand the social and ecological factors that shape these systems.

Together with a team of researchers, I investigated a mangrove restoration project launched in Senegal in 2008. I wanted to see if there were any lessons that could be learned to improve future restoration efforts.

The project's goal was to use a blue carbon financing model. This generates funding for the restoration of coastal and marine ecosystems by selling carbon credits linked to the carbon these ecosystems can capture and store.

As part of the project, millions of mangrove propagules (young mangrove seedlings) were planted. The idea was that, as the mangroves grew, they would absorb carbon dioxide from the atmosphere and store it. The estimated amount of carbon captured by the restored mangroves could then be converted into carbon credits and sold to companies seeking to offset their greenhouse gas emissions.

We recently discovered that more than a third of the restoration plots were a complete failure. There were no surviving mangroves at all in this area. On the rest of the land, the results ranged from less than 5% to a real success (100%).

The failure rate of the trees was not properly taken into consideration when calculating how much carbon dioxide they could capture. This meant the climate benefits were overstated.

Our findings are not an argument against mangrove restoration – far from it. Mangroves remain one of the most effective natural tools for tackling climate change. But for blue carbon projects to play a credible role in climate action, carbon claims must be honest, independently verified and based on sound science.

A flawed design from the start

The original plan was ambitious: to plant millions of mangrove seedlings over more than 10,000 hectares. The plan was to involve the local communities, offset carbon emissions by polluting companies elsewhere, and generate income for decades in the carbon market.

But based on our knowledge of mangrove swamps in Senegal, the project was based on a poor understanding of ecology. The NGO Océanium designed the project, which also involved several French companies and an investment fund focused on climate action and rural development.

The first problem was that in order to guarantee that 10,000 hectares would be replanted, they had to include locations that were not fit for mangroves.

Many mangrove seedlings were planted in salty mudflats, which are unsuitable for their growth, unless they are regularly flooded by the tides. Very large plots of up to 55 hectares were planted, too far away from the tidal channels.

Some plots were planted up to 9km away from existing mangrove areas. These sites had different ecological conditions from natural mangrove habitats. Replanting should have started closest to existing mangroves.

As a result, 36% of the mangrove plots failed completely. The remaining 64% had at least some surviving mangroves, but survival within these plots was low. Overall, only 18.3% to 20.5% of the planted mangroves survived across the project area.

The other problem was revealed in an earlier investigation – only 5% of the US$4.4 million meant to pay communities for their work planting the trees had actually been paid. Communities also lost land, which was “grabbed” by the project to make space for tree planting.

How this affected the plan to sell carbon credits

In 2020, the certification body Verra approved a set of carbon credits from this project. Monitoring was carried out by the company Agresta. They measured the average height of the mangroves, about 70cm, and estimated that only 18% of the planted areas had failed.

This mistake happened because although they carefully measured a sample of trees and used those measurements to calculate how much carbon those trees stored, they then assumed that all the trees across the entire 10,000-hectare project area had survived and grown. Because so many trees died or never grew, the total amount of carbon stored in the trees was overestimated.

This is why, when we analysed the same areas using satellite data, we found a higher failure rate integrated in the carbon estimation. We estimated far less carbon storage: about 11,000 tons in the trees and 65,000 tons in the soil.

The difference between these estimates is around 168,000 tons of CO₂. This matters because carbon credits, which are negotiated privately at the beginning of projects, convert these numbers into financial value. At current carbon market prices, this gap could represent roughly US$2 million to US$7 million.

It became clear to us that the carbon credits were calculated based on a full 10,000 hectares of growing seedlings, even though only a small number survived. This led to an overestimation of carbon stored in the trees.

When carbon credits become ghost carbon

Polluting companies around the world are trying to find ways to offset the carbon dioxide they emit, so that they can meet the global target of net zero (zero human-caused emissions) by 2050. Carbon markets are expanding rapidly.

Nature-based solutions such as forest regeneration are increasingly important. But these projects must be credible. The only way that can happen is through reliable ecological monitoring. 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.

Local restoration initiatives could also lose the trust the public has in them, even though they have real ecological and social benefits.

In future, institutions that certify projects to sell carbon credits and those that monitor the projects must take steps to assess the failure rate. If not, they are selling “ghost carbon” and this has no benefit to people or the environment.

Julien Andrieu receives funding from CNRS and ANR.

By Julien Andrieu, Professor of Geography, Université Côte d'Azur

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