The problem with trees-for-carbon

Programmes to encourage tree-planting have been hailed as a solution in the fight to reduce greenhouse gases and global warming. But new research casts doubt on the likely success of trading trees for carbon. 

Africa is the grassiest continent. These grasses support birds, reptiles, plants, insects and the last remaining herds of large animals that lived during the Pleistocene epoch – an invaluable asset for the continent and the world. Africa’s grasslands were the cradle of our ancestors and today are home to more than 300 million people.

But these open landscapes could be transformed if trees-for-carbon projects inappropriately target them for ‘restoration’ according to University of Cape Town (UCT) Emeritus Professor William Bond, lead author of new research on the topic.

“We challenge the popular view that planting trees is a credible way of slowing global warming,” says Bond.

The suggestion that Africa’s grasslands might be transformed to forestry plantations is not theoretical: the Bonn Challenge is an example project that proposes to ‘restore’ forest across 3.5 million square kilometres – an area covered by Europe’s 10 largest countries – by 2030. Much of the land it’s targeting lies in Africa.

Bond collaborated with Dr Nicola Stevens and Professor Guy Midgley from Stellenbosch University, and Dr Caroline Lehmann of the Royal Botanic Garden Edinburgh to look critically at such trees-for-carbon projects that propose to forest landscapes to capture carbon dioxide from the atmosphere.

“We challenge the popular view that planting trees is a credible way of slowing global warming.”

“We found that the benefits of afforestation for reducing atmospheric carbon are paltry,” he says, “while the costs to Africa in lost land for agriculture, livestock, conservation, and in managing vast plantations will have to be borne for the foreseeable future.”

Restoration or distraction?

The researchers’ focus was the ambitious AFR100 plan to plant 100 million hectares of trees in Africa by 2030 – and an offshoot of the Bonn Challenge. That vast area – more than four times the size of Britain – is the subject of a pledge by 28 African countries.

Mozambique, for example, has committed to planting 1 million hectares, South Africa to 3.6 million hectares. Cameroon’s pledge requires converting a quarter of the country to plantations, and Nigeria’s requires almost one-third.

To assess the impact of AFR100, the researchers looked at how much it would cost to plant enough trees to balance out one year’s growth in atmospheric carbon dioxide at current rates of emissions.

Their results suggest, that, far from offering hope, such trees-for-carbon projects may be detrimental for grasslands in Africa and distract attention from the more urgent problem: lessening emissions from fossil fuels.

Trees for carbon

We know that the amount of carbon dioxide in the atmosphere is increasing by about 4.7 gigatonnes per year (1 gigatonne = 1 000 000 000 tonnes).

“We found that the benefits of afforestation for reducing atmospheric carbon are paltry.”

Assuming it would cost USD10 to sequester a tonne of carbon – a “very conservative” rate according to Bond – the total amount required to balance out this yearly increase would be USD47 billion.

“The World Bank’s contribution of a billion dollars [to the Bonn Challenge] is less than 0.5% of what would be needed over the next 10 years,” explains Bond. “And that billion dollars, spread over 100 million hectares of Africa, works out at USD10 per hectare – a bargain for the industrial countries of the world.”

The researchers’ calculations show that should Africa reach its target of foresting 100 million hectares, only 2.7% less carbon dioxide would enter the atmosphere each year.

“If that seems small, consider that the coal used in the industrial revolution took 400 million years to accumulate,” adds Stevens, a co-author on the study. “Can you really expect to stuff it all back again in the next few decades?”

The team concludes that converting Africa’s grassy landscapes to tree plantations will not only do little to reduce greenhouse gases but that the funding for the programme is a small fraction of what’s needed. African countries could also be locking themselves into plantation forestry for decades at the expense of other industries, such as food crops, livestock farming and conservation.

“What concerns us is that the trees-for-carbon projects distract us from the real issue: the urgent and immediate need to reduce carbon emissions, especially by reducing fossil fuel use.”

Furthermore, the amount of carbon that tree plantations store depends on intensive management: supressing fires, felling trees and then storing the carbon, and replanting every decade or two for the foreseeable future. This is something African countries would need to deliver on.

Exporting emission problems

The researchers also raise the point that there isn’t even scientific agreement on whether such tree plantations will warm or cool the planet. Overall, trees canopies are darker than grassy vegetation, and thus absorb more sunlight and heat – this could lead to warming.

Advocates of trees-for-carbon projects have not taken this into account.  

“What concerns us is that the trees-for-carbon projects distract us from the real issue: the urgent and immediate need to reduce carbon emissions, especially by reducing fossil fuel use,” adds Bond.

Indeed, trees-for-carbon projects can be seen as a way for industrialised countries – the major sources of greenhouse gases – to export fossil-fuel emission problems to Africa.

The researchers highlight that they strongly endorse planting trees to restore destroyed forests and in urban areas for shade and enjoyment. They also support retaining the intact forests that remain. But that trees-for-carbon projects are based on wrong assumptions.

“For tree planting to be positive, it needs to be the right trees in the right places,” says Lehmann.

A better way of supporting Africa’s transition to a warmer future might be to promote energy efficient cities in this rapidly urbanising continent so that Africa follows a less carbon-intensive trajectory without destroying its grassy landscapes.

• Bond WJ et al. (2019) The Trouble with Trees: Afforestation Plans for Africa, Trends in Ecology and Evolution.