February 22, 2024

expert reaction to impact of large-scale forestation on CO2 removal benefits

A study published in Science looks at the impact of large-scale forestation on CO2 removal benefits. 

Dr Cat Scott, School of Earth and Environment, University of Leeds, said:

“This new paper is a valuable contribution to our understanding around the climate impacts of future changes to forest area but we must interpret the headline number carefully. The one-third ‘offset’ figure comes from comparing a future scenario in which large areas of tropical rainforest have been deforested to one in which they (and additional) forests are present. As the authors show, when looking at a more sustainable future scenario in which there is less change in forest area, and it occurs in different regions, a smaller proportion (14%) of the CO2 effect is ‘offset’ by the other effects considered here. 

“It’s well known that changes to forest area affect more than just carbon storage and the relative importance of these effects differs depending on where in the world the forest is. When modelling these scenarios, we’re still very often only able to look at only part of this picture. As the authors acknowledge, their experiments don’t include the cooling effect that forests provide through evapotranspiration – this makes a big difference to the overall climate impact of forests in tropical regions.

“Afforestation and reforestation are certainly not get out of jail free cards in terms of climate change mitigation but we shouldn’t be putting down our spades just yet. As a society we are going to need every tool available to us to limit climate warming and for me what this study really highlights is that to maximise the climate benefits of any future reforestation we must limit our emissions of both greenhouse gases and other air pollutants.”

Prof Yadvinder Malhi, Professor of Ecosystem Science, University of Oxford, said:

“Increasing tree cover can absorb carbon that helps mitigate climate change but can also make the land surface darker and change atmospheric chemistry in ways that change greenhouse gases and atmospheric aerosols. This model-based study brings all these factors together to look at the net effect of reforestation on climate. It finds that the climate benefits of reforestation effects are weaker than when considering carbon alone, especially in a much warmer world where the atmospheric chemistry effects get stronger.

“Some of the details of the study can be questioned – much of the proposed modelled afforestation is in natural savannas where such increase in tree cover is not ecologically appropriate and would be very vulnerable to wildfires, and we still don’t understand well how tree cover increases might possibly increase daytime cloud cover, which reflects sunlight and have a cooling effect. But by adding and modelling atmospheric chemistry this study is a step forward in our understanding and provides new analysis that reinforces what we already know to be true: that forest restoration has climate change mitigation benefits, but these benefits become weaker and more uncertain the warmer the world becomes.

“Nature-based solutions can only help us tackle climate in tandem with strong reductions in fossil fuel carbon dioxide emissions – tree planting must never be seen as an alternative to leaving fossil fuels in the ground.”

Prof Cédric John, Head of Data Science for the Environment and Sustainability, Queen Mary University of London, said:

“This study presents the first quantification of forestation’s impact on CO2 levels, offering a thorough and well-executed analysis. It concludes that reforestation, while beneficial, is not as effective as a carbon sink compared to preventing deforestation. The study highlights that the carbon storage benefits of forests are partially negated by two previously unaccounted factors: forests’ lower albedo due to their darker appearance compared to plains, leading to less heat reflection back into space, and the release of water and organic molecules by forests, which can act as greenhouse gases.

“Despite these drawbacks, forestation at lower latitude can still contribute positively to the carbon budget and ecosystem health. However, the authors note that forestation alone cannot meet our atmospheric carbon reduction goals and acknowledge their model’s limitations, including potential complex feedback mechanisms not accounted for. The study is insightful, particularly in exploring a climate mitigation strategy, but leaves room for further exploration on the long-term benefits in terms of carbon budget of forestation over several centuries.”

Dr Phil Williamson, Honorary Associate Professor, University of East Anglia, said:

“Scientists have been aware for over a decade that forests can cause warming as well as cooling, as a result of the complex interactions between tree canopies, atmospheric chemistry and heat energy exchanges.  Nevertheless, they have continued to use relatively simplistic data on carbon uptake and storage for the purpose of estimating the climatic benefits of forestation.  This new analysis shows the scale of the errors likely to be involved: using sophisticated Earth system models, they found that the carbon offset value of tree-planting to have been greatly overestimated, by as much as 50 per cent in non-tropical regions.  The authors emphasized that increasing tree cover is still worthwhile, particularly through forest restoration; however, they found that avoiding deforestation was a much more efficient way of achieving climate mitigation.

“There are two further important implications of this study.  Firstly, governments need to reduce reliance on future carbon removal using forestation in their net-zero climate planning, with a corresponding increase in urgency of rapidly reducing greenhouse gas emissions, phasing out fossil fuel use and being bold in their green investment plans.  Second, stronger regulation is needed to avoid green-washing when using forestry – or other CO2 removal processes – for carbon credits, with a switch from carbon accounting to climate accounting to include the full range of potential side effects.  Ideally, the true ‘additionality’ of climate benefits needs to be determined on a site-specific basis, erring on the side of caution if that is not possible – to reduce the risk of dependence on risky or unsustainable approaches.”

‘Chemistry-albedo feedbacks offset up to a third of forestation’s CO2 removal benefits’ by James Weber et al. was published in Science at 7pm UK time Thursday 22 February 2024.

DOI: science.org/doi/10.1126/science.adg6196

Declared interests

Yadvinder Mahli: No interests

Cédric John: no interests linked to this paper.

Phil Williamson: No conflict of interests.

Cat Scott: My role at the University of Leeds is funded by the Natural Environment Research Council (NERC) and I act as Scientific Lead for the United Bank of Carbon.