Side effects of wide scale forestation could reduce carbon removal

The side effects of large-scale forestation initiatives could reduce the CO2 removal benefits by up to a third.

Researchers at the University of Leeds and the University of Sheffield used computer models, which simulate the land, ocean and atmosphere, to investigate the impact of forestation under future climate scenarios.

They found that while forestation increases atmospheric CO₂ removal, it also changes atmospheric composition and darkens the land surface, reducing its potential to tackle climate change.

The research which is published today in the journal Science, provides a new insight into the broader impacts of forestation on the Earth's climate, indicating that its positive impact is potentially smaller than previously thought.

Climate change

Carbon removal strategies, such as forestation, alongside greenhouse gas emissions reduction efforts, have been recognised by the Intergovernmental Panel on Climate Change as essential measures to mitigate the risk of dangerous future climate change.

By simulating global forest expansion with advanced computer modelling techniques, academics from the University of Sheffield, in collaboration with the Universities of Leeds and Cambridge, the National Centre for Climate Research and WWF, found that while forestation increases absorption of carbon dioxide from the atmosphere, other complex Earth System responses could together partially offset these benefits by up to a third.

Co-author of the study, Dr Daniel Grosvenor from the University of Leeds School of Earth and Environment and the Met Office, said: “What's interesting about this study is that it examines the side effects of forestation that occur via changes in atmospheric chemistry, aerosol particles and surface reflectivity.

“It shows that the cooling impact of carbon dioxide removal from an extensive, but feasible, global forest expansion could be considerably reduced due to those side effects. This would make it harder than expected to mitigate climate change and to reach the Paris agreement target.”

The study, which simulated wide scale forestation under two future scenarios – one with minimal climate change efforts and another with extensive mitigation measures alongside forestation – found that forestation leads to increased CO₂ removal.

However, it also reduces the reflectivity of the land surface (as trees are darker than grassland) and changes the atmospheric concentrations of other greenhouse gases (methane and ozone) and tiny particles called aerosols. Altogether, these indirect effects partially offset the CO₂ reduction benefits, by up to 30 per cent.

The study also found that when forestation is implemented alongside other strategies to tackle climate change, such as reducing fossil fuel emissions, the negative impacts of these indirect effects are lower. This highlights the importance of combining forestation efforts with complementary climate change mitigation strategies for more effective long-term climate action.

Dr James Weber from the University of Sheffield’s School of Biosciences and lead author of the study said: “The public are bombarded with messages about climate change, and the suggestion that you can plant trees to offset your carbon emissions is widespread. Many businesses now offer to plant a tree with a purchase, and some countries plan to expand, conserve, and restore forests.

“Trees can help tackle climate change, but we need to be careful about relying on them. We need to evaluate forestation, and other climate change mitigation strategies, in detail. This will help to identify limitations and unintended consequences so these can be minimised where possible.”

Professor Dominick Spracklen, Professor of Biosphere-Atmosphere Interactions at the University of Leeds and holder of a European Research Council Consolidator Fellowship to study the impacts of tropical deforestation on regional climate, added: "This study highlights the amazingly complex role of forests in our climate system. Through calculating how forests alter atmospheric composition, this study provides one of the most comprehensive assessments of the climate impacts of large-scale forestation.

"Stopping climate change requires urgent and rapid reductions in fossil fuel emissions alongside protecting and restoring forests. The analysis suggests that the climate benefit of forests is greater when fossil fuel emissions are rapidly reduced - a win-win outcome for the fight against climate change."