Not every forest cools the Earth

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https://ethz.ch/en/news-and-events/eth-news/news/2026/03/not-every-forest-cools-the-earth.html

Climate sciences
Research

In the fight against the climate crisis, countries are pinning great hope in reforestation projects. In a new study, ETH Zurich researchers show that the location in which reforestation is taking place is usually more important than the number of trees planted. If forests are strategically positioned, the same cooling effect could be achieved using half the area of land.

16.03.2026 by Samuel Schlaefli, freelance author

In brief

• Climate researchers at ETH Zurich show where planting trees makes the most sense with a view to achieving the greatest possible cooling effect on the climate.
• Reforestation in tropical regions has the greatest cooling effect. Tree planting in the northern hemisphere, on the other hand, reduces the reflection of sunlight and has no effect or even contributes to global warming.
• The cooling effect on the climate will be a maximum of 0.25°C by 2100. This contribution is important, but it cannot replace the urgently required reduction in greenhouse gas emissions.

Trees are much beloved, and reforestation enjoys broad support in society, in the political sphere and, to some extent, within the scientific community. Widespread campaigns such as the Trillion Tree Campaign, launched by the United Nations Environment Programme, promise climate protection through the planting of billions of trees.

Such initiatives aim to increase the number of trees worldwide as quickly as possible to absorb climate-damaging carbon dioxide. How much land is available for reforestation globally remains a highly contested topic of debate to this day. Depending on the individual study, between 150 and 1,000 million hectares could absorb between 130 and 750 gigatonnes of carbon dioxide.

Various reforestation scenarios compared
Previous studies have generally only examined individual, often highly idealised reforestation scenarios or have worked with simplified models. In a recently published article, researchers led by Robert Jnglin Wills, Professor of Climate Dynamics at ETH Zurich, simulated and compared the climate impact of three global reforestation scenarios in a complex Earth system model for the first time.

In order to do this, they considered not only the biochemical effects of reforestation, i.e. the absorption of carbon dioxide through photosynthesis by trees, but also the biophysical effects. These include the changed albedo, i.e. the ability to reflect sunlight, as well as the effects on water evapotranspiration and the altered surface properties of reforested areas, caused for example by the presence of leaves rather than grasses.

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Fahrenbach, N.L.S., De Hertog, S.J., Jäger, F. et al. Reforestation scenarios shape global and regional temperature outcomes. Commun Earth Environ 7, 204 (2026). https://doi.org/10.1038/s43247-026-03331-3