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. 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.