Vivek K. Arora & Alvaro Montenegro. Nature Geoscience (2011) doi:10.1038/ngeo1182 Published online 19 June 2011
Afforestation, the conversion of croplands or marginal lands into forests, results in the sequestration of carbon. As a result, afforestation is considered one of the key climate-change mitigation strategies available to governments by the United Nations1. However, forests are also less reflective than croplands, and the absorption of incoming solar radiation is greater over afforested areas. Afforestation can therefore result in net climate warming, particularly at high latitudes2, 3, 4, 5. Here, we use a comprehensive Earth system model to assess the climate-change mitigation potential of five afforestation scenarios, with afforestation carried out gradually over a 50-year period. Complete (100%) and partial (50%) afforestation of the area occupied at present by crops leads to a reduced warming of around 0.45 and 0.25 °C respectively, during the period 2081–2100. Temperature benefits associated with more realistic global afforestation efforts, where less than 50% of cropland is converted, are expected to be even smaller, indicating that afforestation is not a substitute for reduced greenhouse-gas emissions. We also show that warming reductions per unit afforested area are around three times higher in the tropics than in the boreal and northern temperate regions, suggesting that avoided deforestation and continued afforestation in the tropics are effective forest-management strategies from a climate perspective.