Small temperature benefits provided by realistic afforestation efforts

Abstract

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.

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Figure 1: Effect of cropland afforestation on land carbon uptake, atmospheric CO2 and temperature.
Figure 2: Temperature effectiveness of afforestation (TEA).
Figure 3: Geographical pattern of temperature change owing to cropland afforestation and its separation into biogeophysical and biogeochemical components.

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Acknowledgements

We would like to thank G. Flato, J. Fyfe and D. Blain and the two anonymous reviewers for their helpful comments. A.M. is grateful for funding from the Natural Sciences and Engineering Research Council. We also acknowledge the work of Canadian Centre for Climate Modelling and Analysis members who developed CanESM1 including, as well as the first author, G. J. Boer, C. L. Curry, J. R. Christian, K. Zahariev, K. L. Denman, G. M. Flato, J. F. Scinocca, W. J. Merryfield, W. G. Lee and D. Yang for help with processing model output.

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V.K.A. carried out the model simulations, analysed model output, conceived the CO2-only experiments and wrote most of the paper. A.M. conceived the primary experiments, put together land-cover data and helped with the manuscript text.

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Correspondence to Vivek K. Arora.

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The authors declare no competing financial interests.

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Arora, V., Montenegro, A. Small temperature benefits provided by realistic afforestation efforts. Nature Geosci 4, 514–518 (2011). https://doi.org/10.1038/ngeo1182

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