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Human land-use-driven reduction of forest volatiles cools global climate


Human conversion of forest ecosystems to agriculture is a major driver of global change. Conventionally, the impacts of the historical cropland expansion on Earth’s radiation balance have been quantified through two opposing effects: the release of stored carbon to the atmosphere as CO2 (warming) versus the increase in surface albedo (cooling)1. Changing forest cover has a third effect on the global radiation balance by altering emissions of biogenic volatile organic compounds (BVOCs) that control the loadings of multiple warming and cooling climate pollutants: tropospheric ozone (O3), methane (CH4) and aerosols. Although human land cover change has dominated BVOC emission variability over the past century2,3,4, the net effect on global climate has not been quantified. Here, I show that the effects of the global cropland expansion between the 1850s and 2000s on BVOC emissions and atmospheric chemistry have imposed an additional net global radiative impact of −0.11 ± 0.17 W m−2 (cooling). This magnitude is comparable to that of the surface albedo and land carbon release effects. I conclude that atmospheric chemistry must be considered in climate impact assessments of anthropogenic land cover change and in forestry for climate protection strategies.

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Figure 1: Global climate effects of historical cropland expansion (W m−2).
Figure 2: Spatial distribution of radiative forcing by climate pollutants due to historical cropland expansion (mW m−2).


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Funding for this research was provided by the Leverhulme Trust (UK). This project was supported in part by the facilities and staff of the Yale University Faculty of Arts and Sciences High Performance Computing Center. The author thanks P. A. Raymond and O. J. Schmitz.

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N.U. conceived the study, conducted the global model simulations, analysed the model output and wrote the paper.

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Correspondence to Nadine Unger.

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The author declares no competing financial interests.

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Unger, N. Human land-use-driven reduction of forest volatiles cools global climate. Nature Clim Change 4, 907–910 (2014).

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