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Earth System Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/esd-2019-94
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esd-2019-94
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 02 Apr 2020

Submitted as: research article | 02 Apr 2020

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This preprint is currently under review for the journal ESD.

Biases in the albedo sensitivity to deforestation in CMIP5 models and their impacts on the associated historical Radiative Forcing

Quentin Lejeune1,2, Edouard L. Davin2, Grégory Duveiller3, Bas Crezee2, Ronny Meier2, Alessandro Cescatti3, and Sonia I. Seneviratne2 Quentin Lejeune et al.
  • 1Climate Analytics, Berlin, 10969, Germany
  • 2Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, 8092, Switzerland
  • 3European Commission Joint Research Centre, Ispra (VA), 21027, Italy

Abstract. Climate model biases in the representation of albedo variations between land cover types contribute to uncertainties on the climate impact of land cover changes since pre-industrial times, and especially on the associated Radiative Forcing. The recent publications of new observation-based datasets offer opportunities to investigate these biases and their impact on historical albedo changes in simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Conducting such an assessment is however complicated by the non-availability of albedo values for specific land cover types, as well as the limited number of simulations isolating the land use forcing in CMIP. In this study, we demonstrate the suitability of a new methodology to extract the albedo of trees and crops/grasses in standard climate model simulations. We then apply it to historical runs from 13 CMIP5 models and compare the obtained results to satellite-derived reference data. This allows us to identify substantial biases in the representation of the albedo of trees, crops/grasses, and the albedo change due to the transition between these two land cover types in the analysed models. Additionally, we reconstruct the local albedo changes induced by historical conversions between trees and crops/grasses for 15 CMIP5 models. This allows us to derive estimates of the Radiative Forcing from land cover changes since pre-industrial times ranging between 0 and −0.22 W/m2, with a mean value of −0.07 W/m2. Constraining the albedo response to transitions between trees and crops/grasses from the models with satellite-derived data leads to an increase in this range, however after excluding two models with unrealistic conversion rates from trees to crops/grasses we obtain a revised model mean estimate of −0.11 W/m2 (with individual model results between −0.04 and −0.16 W/m2). These numbers are at the lower end of the range provided by the IPCC AR5 (−0.15 ± 0.10 W/m2). The approach described in this study can be applied on other model simulations, such as those from CMIP6, especially as a diagnostic enabling the reproduction of the model evaluation part has been included in the ESMValTool v2.0.

Quentin Lejeune et al.

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Short summary
Trees are darker than crops or grasses, hence they absorb more solar radiation. Therefore land-cover changes modify the fraction of solar radiation reflected by the land surface (its albedo), with consequences for the climate. In this study we apply a new statistical method to simulations conducted with 13 recent climate models, and find that albedo variations due to land-cover changes since 1860 have led to a decrease in the net amount of energy entering the atmosphere by −0.11 W/m2 on average.
Trees are darker than crops or grasses, hence they absorb more solar radiation. Therefore...
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