Climatic Impact of Global-Scale Deforestation: Radiative versus Nonradiative Processes. Davin, E. L. & de Noblet-Ducoudré , N. Journal of Climate, 23(1):97–112, January, 2010.
doi  abstract   bibtex   
A fully coupled land-ocean-atmosphere GCM is used to explore the biogeophysical impact of large-scale deforestation on surface climate. By analyzing the model sensitivity to global-scale replacement of forests by grassland, it is shown that the surface albedo increase owing to deforestation has a cooling effect of -1.36 K globally. On the other hand, forest removal decreases evapotranspiration efficiency and decreases surface roughness, both leading to a global surface warming of 0.24 and 0.29 K, respectively. The net biogeophysical impact of deforestation results from the competition between these effects. Globally, the albedo effect is dominant because of its wider-scale impact, and the net biogeophysical impact of deforestation is thus a cooling of -1 K. Over land, the balance between the different processes varies with latitude. In temperate and boreal zones of the Northern Hemisphere the albedo effect is stronger and deforestation thus induces a cooling. Conversely, in the tropics the net impact of deforestation is a warming, because evapotranspiration efficiency and surface roughness provide the dominant influence. The authors also explore the importance of the ocean coupling in shaping the climate response to deforestation. First, the temperature over ocean responds to the land cover perturbation. Second, even the temperature change over land is greatly affected by the ocean coupling. By assuming fixed oceanic conditions, the net effect of deforestation, averaged over all land areas, is a warming, whereas taking into account the coupling with the ocean leads, on the contrary, to a net land cooling. Furthermore, it is shown that the main parameter involved in the coupling with the ocean is surface albedo. Indeed, a change in albedo modifies temperature and humidity in the whole troposphere, thus enabling the initially land-confined perturbation to be transferred to the ocean. Finally, the radiative forcing framework is discussed in the context of land cover change impact on climate. The experiments herein illustrate that deforestation triggers two opposite types of forcingmechanisms-radiative forcing (owing to surface albedo change) and nonradiative forcing (owing to change in evapotranspiration efficiency and surface roughness)-that exhibit a similar magnitude globally. However, when applying the radiative forcing concept, nonradiative processes are ignored, which may lead to a misrepresentation of land cover change impact on climate. [Excerpt: Conclusions] In this study we addressed the biogeophysical impact of deforestation with a fully coupled land-ocean-atmosphere GCM. We contrasted the climate of a maximally forested earth with the climate resulting from the replacement of forest by grass. Our experimental design allows us to separate the respective roles of surface albedo, evapotranspiration efficiency, and surface roughness in shaping the net biogeophysical effect of deforestation. Whereas our main focus here was on the energy budget and surface temperature, investigations of the response of the hydrological cycle will be conducted in the future. [\n] Increase in surface albedo owing to complete deforestation has a cooling effect on climate (-1.36 K globally). On the other hand, forest removal decreases evapotranspiration efficiency and surface roughness, which warms surface climate (respectively, by 0.24 and 0.29 K globally). The magnitude of these different effects varies regionally. The cooling effect due to albedo change is stronger at high latitudes and affects both land and ocean. Conversely, the warming effect from change in evapotranspiration efficiency and surface roughness is stronger at low latitudes and does not affect the oceans. [\n] The net biogeophysical impact of deforestation results from the competition between these effects. Globally, the albedo effect is dominant and the net biogeophysical impact of deforestation is a cooling of -1 K. This is mainly because the albedo effect spreads over the ocean, whereas the other effects do not. On continents, however, the balance between the different processes changes with latitude. In temperate and boreal zones of the Northern Hemisphere the albedo effect is stronger and deforestation thus induces a cooling, as has already been noticed in previous studies (e.g., Betts 2001; Bounoua et al. 2002). Conversely, in the tropics the net impact of deforestation is a warming because evapotranspiration efficiency and surface roughness provide the dominant influence in these regions. [\n] This study also highlights the importance of the coupling with the ocean. Up to now, most of our knowledge concerning the impact of land cover change on climate comes from atmospheric models not coupled to an ocean model but instead assuming fixed oceanic conditions (e.g., Dickinson and Henderson-Sellers 1988; Nobre et al. 1991; Bonan 1997; Lean and Rowntree 1997; Chase et al. 2000; Gedney and Valdes 2000; Betts 2001; Bounoua et al. 2002; DeFries et al. 2002; Voldoire 2006). Implicitly, this assumption was justified be the fact that the perturbation owing to land cover change is applied to land and not to the ocean. However, our experiments show that taking into account the coupling with the ocean greatly affect the simulated response to deforestation. First, we noted that the ocean surface responds to deforestation by a cooling. Second, even the temperature change over land is strongly affected by the ocean coupling. By not taking into account the coupling with the ocean we would have concluded that the net effect of deforestation, averaged over all land areas, is a warming. By accounting for the ocean coupling, this net effect is of opposite sign. We also further demonstrated that the main parameter involved in the coupling with the ocean is surface albedo. This is because change in albedo modifies temperature and humidity in the whole troposphere, thus enabling the initially land-confined perturbation to be transferred to the ocean. [\n] Finally, the results presented here give some insight concerning the nature of the forcing owing to land cover change. Supporting earlier hypothesis (Pielke et al. 2002; NRC 2005; Davin et al. 2007), we showed that deforestation involves two opposite types of forcing mechanisms: a radiative forcing (owing to surface albedo change) and a nonradiative forcing (owing to change in evapotranspiration efficiency and surface roughness). We quantified the relative importance of these opposite forcings in the context of our complete deforestation experiments and found that, globally, they are of similar magnitude. This result highlights the limitation of the classical radiative forcing framework in which equilibrium temperature change is viewed as a response to a radiative forcing perturbation. Land cover change can also affect equilibrium temperature through nonradiative processes. Historical deforestation took place mostly in temperate regions, and therefore radiative forcing was roughly acceptable in quantifying its effect. Future deforestation, however, is expected to take place in the tropics where nonradiative effects are dominant. Hence, using the radiative forcing framework in the context of future land cover change may lead to a misrepresentation of its impact on climate.
@article{davinClimaticImpactGlobalscale2010,
  title = {Climatic Impact of Global-Scale Deforestation: Radiative versus Nonradiative Processes},
  author = {Davin, Edouard L. and {de Noblet-Ducoudr{\'e}}, Nathalie},
  year = {2010},
  month = jan,
  volume = {23},
  pages = {97--112},
  issn = {1520-0442},
  doi = {10.1175/2009jcli3102.1},
  abstract = {A fully coupled land-ocean-atmosphere GCM is used to explore the biogeophysical impact of large-scale deforestation on surface climate. By analyzing the model sensitivity to global-scale replacement of forests by grassland, it is shown that the surface albedo increase owing to deforestation has a cooling effect of -1.36 K globally. On the other hand, forest removal decreases evapotranspiration efficiency and decreases surface roughness, both leading to a global surface warming of 0.24 and 0.29 K, respectively. The net biogeophysical impact of deforestation results from the competition between these effects. Globally, the albedo effect is dominant because of its wider-scale impact, and the net biogeophysical impact of deforestation is thus a cooling of -1 K. Over land, the balance between the different processes varies with latitude. In temperate and boreal zones of the Northern Hemisphere the albedo effect is stronger and deforestation thus induces a cooling. Conversely, in the tropics the net impact of deforestation is a warming, because evapotranspiration efficiency and surface roughness provide the dominant influence. The authors also explore the importance of the ocean coupling in shaping the climate response to deforestation. First, the temperature over ocean responds to the land cover perturbation. Second, even the temperature change over land is greatly affected by the ocean coupling. By assuming fixed oceanic conditions, the net effect of deforestation, averaged over all land areas, is a warming, whereas taking into account the coupling with the ocean leads, on the contrary, to a net land cooling. Furthermore, it is shown that the main parameter involved in the coupling with the ocean is surface albedo. Indeed, a change in albedo modifies temperature and humidity in the whole troposphere, thus enabling the initially land-confined perturbation to be transferred to the ocean. Finally, the radiative forcing framework is discussed in the context of land cover change impact on climate. The experiments herein illustrate that deforestation triggers two opposite types of forcingmechanisms-radiative forcing (owing to surface albedo change) and nonradiative forcing (owing to change in evapotranspiration efficiency and surface roughness)-that exhibit a similar magnitude globally. However, when applying the radiative forcing concept, nonradiative processes are ignored, which may lead to a misrepresentation of land cover change impact on climate.

[Excerpt: Conclusions]

In this study we addressed the biogeophysical impact of deforestation with a fully coupled land-ocean-atmosphere GCM. We contrasted the climate of a maximally forested earth with the climate resulting from the replacement of forest by grass. Our experimental design allows us to separate the respective roles of surface albedo, evapotranspiration efficiency, and surface roughness in shaping the net biogeophysical effect of deforestation. Whereas our main focus here was on the energy budget and surface temperature, investigations of the response of the hydrological cycle will be conducted in the future.

[\textbackslash n] Increase in surface albedo owing to complete deforestation has a cooling effect on climate (-1.36 K globally). On the other hand, forest removal decreases evapotranspiration efficiency and surface roughness, which warms surface climate (respectively, by 0.24 and 0.29 K globally). The magnitude of these different effects varies regionally. The cooling effect due to albedo change is stronger at high latitudes and affects both land and ocean. Conversely, the warming effect from change in evapotranspiration efficiency and surface roughness is stronger at low latitudes and does not affect the oceans.

[\textbackslash n] The net biogeophysical impact of deforestation results from the competition between these effects. Globally, the albedo effect is dominant and the net biogeophysical impact of deforestation is a cooling of -1 K. This is mainly because the albedo effect spreads over the ocean, whereas the other effects do not. On continents, however, the balance between the different processes changes with latitude. In temperate and boreal zones of the Northern Hemisphere the albedo effect is stronger and deforestation thus induces a cooling, as has already been noticed in previous studies (e.g., Betts 2001; Bounoua et al. 2002). Conversely, in the tropics the net impact of deforestation is a warming because evapotranspiration efficiency and surface roughness provide the dominant influence in these regions.

[\textbackslash n] This study also highlights the importance of the coupling with the ocean. Up to now, most of our knowledge concerning the impact of land cover change on climate comes from atmospheric models not coupled to an ocean model but instead assuming fixed oceanic conditions (e.g., Dickinson and Henderson-Sellers 1988; Nobre et al. 1991; Bonan 1997; Lean and Rowntree 1997; Chase et al. 2000; Gedney and Valdes 2000; Betts 2001; Bounoua et al. 2002; DeFries et al. 2002; Voldoire 2006). Implicitly, this assumption was justified be the fact that the perturbation owing to land cover change is applied to land and not to the ocean. However, our experiments show that taking into account the coupling with the ocean greatly affect the simulated response to deforestation. First, we noted that the ocean surface responds to deforestation by a cooling. Second, even the temperature change over land is strongly affected by the ocean coupling. By not taking into account the coupling with the ocean we would have concluded that the net effect of deforestation, averaged over all land areas, is a warming. By accounting for the ocean coupling, this net effect is of opposite sign. We also further demonstrated that the main parameter involved in the coupling with the ocean is surface albedo. This is because change in albedo modifies temperature and humidity in the whole troposphere, thus enabling the initially land-confined perturbation to be transferred to the ocean.

[\textbackslash n] Finally, the results presented here give some insight concerning the nature of the forcing owing to land cover change. Supporting earlier hypothesis (Pielke et al. 2002; NRC 2005; Davin et al. 2007), we showed that deforestation involves two opposite types of forcing mechanisms: a radiative forcing (owing to surface albedo change) and a nonradiative forcing (owing to change in evapotranspiration efficiency and surface roughness). We quantified the relative importance of these opposite forcings in the context of our complete deforestation experiments and found that, globally, they are of similar magnitude. This result highlights the limitation of the classical radiative forcing framework in which equilibrium temperature change is viewed as a response to a radiative forcing perturbation. Land cover change can also affect equilibrium temperature through nonradiative processes. Historical deforestation took place mostly in temperate regions, and therefore radiative forcing was roughly acceptable in quantifying its effect. Future deforestation, however, is expected to take place in the tropics where nonradiative effects are dominant. Hence, using the radiative forcing framework in the context of future land cover change may lead to a misrepresentation of its impact on climate.},
  journal = {Journal of Climate},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-6519442,~to-add-doi-URL,albedo,boreal-forests,climate,complexity,deforestation,evapotranspiration,feedback,forest-resources,global-climate-models,humidity,land-cover,large-vs-wide-scale,oceans,off-site-effects,surface-roughness,temperate-forests,temperature,trade-offs,tropical-forests,wide-scale},
  lccn = {INRMM-MiD:c-6519442},
  number = {1}
}
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