Carbon Storage versus Albedo Change: Radiative Forcing of Forest Expansion in Temperate Mountainous Regions of Switzerland. Schwaab, J., Bavay, M., Davin, E., Hagedorn, F., Hüsler, F., Lehning, M., Schneebeli, M., Thürig, E., & Bebi, P. 12(2):467–487.
Carbon Storage versus Albedo Change: Radiative Forcing of Forest Expansion in Temperate Mountainous Regions of Switzerland [link]Paper  doi  abstract   bibtex   
In this study, we assess the climate mitigation potential from afforestation in a mountainous snow-rich region (Switzerland) with strongly varying environmental conditions. Using radiative forcing calculations, we quantify both the carbon sequestration potential and the effect of albedo change at high resolution. We calculate the albedo radiative forcing based on remotely sensed data sets of albedo, global radiation and snow cover. Carbon sequestration is estimated from changes in carbon stocks based on national inventories. We first estimate the spatial pattern of radiative forcing (RF) across Switzerland assuming homogeneous transitions from open land to forest. This highlights where forest expansion still exhibits climatic benefits when including the radiative forcing of albedo change. Second, given that forest expansion is currently the dominant land-use change process in the Swiss Alps, we calculate the radiative forcing that occurred between 1985 and 1997. Our results show that the net RF of forest expansion ranges from -24 W m-2 at low elevations of the northern Prealps to 2 W m-2 at high elevations of the Central Alps. The albedo RF increases with increasing altitude, which offsets the CO2 RF at high elevations with long snow-covered periods, high global radiation and low carbon sequestration. Albedo RF is particularly relevant during transitions from open land to open forest but not in later stages of forest development. Between 1985 and 1997, when overall forest expansion in Switzerland was approximately 4\,%, the albedo RF offset the CO2 RF by an average of 40\,%. We conclude that the albedo RF should be considered at an appropriately high resolution when estimating the climatic effect of forestation in temperate mountainous regions. [Excerpt: Conclusions] In the temperate mountainous regions of Switzerland, the net RF associated with changes in forest cover varies greatly on small spatial scales. At low elevations, with low to moderate snow cover, RF is strongly negative due to a dominance of CO2 RF. At high elevations in continental regions with persistent snow cover, a very high global radiation, low carbon sequestration and low albedos of mostly evergreen tree species, RF can be positive. As a consequence, both clearly negative and positive values of RF can be found within a horizontal distance of 5 km in alpine valleys. Therefore, the climatic benefits of changes in forest cover can only be properly assessed using data at a high spatial resolution. [\n] Our results indicate that it is very important to include albedo RF when estimating the impact on climate of changes in forest cover. Maps of RF, such as those produced in this study, indicate where climatic benefits from changes in forest cover can be expected and where not. In the Swiss Alps, the relevance of albedo RF is especially high because most transitions from open land to forest occur in regions where albedo RF causes a strong offset of CO2 RF. Practitioners and politicians who need information about ecosystem services on local and regional scales should take into account that RF in the Swiss Alps mainly depends on the persistence of snow cover and the potential for carbon sequestration. Moreover, late successional forest-cover changes from open forests to closed forests are more beneficial for climate than early successional changes. [\n] Our results could be improved if changes in evapotranspiration, surface roughness, aerosols and gases other than CO2 were included. To determine the impacts of RF better, however, further advances in climate modelling are necessary. A promising approach could thus be the coupling of regional climate models with global climate models. Regional models are able to simulate all the effects of changes in land use on climate (including evapotranspiration, surface roughness and so on) on a small scale. Coupling regional models with global models allows the integration of feedbacks with the global circulation. This could help to close the gap between RF and temperature changes, and thus answer the question about where temperature changes caused by RF can be expected and how much change is likely.
@article{schwaabCarbonStorageAlbedo2015,
  title = {Carbon Storage versus Albedo Change: Radiative Forcing of Forest Expansion in Temperate Mountainous Regions of {{Switzerland}}},
  author = {Schwaab, J. and Bavay, M. and Davin, E. and Hagedorn, F. and Hüsler, F. and Lehning, M. and Schneebeli, M. and Thürig, E. and Bebi, P.},
  date = {2015-01},
  journaltitle = {Biogeosciences},
  volume = {12},
  pages = {467--487},
  issn = {1726-4189},
  doi = {10.5194/bg-12-467-2015},
  url = {https://doi.org/10.5194/bg-12-467-2015},
  abstract = {In this study, we assess the climate mitigation potential from afforestation in a mountainous snow-rich region (Switzerland) with strongly varying environmental conditions. Using radiative forcing calculations, we quantify both the carbon sequestration potential and the effect of albedo change at high resolution. We calculate the albedo radiative forcing based on remotely sensed data sets of albedo, global radiation and snow cover. Carbon sequestration is estimated from changes in carbon stocks based on national inventories. We first estimate the spatial pattern of radiative forcing (RF) across Switzerland assuming homogeneous transitions from open land to forest. This highlights where forest expansion still exhibits climatic benefits when including the radiative forcing of albedo change. Second, given that forest expansion is currently the dominant land-use change process in the Swiss Alps, we calculate the radiative forcing that occurred between 1985 and 1997. Our results show that the net RF of forest expansion ranges from -24 W m-2 at low elevations of the northern Prealps to 2 W m-2 at high elevations of the Central Alps. The albedo RF increases with increasing altitude, which offsets the CO2 RF at high elevations with long snow-covered periods, high global radiation and low carbon sequestration. Albedo RF is particularly relevant during transitions from open land to open forest but not in later stages of forest development. Between 1985 and 1997, when overall forest expansion in Switzerland was approximately 4\,\%, the albedo RF offset the CO2 RF by an average of 40\,\%. We conclude that the albedo RF should be considered at an appropriately high resolution when estimating the climatic effect of forestation in temperate mountainous regions.

[Excerpt: Conclusions] In the temperate mountainous regions of Switzerland, the net RF associated with changes in forest cover varies greatly on small spatial scales. At low elevations, with low to moderate snow cover, RF is strongly negative due to a dominance of CO2 RF. At high elevations in continental regions with persistent snow cover, a very high global radiation, low carbon sequestration and low albedos of mostly evergreen tree species, RF can be positive. As a consequence, both clearly negative and positive values of RF can be found within a horizontal distance of 5 km in alpine valleys. Therefore, the climatic benefits of changes in forest cover can only be properly assessed using data at a high spatial resolution.

[\textbackslash n] Our results indicate that it is very important to include albedo RF when estimating the impact on climate of changes in forest cover. Maps of RF, such as those produced in this study, indicate where climatic benefits from changes in forest cover can be expected and where not. In the Swiss Alps, the relevance of albedo RF is especially high because most transitions from open land to forest occur in regions where albedo RF causes a strong offset of CO2 RF. Practitioners and politicians who need information about ecosystem services on local and regional scales should take into account that RF in the Swiss Alps mainly depends on the persistence of snow cover and the potential for carbon sequestration. Moreover, late successional forest-cover changes from open forests to closed forests are more beneficial for climate than early successional changes.

[\textbackslash n] Our results could be improved if changes in evapotranspiration, surface roughness, aerosols and gases other than CO2 were included. To determine the impacts of RF better, however, further advances in climate modelling are necessary. A promising approach could thus be the coupling of regional climate models with global climate models. Regional models are able to simulate all the effects of changes in land use on climate (including evapotranspiration, surface roughness and so on) on a small scale. Coupling regional models with global models allows the integration of feedbacks with the global circulation. This could help to close the gap between RF and temperature changes, and thus answer the question about where temperature changes caused by RF can be expected and how much change is likely.},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13928362,~to-add-doi-URL,albedo,carbon-stock,climate,conifers,feedback,forest-management,forest-resources,snow,sudden-transition,switzerland,temperate-forests,temperate-mountain-system,temperature,trade-offs},
  number = {2}
}
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