Offset of the Potential Carbon Sink from Boreal Forestation by Decreases in Surface Albedo. Betts, R. A. 408(6809):187–190.
Offset of the Potential Carbon Sink from Boreal Forestation by Decreases in Surface Albedo [link]Paper  doi  abstract   bibtex   
Carbon uptake by forestation is one method proposed1 to reduce net carbon dioxide emissions to the atmosphere and so limit the radiative forcing of climate change2. But the overall impact of forestation on climate will also depend on other effects associated with the creation of new forests. In particular, the albedo of a forested landscape is generally lower than that of cultivated land, especially when snow is lying3, 4, 5, 6, 7, 8, 9, and decreasing albedo exerts a positive radiative forcing on climate. Here I simulate the radiative forcings associated with changes in surface albedo as a result of forestation in temperate and boreal forest areas, and translate these forcings into equivalent changes in local carbon stock for comparison with estimated carbon sequestration potentials10, 11, 12. I suggest that in many boreal forest areas, the positive forcing induced by decreases in albedo can offset the negative forcing that is expected from carbon sequestration. Some high-latitude forestation activities may therefore increase climate change, rather than mitigating it as intended. [Excerpt] [] [...] In the boreal forest regions, the replacement of crops with carbon-sequestering forests induced a positive albedo forcing equivalent to the emission of 50-140 t C ha -1 (Fig. 2a). The greatest effect was in eastern Siberia, where the EESF was 90-140 t C ha -1. The carbon sequestration potential (SP) in the former Soviet Union is estimated to be 80-120 t C ha-1 (Table 2), so if this is appropriate to eastern Siberia then forestation in this region could exert a net warming influence (Fig. 2b). In the intensive agricultural regions of western Russia, the EESF of 80-90 t C ha-1 could again largely counteract or overcome the estimated carbon sink (Fig. 2). Similarly in Canada, the EESF was 60-110 t C ha -1, which significantly outweighs the mean sink potential of 60 t C ha -1 estimated for most of this region (Fig. 2, Table 2). This suggests that in many boreal forest areas, forestation could therefore exert a net positive radiative forcing of climate, rather than a negative forcing as intended. [] [...] [] In temperate regions, the EESF ranged from 20 to 80 t C ha -1 (Fig. 2a). The smallest effects were in western Europe, the western USA and southern China, where infrequent snow cover and/or low soil albedo17 suppressed the albedo change when the vegetation was modified. The effects were largest where snow lies for a significant part of the year; the EESF reached 80 t C ha -1 in the northern USA, and exceeded 100 t C ha -1 in the Rocky Mountains, northern China and the fringes of the Tibetan plateau. [...] Despite such uncertainties, these results show that although temperate forestation should still exert a net negative radiative forcing, the effect cannot be adequately quantified by simple carbon accounting. [] [...] [] Here I have considered forestation under present-day conditions, but the effects of future CO2 rise and climate change are likely to affect the magnitude of both radiative forcing terms, due to dependencies on time-varying quantities such as the atmospheric CO2 concentration, snow extent and vegetation structure and leafiness. As the atmospheric CO2 concentration increases, CO2 fertilization is likely to increase carbon uptake22 so the magnitude of the negative sequestration forcing should therefore increase, although associated climate changes may exert additional positive or negative effects on sequestration. Warmer temperatures may reduce the extent of snow cover23, but the leaf area index (LAI) of potential vegetation may increase24, 25, so the albedo forcing could either increase or decrease. The effect of vegetation on surface albedo is not necessarily proportional to biomass, so the net contribution to radiative forcing may not evolve linearly throughout a forest's development; albedo depends on canopy density and architecture, and can become low rapidly, whereas carbon sequestration depends largely on woody biomass which is more gradually accumulated. Other contributions to forcing may also require consideration; for example, the longwave radiation budget could be affected by modified surface emissivity25, although the sign of such changes is uncertain25, 26. [] The work I report here has focused on perturbations to the Earth's radiation budget, which is the fundamental driver of the climate system. Forestation may also influence the climate by modifying the fluxes of heat, moisture and momentum between the land surface and atmosphere. Whereas boreal forests warm their local climate through reduced albedo, tropical forests tend to cool and moisten their local climates by greatly enhancing evaporation. Both may also influence distant regional climates via the atmospheric circulation9, 27. Assessment of the effect of forestation on climate at a given time in the future will require simulations with a climate model that incorporates vegetation dynamics25, 28 and other atmospheric, terrestrial and oceanic components of the carbon cycle28, in which forest growth occurs at appropriate rates in relation to changes in atmospheric CO 2 and snow cover. Nevertheless, my results suggest that high-latitude forestation would exert a positive radiative forcing through reduced albedo that in many places could outweigh the negative forcing through carbon sequestration. If afforestation and reforestation are required to decrease radiative forcing rather than simply to reduce net CO2 emissions, then changes in surface albedo must also be considered.
@article{bettsOffsetPotentialCarbon2000,
  title = {Offset of the Potential Carbon Sink from Boreal Forestation by Decreases in Surface Albedo},
  author = {Betts, Richard A.},
  date = {2000-11},
  journaltitle = {Nature},
  volume = {408},
  pages = {187--190},
  issn = {0028-0836},
  doi = {10.1038/35041545},
  url = {https://doi.org/10.1038/35041545},
  abstract = {Carbon uptake by forestation is one method proposed1 to reduce net carbon dioxide emissions to the atmosphere and so limit the radiative forcing of climate change2. But the overall impact of forestation on climate will also depend on other effects associated with the creation of new forests. In particular, the albedo of a forested landscape is generally lower than that of cultivated land, especially when snow is lying3, 4, 5, 6, 7, 8, 9, and decreasing albedo exerts a positive radiative forcing on climate. Here I simulate the radiative forcings associated with changes in surface albedo as a result of forestation in temperate and boreal forest areas, and translate these forcings into equivalent changes in local carbon stock for comparison with estimated carbon sequestration potentials10, 11, 12. I suggest that in many boreal forest areas, the positive forcing induced by decreases in albedo can offset the negative forcing that is expected from carbon sequestration. Some high-latitude forestation activities may therefore increase climate change, rather than mitigating it as intended.

[Excerpt]

[] [...]

In the boreal forest regions, the replacement of crops with carbon-sequestering forests induced a positive albedo forcing equivalent to the emission of 50-140 t C ha -1 (Fig. 2a). The greatest effect was in eastern Siberia, where the EESF was 90-140 t C ha -1. The carbon sequestration potential (SP) in the former Soviet Union is estimated to be 80-120 t C ha-1 (Table 2), so if this is appropriate to eastern Siberia then forestation in this region could exert a net warming influence (Fig. 2b). In the intensive agricultural regions of western Russia, the EESF of 80-90 t C ha-1 could again largely counteract or overcome the estimated carbon sink (Fig. 2). Similarly in Canada, the EESF was 60-110 t C ha -1, which significantly outweighs the mean sink potential of 60 t C ha -1 estimated for most of this region (Fig. 2, Table 2). This suggests that in many boreal forest areas, forestation could therefore exert a net positive radiative forcing of climate, rather than a negative forcing as intended.

[] [...]

[] In temperate regions, the EESF ranged from 20 to 80 t C ha -1 (Fig. 2a). The smallest effects were in western Europe, the western USA and southern China, where infrequent snow cover and/or low soil albedo17 suppressed the albedo change when the vegetation was modified. The effects were largest where snow lies for a significant part of the year; the EESF reached 80 t C ha -1 in the northern USA, and exceeded 100 t C ha -1 in the Rocky Mountains, northern China and the fringes of the Tibetan plateau. [...] Despite such uncertainties, these results show that although temperate forestation should still exert a net negative radiative forcing, the effect cannot be adequately quantified by simple carbon accounting.

[] [...]

[] Here I have considered forestation under present-day conditions, but the effects of future CO2 rise and climate change are likely to affect the magnitude of both radiative forcing terms, due to dependencies on time-varying quantities such as the atmospheric CO2 concentration, snow extent and vegetation structure and leafiness. As the atmospheric CO2 concentration increases, CO2 fertilization is likely to increase carbon uptake22 so the magnitude of the negative sequestration forcing should therefore increase, although associated climate changes may exert additional positive or negative effects on sequestration. Warmer temperatures may reduce the extent of snow cover23, but the leaf area index (LAI) of potential vegetation may increase24, 25, so the albedo forcing could either increase or decrease. The effect of vegetation on surface albedo is not necessarily proportional to biomass, so the net contribution to radiative forcing may not evolve linearly throughout a forest's development; albedo depends on canopy density and architecture, and can become low rapidly, whereas carbon sequestration depends largely on woody biomass which is more gradually accumulated. Other contributions to forcing may also require consideration; for example, the longwave radiation budget could be affected by modified surface emissivity25, although the sign of such changes is uncertain25, 26.

[] The work I report here has focused on perturbations to the Earth's radiation budget, which is the fundamental driver of the climate system. Forestation may also influence the climate by modifying the fluxes of heat, moisture and momentum between the land surface and atmosphere. Whereas boreal forests warm their local climate through reduced albedo, tropical forests tend to cool and moisten their local climates by greatly enhancing evaporation. Both may also influence distant regional climates via the atmospheric circulation9, 27. Assessment of the effect of forestation on climate at a given time in the future will require simulations with a climate model that incorporates vegetation dynamics25, 28 and other atmospheric, terrestrial and oceanic components of the carbon cycle28, in which forest growth occurs at appropriate rates in relation to changes in atmospheric CO 2 and snow cover. Nevertheless, my results suggest that high-latitude forestation would exert a positive radiative forcing through reduced albedo that in many places could outweigh the negative forcing through carbon sequestration. If afforestation and reforestation are required to decrease radiative forcing rather than simply to reduce net CO2 emissions, then changes in surface albedo must also be considered.},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-11975999,~to-add-doi-URL,afforestation,albedo,boreal-forests,carbon-cycle,feedback,forest-resources,ghg,global-warming,temperature},
  number = {6809}
}
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