2.5.3 Radiative Forcing by Anthropogenic Surface Albedo Change: Land Use
Since the TAR, a number of estimates of the RF from land use changes over the industrial era have been made (Table 2.8). Unlike the main TAR estimate, most of the more recent studies are ‘pure’ RF calculations with the only change being land cover; feedbacks such as changes in snow cover are excluded. Brovkin et al. (2006) estimated the global mean RF relative to 1700 to be –0.15 W m–2, considering only cropland changes (Ramankutty and Foley, 1999) and not pastures. Hansen et al. (2005) also considered only cropland changes (Ramankutty and Foley, 1999) and simulated the RF relative to 1750 to be –0.15 W m–2. Using historical reconstructions of both croplands (Ramankutty and Foley, 1999) and pasturelands (Klein Goldewijk, 2001), Betts et al. (2007) simulated an RF of –0.18 W m–2 since 1750. This study also estimated the RF relative to PNV to be –0.24 W m–2. Other studies since the TAR have also estimated the RF at the present day relative to PNV (Table 2.8). Govindasamy et al. (2001a) estimated this RF as –0.08 W m–2. Myhre et al. (2005a) used land cover and albedo data from MODIS (Friedl et al., 2002; Schaaf et al., 2002) and estimated this RF as –0.09 W m–2. The results of Betts et al. (2007) and Brovkin et al. (2006) suggest that the RF relative to 1750 is approximately 75% of that relative to PNV. Therefore, by employing this factor published RFs relative to PNV can be used to estimate the RF relative to 1750 (Table 2.8).
In all the published studies, the RF showed a very high degree of spatial variability, with some areas showing no RF in 1990 relative to 1750 while values more negative than –5 W m–2 were typically found in the major agricultural areas of North America and Eurasia. The local RF depends on local albedo changes, which depend on the nature of the PNV replaced by agriculture (see top panel of Figure 2.15). In historical simulations, the spatial patterns of RF relative to the PNV remain generally similar over time, with the regional RFs in 1750 intensifying and expanding in the area covered. The major new areas of land cover change since 1750 are North America and central and eastern Russia.
Changes in the underlying surface albedo could affect the RF due to aerosols if such changes took place in the same regions. Similarly, surface albedo RF may depend on aerosol concentrations. Estimates of the temporal evolution of aerosol RF and surface albedo RF may need to consider changes in each other (Betts et al., 2007).
Table 2.8. Estimates of forest area, contribution to CO2 increase from anthropogenic land cover change, and RF due to the land use change-induced CO2 increase and surface albedo change, relative to pre-industrial vegetation and PNV. The CO2 RFs are for 2000 relative to 1850, calculated from the land use change contribution to the total increase in CO2 from 1850 to 2000 simulated with both land use and fossil fuel emissions by the carbon cycle models. Carbon emissions from land cover change for the 1980s and 1990s are discussed in Section 7.3 and Table 7.2.
|Main Source of Land Cover Data ||Forest Area PNV 106 km2 ||Forest Area circa 1700 106 km2 ||Forest Area circa 1990 106 km2 ||Contribution to CO2 Increase 1850–2000a (ppm) ||CO2 RF (W m–2) ||Albedo RF vs. PNV (W m–2) ||Albedo RF vs. 1750 (W m–2) |
|Ramankutty and Foley (1999) ||55.27 ||52.77b ||43.97c ||16d ||0.27 ||–0.24e ||–0.18e |
| || || || || ||–0.29 to +0.02f ||–0.22 to +0.02h |
| || || || || ||–0.2g ||–0.14g,i |
| || || || || || ||–0.15 to –0.28i,j |
| || || || || || ||–0.15k |
| || || || || || ||–0.075 to –0.325i,l |
|Klein Goldewijk (2001) ||58.6 ||54.4 ||41.5 ||12d ||0.20 ||–0.66 to +0.1f ||–0.50 to +0.08h |
| || || || || || ||–0.275i,l |
|Houghton (1983m, 2003) || ||62.15 ||50.53n ||35d ||0.57 || || |
| || || ||26o ||0.44 || || |
|MODIS (Schaaf et al., 2002) || || || || || ||–0.09p ||–0.07h |
|Wilson and Henderson-Sellers (1985) || || || || || ||–0.2q ||–0.15h |
| || || || || ||–0.29f ||–0.22h |
|SARBr || || || || || ||–0.11 to –0.55f ||–0.08 to –0.41h |
|Matthews (1983) || || || || || ||–0.12f–0.4s ||–0.09h–0.3h |
| || || || || ||–0.4s ||–0.3h |
| || || || || ||–0.08t ||–0.06h |