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	IPCC Fourth Assessment Report: Climate Change 2007

Climate Change 2007: Working Group I: The Physical Science Basis

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 1850–2000^a (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	52.77^b	43.97^c	16^d	0.27	–0.24^e	–0.18^e
						–0.29 to +0.02^f	–0.22 to +0.02^h
						–0.2^g	–0.14^g,i
							–0.15 to –0.28^i,j
							–0.15^k
							–0.075 to –0.325^i,l
Klein Goldewijk (2001)	58.6	54.4	41.5	12^d	0.20	–0.66 to +0.1^f	–0.50 to +0.08^h
Klein Goldewijk (2001)							–0.275^i,l
Houghton (1983^m, 2003)		62.15	50.53ⁿ	35^d	0.57
Houghton (1983^m, 2003)				26^o	0.44
MODIS (Schaaf et al., 2002)						–0.09^p	–0.07^h
Wilson and Henderson-Sellers (1985)						–0.2^q	–0.15^h
Wilson and Henderson-Sellers (1985)						–0.29^f	–0.22^h
SARB^r						–0.11 to –0.55^f	–0.08 to –0.41^h
Matthews (1983)						–0.12^f–0.4^s	–0.09^h–0.3^h
						–0.4^s	–0.3^h
						–0.08^t	–0.06^h

Notes:

^a The available literature simulates CO₂ rises with and without land use relative to 1850.

^b 1750 forest area reported as 51.85 x 106 km².

^c 1992 forest area.

^d Land use contribution CO₂ rise from Brovkin et al. (2004).

^e Albedo RF from Betts et al. (2007). Land cover data combined from Ramankutty and Foley (1999), Klein Goldelwijk (2001) and Wilson and Henderson-Sellers (1985).

^f Albedo RF from Myhre and Myhre (2003). Range of estimate for each land cover data set arises from use of different albedo values.

^g Albedo RF from model of Goosse et al. (2005) in Brovkin et al. (2006).

^h RF relative to 1750 estimated here as 0.75 of RF relative to PNV following Betts et al. (2007) and Brovkin et al. (2006).

ⁱ Estimate relative to 1700.

^j Albedo RF from Matthews et al. (2003).

^k Albedo RF from Hansen et al. (2005).

l Albedo RF from Matthews et al. (2004).

m Forest areas aggregated by Richards (1990).

n 1980 forest area.

o Land use contribution to CO2 rise from Matthews et al. (2004). Estimate only available relative to 1850 not 1750.

p Albedo RF from Myhre et al. (2005a).

q Albedo RF from Betts (2001).

r Surface and Atmosphere Radiation Budget; http://www-surf.larc.nasa.gov/surf/.

s Albedo RF from Hansen et al. (1997).

t Albedo RF from Govindasamy et al. (2001a).