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Page 1© Crown copyright 2007 The influence of land use changes on pre-industrial and 20 th Century climate Richard Betts With thanks to Simon Tett (Hadley.

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Presentation on theme: "Page 1© Crown copyright 2007 The influence of land use changes on pre-industrial and 20 th Century climate Richard Betts With thanks to Simon Tett (Hadley."— Presentation transcript:

1 Page 1© Crown copyright 2007 The influence of land use changes on pre-industrial and 20 th Century climate Richard Betts With thanks to Simon Tett (Hadley Centre), Kees Klein Goldewijk (RIVM), Navin Ramankutty (University of Montreal) C20C Workshop, Met Office Hadley Centre, Exeter, 13-15 March, 2007

2 Page 2© Crown copyright 2007 Historical reconstructions of land use SAGE: Centre for Sustainability And the Global Environment, University of Wisconsin, USA HYDE: HistorY Database of the Environment, RIVM, Netherlands

3 Page 3© Crown copyright 2007 How has land cover change affected climate through biogeophysical effects?  Performed two 20-year HadAM3 GCM simulations with different land surface parameter input fields  (i) 1990 vegetation  (ii) Potential natural vegetation  vegetation dataset at 1º resolution  in gridcells which are partially deforested in 1990, extrapolate remaining natural vegetation  in gridcells which are totally deforested in 1990, replace 1990 vegetation with natural vegetation from a model (Woodward et al, 1995)

4 Page 4© Crown copyright 2007 Differences in land surface parameter fields: 1990 (ACTual) - “Natural” (NAT) veg

5 Page 5© Crown copyright 2007 Differences in land surface parameter fields: 1990 (ACTual) - “Natural” (NAT) veg

6 Page 6© Crown copyright 2007 Simulated annual mean 1.5m temperature difference (K): 1990 - “Natural” vegetation

7 Page 7© Crown copyright 2007 Simulated seasonal differences in 1.5m temperature (K): 1990 (ACTual) - “Natural” (NAT) veg

8 Page 8© Crown copyright 2007 Simulated seasonal differences due to albedo change alone “ALBNAT” = albedo of natural veg

9 Page 9© Crown copyright 2007 Changes in fraction of land disturbed by agriculture Ramankutty & Foley 1999 Klein Goldewijk 2000

10 Page 10© Crown copyright 2007 Effects of pre-industrial (1750) vs. current (1990) land use in HadCM3 OAGCM Surface temperature difference (K), “NATURAL” simulation – “CONTROL” simulation “NATURAL”: land use = 1750, CO 2 = 277.5 ppmv, historical solar forcing and volcanic aerosol “CONTROL”: land use = 1990, CO 2 = 289.6 ppmv, solar irradiance = 1365 Wm -2, volcanic aerosol = average 20 th Century values Tett et al, 2006

11 Page 11© Crown copyright 2007 Snow cover feedbacks due to land use effects on pre-industrial climate Tett et al, 2006 Snow cover difference (%), “NATURAL” simulation – “CONTROL” simulation “NATURAL”: land use = 1750, CO 2 = 277.5 ppmv, historical solar forcing and volcanic aerosol “CONTROL”: land use = 1990, CO 2 = 289.6 ppmv, solar irradiance = 1365 Wm -2, volcanic aerosol = average 20 th Century values

12 Page 12© Crown copyright 2007 Source: IPCC Global radiative forcings at 2005 relative to 1750

13 Page 13© Crown copyright 2007 Simulating radiative forcing due to surface albedo change  Use radiative transfer model (Edwards and Slingo) in HadAM3 GCM  6 sets of radiation calculations (only one feeds back to climate)  Simulate surface albedo and outgoing SW flux with vegetation state for:  Potential natural vegetation (PNV)  1750  1850  1900  1950  1990 - only 1990 surface SW budget feeds back to climate  Difference in outgoing SW flux between any date and PNV is radiative forcing (RF) relative to PNV  Easy to calculate RF relative to any reference state (eg: 1750 as usually examined by IPCC)

14 Page 14© Crown copyright 2007 Surface albedo forcing (Wm -2 ): 1750 - “natural” Global mean: - 0.06 Wm -2

15 Page 15© Crown copyright 2007 Global mean: - 0.10 Wm -2 Surface albedo forcing (Wm -2 ): 1850 - “natural”

16 Page 16© Crown copyright 2007 Global mean: - 0.14 Wm -2 Surface albedo forcing (Wm -2 ): 1900 - “natural”

17 Page 17© Crown copyright 2007 Global mean: - 0.18 Wm -2 Surface albedo forcing (Wm -2 ): 1950 - “natural”

18 Page 18© Crown copyright 2007 Surface albedo forcing (Wm -2 ): 1990 - “natural” Global mean: - 0.24 Wm -2

19 Page 19© Crown copyright 2007 1990 forcing relative to “natural” global mean: -0.24 Wm -2 1990 forcing relative to 1750 global mean: -0.18 Wm -2 Wm -2

20 Page 20© Crown copyright 2007 Conclusions  Model results suggest that past deforestation has affected global climate mainly through surface albedo change  Current major temperate agricultural areas may be approximately 1K cooler in annual mean than they would otherwise be if still under potential natural vegetation, due to increased surface albedo following deforestation  Much of this regional cooling likely to have occurred since Industrial Revolution  Local and global radiative forcings due to land use change continue to increase over time between 1750 and 1990  Land use change therefore may influence 20 th Century climate change

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