page 1© crown copyright 2007 the influence of land use changes on pre-industrial and 20 th century...

© Crown copyright 2007

The influence of land use changes on pre-industrial and 20th 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


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


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)


Differences in land surface parameter fields: 1990 (ACTual) - “Natural” (NAT) veg


Simulated annual mean 1.5m temperature difference (K): 1990 - “Natural” vegetation


Simulated seasonal differences in 1.5m temperature (K): 1990 (ACTual) - “Natural” (NAT) veg


Simulated seasonal differences due to albedo change alone

“ALBNAT” = albedo of natural veg


Changes in fraction of land disturbed by agriculture

Ramankutty & Foley 1999

Klein Goldewijk 2000


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, CO2 = 277.5 ppmv, historical solar forcing and volcanic aerosol

“CONTROL”: land use = 1990, CO2 = 289.6 ppmv, solar irradiance = 1365 Wm-2, volcanic aerosol = average 20th Century values

Tett et al, 2006


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, CO2 = 277.5 ppmv, historical solar forcing and volcanic aerosol

“CONTROL”: land use = 1990, CO2 = 289.6 ppmv, solar irradiance = 1365 Wm-2, volcanic aerosol = average 20th Century values


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)


Surface albedo forcing (Wm-2): 1750 - “natural”

Global mean: - 0.06 Wm-2



Global mean: - 0.24 Wm-2


1990 forcing relative to “natural”

global mean: -0.24 Wm-2

1990 forcing relative to 1750

global mean: -0.18 Wm-2

Wm-2


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 20th Century climate change

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