1 Hadley Centre for Climate Prediction and Research Biophysical forcing of climate by anthropogenic vegetation change Richard A. Betts & Pete Falloon Hadley Centre for Climate Prediction and Research Met Office Expert Meeting on the Contribution of Agriculture to the State of Climate Ottawa, Canada 28 September 2004

2 Hadley Centre for Climate Prediction and Research IPCC 2001

3 Hadley Centre for Climate Prediction and Research Photo: Centre for Ecology and Hydrology

4 Hadley Centre for Climate Prediction and Research Photo: Tim Hewison

5 Hadley Centre for Climate Prediction and Research Fraction of land used by agriculture (crops + grazing) Ramankutty & Foley 1999 Klein Goldewijk 2000

6 Hadley Centre for Climate Prediction and Research Forest and grassland albedo Delta Junction, Alaska, Sharratt 1998

7 Hadley Centre for Climate Prediction and Research Modelling surface albedo α α = α 0 + (α D - α 0 )(1 - e -0.2S ) α 0 = snow-free albedo Forest: 0.15 Grassland: 0.2 α D = deep-snow albedo Forest: 0.25 Grassland: 0.8 (also some temperature dependence) S = snow amount (kg m -2 ) Hansen et al 1983

8 Hadley Centre for Climate Prediction and Research Radiative forcing (Wm -2 ) by surface albedo change: “actual” - “natural” vegetation Global mean: Wm -2

9 Hadley Centre for Climate Prediction and Research Vegetation-atmosphere interactions Surface albedo Latent heat / moisture Sensible heat LW emissivity CO 2 Aerodynamic roughness Moisture availability

10 Hadley Centre for Climate Prediction and Research Simulated 1.5m temperature difference (K) “Actual” - “Natural” vegetation (annual mean)

11 Hadley Centre for Climate Prediction and Research Simulated seasonal differences “Actual” (ACT) - “Natural” (NAT) vegetation

12 Hadley Centre for Climate Prediction and Research Simulated seasonal differences due to albedo change alone “ALBNAT” = albedo of natural veg

13 Hadley Centre for Climate Prediction and Research Changes in fraction of land disturbed by agriculture Ramankutty & Foley 1999 Klein Goldewijk 2000

14 Hadley Centre for Climate Prediction and Research Surface albedo forcing (Wm -2 ): “natural” Global mean: Wm -2

15 Hadley Centre for Climate Prediction and Research Global mean: Wm -2 Surface albedo forcing (Wm -2 ): “natural”

16 Hadley Centre for Climate Prediction and Research Global mean: Wm -2 Surface albedo forcing (Wm -2 ): “natural”

17 Hadley Centre for Climate Prediction and Research Global mean: Wm -2 Surface albedo forcing (Wm -2 ): “natural”

18 Hadley Centre for Climate Prediction and Research Surface albedo forcing (Wm -2 ): “natural” Global mean: Wm -2

19 Hadley Centre for Climate Prediction and Research 1990 forcing relative to “natural” global mean: Wm forcing relative to 1750 global mean: Wm -2 Wm -2

20 Hadley Centre for Climate Prediction and Research Time evolution of shortwave radiative forcings (Wm -2 ) Surface albedo Ramaswamy et al 2001

21 Hadley Centre for Climate Prediction and Research But what about “Cool the Earth - plant a tree?”

22 Hadley Centre for Climate Prediction and Research Carbon sink plantations: estimated sequestration potentials From regional/national estimates of annual increment (Nilsson & Schopfhauser 1995, Nabuurs & Mohren 1995) Carbon uptake (trees + soil) over 1 harvest rotation period

23 Hadley Centre for Climate Prediction and Research Radiative forcing due to carbon sequestration

24 Hadley Centre for Climate Prediction and Research Radiative forcing due to surface albedo change

25 Hadley Centre for Climate Prediction and Research Net forcing due to “carbon sink” plantations

26 Hadley Centre for Climate Prediction and Research Rates of change of forest cover Temperate forests: million ha yr -1 Tropical forests: million ha yr -1 UN Food and Agriculture Organization 1997

27 Hadley Centre for Climate Prediction and Research Temperature change (K) due to Amazon deforestation Kleidon and Heimann Contour interval 0.25K

28 Hadley Centre for Climate Prediction and Research 150hpa circulation response to Amazon deforestation Streamfunction deviation from zonal mean Contour interval 5×10 5 m 2 s -1 Gedney & Valdes 2000

29 Hadley Centre for Climate Prediction and Research Conclusions (i) Model results suggest that past deforestation has affected global climate mainly through surface albedo change Radiative forcing (-0.18 Wm -2 since 1750) therefore seems to be a reasonable indicator of land use effects on global climate Surface albedo change may affect estimates of aerosol forcing

30 Hadley Centre for Climate Prediction and Research Conclusions (ii) “Carbon sink” afforestation will also affect climate via surface albedo –carbon accounting may overestimate negative forcing –in cold regions, forcing may even be positive! Tropical deforestation forces climate non-radiatively –how do we quantify this?

31 Hadley Centre for Climate Prediction and Research