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Biophysical forcing of climate by anthropogenic vegetation change Richard A. Betts & Pete Falloon

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. IPCC 2001.

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Biophysical forcing of climate by anthropogenic vegetation change Richard A. Betts & Pete Falloon

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  1. 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. IPCC 2001

  3. Photo: Centre for Ecology and Hydrology

  4. Photo: Tim Hewison

  5. Fraction of land used by agriculture(crops + grazing) Ramankutty & Foley 1999 Klein Goldewijk 2000

  6. Forest and grassland albedo Delta Junction, Alaska, 1991-1993 Sharratt 1998

  7. Modelling surface albedo α α = α0 + (αD - α0)(1 - e-0.2S) α0= snow-free albedo Forest: 0.15 Grassland: 0.2 αD= deep-snow albedo Forest: 0.25 Grassland: 0.8 (also some temperature dependence) S = snow amount (kg m-2) Hansen et al 1983

  8. Radiative forcing (Wm-2) by surface albedo change:“actual” - “natural” vegetation Global mean: - 0.24 Wm-2

  9. Vegetation-atmosphere interactions CO2 Latent heat / moisture Sensible heat Surface albedo LW emissivity Aerodynamic roughness Moisture availability

  10. Simulated 1.5m temperature difference (K) “Actual” - “Natural” vegetation (annual mean)

  11. Simulated seasonal differences“Actual” (ACT) - “Natural” (NAT) vegetation

  12. Simulated seasonal differencesdue to albedo change alone “ALBNAT” = albedo of natural veg

  13. Changes in fraction of land disturbed by agriculture Ramankutty & Foley 1999 Klein Goldewijk 2000

  14. Surface albedo forcing (Wm-2): 1750 - “natural” Global mean: - 0.06 Wm-2

  15. Surface albedo forcing (Wm-2): 1850 - “natural” Global mean: - 0.10 Wm-2

  16. Surface albedo forcing (Wm-2): 1900 - “natural” Global mean: - 0.14 Wm-2

  17. Surface albedo forcing (Wm-2): 1950 - “natural” Global mean: - 0.18 Wm-2

  18. Surface albedo forcing (Wm-2): 1990 - “natural” Global mean: - 0.24 Wm-2

  19. 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. Time evolution of shortwave radiative forcings (Wm-2) Surface albedo Ramaswamy et al 2001

  21. But what about “Cool the Earth - plant a tree?”

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

  23. Radiative forcing due to carbon sequestration

  24. Radiative forcing due to surface albedo change

  25. Net forcing due to “carbon sink” plantations

  26. Rates of change of forest cover Temperate forests: + 1.3 million ha yr-1 Tropical forests: - 12.6 million ha yr-1 UN Food and Agriculture Organization 1997

  27. Temperature change (K) due to Amazon deforestation 1.25 Contour interval 0.25K Kleidon and Heimann 2000

  28. 150hpa circulation response to Amazon deforestation Streamfunction deviation from zonal mean Contour interval 5×105 m2s-1 Gedney & Valdes 2000

  29. 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. 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?

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