Biophysical forcing of climate by anthropogenic vegetation change Richard A. Betts & Pete Falloon
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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
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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. - PowerPoint PPT Presentation
Transcript of Biophysical forcing of climate by anthropogenic vegetation change Richard A. Betts & Pete Falloon
1Hadley 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 ResearchMet Office
Expert Meeting on the Contribution of Agriculture to the State of Climate
Ottawa, Canada28 September 2004
2Hadley Centre for Climate Prediction and Research IPCC 2001
3Hadley Centre for Climate Prediction and ResearchPhoto: Centre for Ecology and Hydrology
4Hadley Centre for Climate Prediction and ResearchPhoto: Tim Hewison
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Fraction of land used by agriculture(crops + grazing)
Ramankutty & Foley 1999 Klein Goldewijk 2000
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Forest and grassland albedo Delta Junction, Alaska, 1991-1993
Sharratt 1998
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Modelling surface albedo α
α = α0 + (αD - α0)(1 - e-0.2S)
α0 = snow-free albedoForest: 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
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Radiative forcing (Wm-2) by surface albedo change:“actual” - “natural” vegetation
Global mean: - 0.24 Wm-2
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Vegetation-atmosphere interactions
Surface albedo
Latent heat / moisture Sensible
heat LW emissivity
CO2
Aerodynamic roughness
Moisture availability
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Simulated 1.5m temperature difference (K) “Actual” - “Natural” vegetation
(annual mean)
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Simulated seasonal differences“Actual” (ACT) - “Natural” (NAT) vegetation
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Simulated seasonal differencesdue to albedo change alone
“ALBNAT” = albedo of natural veg
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Changes in fraction of land
disturbed by agriculture
Ramankutty & Foley 1999
Klein Goldewijk 2000
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Surface albedo forcing (Wm-2): 1750 - “natural”
Global mean: - 0.06 Wm-2
15Hadley Centre for Climate Prediction and Research Global mean: - 0.10 Wm-2
Surface albedo forcing (Wm-2): 1850 - “natural”
16Hadley Centre for Climate Prediction and Research Global mean: - 0.14 Wm-2
Surface albedo forcing (Wm-2): 1900 - “natural”
17Hadley Centre for Climate Prediction and Research Global mean: - 0.18 Wm-2
Surface albedo forcing (Wm-2): 1950 - “natural”
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Surface albedo forcing (Wm-2): 1990 - “natural”
Global mean: - 0.24 Wm-2
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1990 forcing relative to “natural”
global mean: -0.24 Wm-2
1990 forcing relative to 1750
global mean: -0.18 Wm-2
Wm-2
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Time evolution of shortwave radiative
forcings (Wm-2)
Surface albedo
Ramaswamy et al 2001
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But what about
“Cool the Earth - plant a tree?”
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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
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Radiative forcing due to carbon sequestration
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Radiative forcing due to surface albedo change
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Net forcing due to “carbon sink” plantations
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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
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Temperature change (K) due to Amazon deforestation
Kleidon and Heimann 2000
1.25
Contour interval 0.25K
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150hpa circulation response to Amazon deforestation
Streamfunction deviation from zonal mean
Contour interval 5×105 m2s-1
Gedney & Valdes 2000
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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
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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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