Biophysical forcing of climate by anthropogenic vegetation change Richard A. Betts & Pete Falloon
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Transcript of Biophysical forcing of climate by anthropogenic vegetation change Richard A. Betts & Pete Falloon
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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
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2Hadley Centre for Climate Prediction and Research IPCC 2001
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3Hadley Centre for Climate Prediction and ResearchPhoto: Centre for Ecology and Hydrology
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4Hadley Centre for Climate Prediction and ResearchPhoto: Tim Hewison
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5Hadley Centre for Climate Prediction and Research
Fraction of land used by agriculture(crops + grazing)
Ramankutty & Foley 1999 Klein Goldewijk 2000
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6Hadley Centre for Climate Prediction and Research
Forest and grassland albedo Delta Junction, Alaska, 1991-1993
Sharratt 1998
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7Hadley Centre for Climate Prediction and Research
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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8Hadley Centre for Climate Prediction and Research
Radiative forcing (Wm-2) by surface albedo change:“actual” - “natural” vegetation
Global mean: - 0.24 Wm-2
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9Hadley Centre for Climate Prediction and Research
Vegetation-atmosphere interactions
Surface albedo
Latent heat / moisture Sensible
heat LW emissivity
CO2
Aerodynamic roughness
Moisture availability
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10Hadley Centre for Climate Prediction and Research
Simulated 1.5m temperature difference (K) “Actual” - “Natural” vegetation
(annual mean)
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11Hadley Centre for Climate Prediction and Research
Simulated seasonal differences“Actual” (ACT) - “Natural” (NAT) vegetation
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12Hadley Centre for Climate Prediction and Research
Simulated seasonal differencesdue to albedo change alone
“ALBNAT” = albedo of natural veg
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13Hadley Centre for Climate Prediction and Research
Changes in fraction of land
disturbed by agriculture
Ramankutty & Foley 1999
Klein Goldewijk 2000
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14Hadley Centre for Climate Prediction and Research
Surface albedo forcing (Wm-2): 1750 - “natural”
Global mean: - 0.06 Wm-2
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15Hadley Centre for Climate Prediction and Research Global mean: - 0.10 Wm-2
Surface albedo forcing (Wm-2): 1850 - “natural”
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16Hadley Centre for Climate Prediction and Research Global mean: - 0.14 Wm-2
Surface albedo forcing (Wm-2): 1900 - “natural”
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17Hadley Centre for Climate Prediction and Research Global mean: - 0.18 Wm-2
Surface albedo forcing (Wm-2): 1950 - “natural”
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18Hadley Centre for Climate Prediction and Research
Surface albedo forcing (Wm-2): 1990 - “natural”
Global mean: - 0.24 Wm-2
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19Hadley Centre for Climate Prediction and Research
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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20Hadley Centre for Climate Prediction and Research
Time evolution of shortwave radiative
forcings (Wm-2)
Surface albedo
Ramaswamy et al 2001
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21Hadley Centre for Climate Prediction and Research
But what about
“Cool the Earth - plant a tree?”
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22Hadley 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
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23Hadley Centre for Climate Prediction and Research
Radiative forcing due to carbon sequestration
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24Hadley Centre for Climate Prediction and Research
Radiative forcing due to surface albedo change
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25Hadley Centre for Climate Prediction and Research
Net forcing due to “carbon sink” plantations
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26Hadley Centre for Climate Prediction and Research
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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27Hadley Centre for Climate Prediction and Research
Temperature change (K) due to Amazon deforestation
Kleidon and Heimann 2000
1.25
Contour interval 0.25K
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28Hadley Centre for Climate Prediction and Research
150hpa circulation response to Amazon deforestation
Streamfunction deviation from zonal mean
Contour interval 5×105 m2s-1
Gedney & Valdes 2000
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29Hadley 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
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30Hadley 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?
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31Hadley Centre for Climate Prediction and Research