douard Davin, Nathalie de NobletDucoudr, Pierre Friedlingstein

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Biophysical impact of land use change on present
day and future climate

• Édouard Davin, Nathalie de Noblet-Ducoudré,
  Pierre Friedlingstein

Laboratoire des Sciences du Climat et de
lEnvironnement France
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How does land cover change influence global
climate?

• Biogeochemical impact
• Change in GHG emissions
• Biophysical impact
• Change in energy availability (albedo)
• Change in hydrology and energy distribution
  (roughness, evapotranspiration)

Foley et al., 2003

• State of the art
• Historical land use change may have cooled
  northern mid-latitude climate Betts, 2001
  Brovkin et al., 1999
• Tropical deforestation is expected to produce a
  warming of the deforested area Claussen et al.,
  2001 Voldoire Royer, 2004

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Aims of the study

• Examine the biophysical impact of land cover
  change on surface climate (present day future
  climate) with a fully coupled climate model.
• Separate the contribution of the different
  surface properties.
• Discuss the applicability of the radiative
  forcing concept.

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Model description
IPSL-CM4

• The radiation code can be used for off line
  radiative forcing calculations
• The vegetation model can be run in stand alone
  mode with forced climatology
• LAI is a prognostic variable
• Most of the previous studies have used fixed
  ocean conditions

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Land use data sets and climate simulations

• Climate simulations
• With pre-industrial vegetation
• With present day vegetation

Natural vegetation Loveland et al. (2000) Crops
Ramankutty Foley (1999) Pastures Goldewijk
(2001)

• ORCHIDEE forced with IPSL-CM4 climatology
• With pre-industrial vegetation
• With present day vegetation

Crop fraction (present day vs 1860)
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Land use data sets and climate simulations

• Climate simulations
• With 2100 vegetation

Crops IMAGE2 A2 scenario for 2100

• ORCHIDEE forced with IPSL-CM4 climatology
• With 2100 vegetation
• With present day vegetation and albedo
  corresponding to 2100 vegetation

Crop fraction (2100 vs present day)
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Present day VS pre-industrial
Surface temperature change
Global cooling - 0.22 C
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Present day VS pre-industrial
Surface temperature change
Global forcing - 0.26 W/m2
Radiative forcing of albedo (W/m2)
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Present day VS pre-industrial
Surface temperature change
Change in boreal forest LAI (m2/m2)
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Present day VS pre-industrial
Change in fresh water balance (North
Atlantic) Weakening of the hydrological cycle
over continents due to land cover change Less
evaporation, precipitation and runoff Change in
runoff - 15000 m3/s Slight strengthening of
the THC
Surface temperature change
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2100 VS present day
Surface temperature change
Global cooling - 0.17 C
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2100 VS present day
Surface temperature change
Global forcing - 0.55 W/m2
Radiative forcing of albedo (W/m2)
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2100 VS present day
Surface temperature change
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2100 VS present day
Surface temperature change
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Is the radiative forcing concept a relevant tool
to assess land use impact on global climate?
Proportionality between forcing and response ?Ts
? RF
IPSL-CM4 equilibrium climate sensitivity ?
0.85 K/(W.m-2)

• Climate sensitivity to land use change is not
  constant

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Conclusions

• Land use change (historical or future)
  global cooling
• Ocean circulation seems to be affected by land
  cover change through the hydrological cycle.
• Radiative forcing concept may be inappropriate to
  predict land use change impact on climate.

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