past- pre-industrial land-cover (year 1870) pres – present-day land-cover
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Impact ofAnthropogenic Land-Cover Changes
on
1) the global radiative forcing of planet EarthEdouard Davin, Nathalie de Noblet, Pierre Friedlingstein
2) the characteristics of the El-Niño Southern Oscillation
Edouard Davin, Nathalie de Noblet, Christian Laguerre,Pascal Terray, Eric Guilyardi
Laboratoire des Sciences du Climat et de l’EnvironnementUnité mixte CEA-CNRS-UVSQ / Gif-sur-Yvette / France
3 snap-Shot experimentsfully coupled Atmosphere-Ocean (IPSL-CM4)
+ interactive seasonally varying foliage density (LAI)
200-year long simulations, analyses on last 50 years,all with pre-industrial aerosols and GHGs
PAST- pre-industrial land-cover (year 1870)PRES – present-day land-coverFUTU – Future land-cover (year 2100, SRES
A2)
Ramankutty & Foley, 1999 ; Goldewijk, 2001
IMAGE2 SRES A2 (Alcamo et al. 1998)
Change in anthropogenic land fraction (crops + pastures)
PRES - PAST FUTU - PRES
Part I:
Impact of land cover change on surface climate:
relevance of the radiative forcing conceptEdouard Davin, Nathalie de Noblet-Ducoudré,
Pierre Friedlingstein
Submitted to GRL
Question addressed in this paper :
is the radiative forcing concept
applicable to the climatic impacts
of the land-use induced land-cover
changes ?
Reminder: concept of radiative forcing
Instantaneous forcing Adjusted forcing Climatic Change
Quantity (e.g. CO2) ΔQi (change in radiative forcing, W/m-2)
ΔTs = λ * ΔQ
Radiative forcings from 1750 till 2005
175017508-9 millions km8-9 millions km22 of anthropogenic land-cover of anthropogenic land-cover
(6-7% of land areas)(6-7% of land areas)
1990199046-51 millions km46-51 millions km22 of anthropogenic land-cover of anthropogenic land-cover
(35-39% of land areas)(35-39% of land areas)
due to the sole changes in
land-surface albedo
We have computed the radiative forcingdue to land-cover changes from 1870 to
present-dayand from present-day to 2100
Simulatedclimatology
land surface modelORCHIDEE
ΔΔ albedo
ΔΔ evapotranspiration
ΔΔ water vapor content
radiative transfer schemeof our AGCM (LMDz)
ΔΔ F albedoΔΔ F water vapor
Change in land surface characteristics prior to any atmospheric feedbacks
Conversion into radiative forcings
Land cover change
Change in anthropogenic land fraction (crops + pastures)PRES - PASTPRES - PAST FUTU - PRESFUTU - PRES
Annual mean radiative forcing (albedo + water vapor)PRES – PASTPRES – PAST FUTU - PRESFUTU - PRES
Changes in the global annual radiative forcing (W/m-2)
due to albedo changes alonedue to evapotranspiration changes alone
-0,7
-0,6
-0,5
-0,4
-0,3
-0,2
-0,1
0
Preindustrial to Present Present to Future
- 0.29 W/m-2
-0.70 W/m-2
Changes in the global annual mean surface temperature (°C)
due to albedo changes alonedue to evapotranspiration changes alone
-0,14
-0,12
-0,1
-0,08
-0,06
-0,04
-0,02
0
Preindustrial to Present Present to Future
- 0.05°C
- 0.14°C
FUTU – PRESFUTU – PRESPRES - PASTPRES - PAST
ΔT = - 0.05 K ΔT = - 0.14 K
Spatial patterns of the changes in the global annual mean surface temperature (°C)
Derived climate sensitivityΔ Ts / (Δ F - Δ R)
1. Climate exhibits the same sensitivity to both historical and future land cover change.
Simulations Δ F
(W/m2)
Δ Ts
(K)
Δ Ts / (Δ F - Δ R)
K/(W.m-2)
PRES - PAST - 0.29 - 0.05 0.3
FUTU - PAST - 0.7 - 0.14 0.27
Climate sensitivity inferred from a2xCO2 experiment : 1 K/(W.m-2)
2. Climate sensitivity to land cover change is 70 % lower than the sensitivity to CO2 forcing.
2 hypothesized causes: Spatial inhomogeneity
limited poleward extent of land-cover changes limited sea-ice feedback.
but this only explains about 20% of the reduced sensitivity
Non-radiative effectsLand-cover changes applied decreased net radiation absorbed by the land-surface decreased evapotranspiration BUT increased sensible heat (due to increased Bowen Ratio)
Why is the climate sensitivityderived from land-cover changes different from
the one derived from CO2?
(figures taken from Kabat et al.: Vegetation, Water, Humans, and the Climate, IGBP BAHC)
Decreased evapotranspiration
decreased water vapor greenhouse effect
cooling of the troposphere
Increased sensible heat warming of the boundary layer
over deforested areas
Value used to quantify the climate sensitivity
Deforestation, wherever it occurs, leads to global cooling through its biophysical effects on climate
The climate sensitivity derived is the same wherever deforestation occurs the radiative concept may be applicable (similar work needs to be carried out with other models, other deforestation locations and intensities)
But the derived climate sensitivity is 70% lower than the one derived from changes in CO2.
In conclusion
Part II:
Influence of future land-use inducedland-cover change
on the characteristics ofthe El-Niño Southern Oscillation
Edouard Davin, Nathalie de Noblet-Ducoudré,Christian Laguerre, Pascal Terray, Eric Guilyardi
(Prelimirary results)
Large changes in the tropical regions
+Global oceanic cooling
Potential effects on
ENSO
Land-use induced land cover changes between present-day and year 2100
futur
PRES30 %
FUTU34 %
Observed (HADISST)30 %
First EOF of sea-surface temperature% of tropical variability explained by this
first EOF
Question addressed :
what is the impact of this
increased variance on the
frequency and/or characteristics of
the El-Niño events ?
1) No significant change in frequency
Observations
El Niños are more frequent in the model
than in reality
Year-1
Year-1
Power spectrum
Based on the following criterias:
examine anomalous SSTs (with respect to modelled climatology) in the Niño 3 box:
5°S-5°N / 150°W-90°W
Select the years exhibiting anomalous SSTs larger than 1.5* during 3 consecutive months between October and February
2) Selection of modelled El-Niñosover a 100 year-long time period
Selected eventsanomalous SSTs over a 3-year long time
period
El Niño events
Selected eventsanomalous SSTs over a 3-year long time
period
PRESFUTU
SSTs are colder at present, prior to the
event
SSTs are warmer in FUTU at the
peak of the event
PRESFUTU
El Niño compositesanomalous SSTs over a 3-year long time
period
time versus longitude diagram of anomalous SSTs
composite El Niño for each set of simulations
TIM
E
PRES FUTU
COLDER
WARMER
earlier onset of warming
similar timing of termination
Statistically significant spatial patternsof anomalous SSTs through time
PRES FUTU
Earlier warming
of the Indian ocean
Jan
uary
Ap
ril
July
Octo
berM
arc
hJu
ne
Sep
tem
berD
ecem
ber
Niño 3Indian Ocean
Comparing SST changesin the Niño 3 region to those in the Indian
OceanPRES
FUTU
~3.5 months
~3 months
Amplitude of warming in
Indian ocean almost as large
as in Pacific
What can explain the earlier onset of El Niño ?
What can explain the earlier onset of El Niño ?
hypothesis
With a deforested Amazon
Ascending motion reduced over the AmazonShift of the
main convective cell
towards the East Pacific
Reduced Walker
cirulation
What can explain the earlier onset of El Niño ?
The model shows changes in upper-level divergence
PRES
FUTU - PRES
Divergence at 200hPa
Change in divergence at 200hPa
An
nu
al m
ean
s
Reduced upper-level
divergence
decreased upper-level convergen
ce
DJF
MAM
JJA
SON
FU
TU
- P
RES
Change in divergenceat 200hPa
differences in zonal mean windbetween 200hPa and 850hPa. Winter values (DJF)
PR
ES
FU
TU
- P
RES
Weakening of the Walker
circulation
FUTU – PRESFUTU – PRES
Changes in surface temperature in global annualmean already exhibits an El Niño like pattern
Tropical deforestation reduces ascending motions above the Amazon basin reduced subsidence in the eastern side of equatorial Pacific slowing down of the Walker circulation.
The frequency of ENSO events is not modified
But the warm phases of ENSO start earlier, and the maximum temperature anomaly reached is larger.
+ Temperature changes in the Indian ocean are as large as in the eastern Pacific.
In conclusion