Sahel Climate Change

1
Sahel Climate Change in the IPCC AR4 models
Michela Biasutti biasutti_at_ldeo.columbia.edu in
collaboration with Alessandra Giannini, Adam
Sobel, Isaac Held
2
OUTLINE

• 20th Century
• Was the Sahel drought internal noise?
• Forced Signal?
• Anthropogenic? GHG or Aerosols?
• 21st Century
• What is the source of model disagreement?
• Different SST forcing?
• Different response to the same SST forcing?

3
SST-forced Sahel drought natural?
AMIP
coupled CTL
Fig. 5. The 195099 trends of (left) observed and
(middle) atmospheric GCM simulated seasonal
African rainfall for JAS. Plotted is the total
seasonal rainfall change (mm) over the 50-yr
period. (right) The empirical PDFs of JAS 50-yr
rainfall trends averaged over the Sahel region.
The data given by the red curve are from the 80
individual members of the AGCM simulations forced
with the history of global observed SSTs. The
data given by the blue curve are from 15
individual members of unforced coupled
atmosphereocean model simulations. The observed
trend value is indicated by the gray bar.
Hoerling et al., 2006
4
IPCC Simulations
XX
PI
A1B
GCMs
NASA/GISS
5
IPCC Simulations
1950
2000
2050
The ensamble mean fails to simulate the
pattern or amplitude of the twentieth-century
African drying, indicating that the drought
conditions were likely of natural origin.
Hoerling et al., 2006
6
Importance of Internal Variability
? 1950-1985 Trend
? 1950-1999 Trend
? 1930-1999 Trend
60 XX Simulations

• 1. reduced variability
• 2. predominance of
• drying trend

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Forced Signal (1975-1999 mean) minus (PI mean)
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XX-PI Rainfall Change
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XX-PI SST Change
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OUTLINE

• 20th Century
• Was the Sahel drought internal noise?
• Forced?
• Anthropogenic? GHG or Aerosols?
• 21st Century
• What is the source of model disagreement?
• Different SST forcing?
• Different response to the same SST forcing?

11
Effect of GHG 4x(yrs5070)-PI
Surface Temperature
Mean Rainfall Change
Robustness of Rainfall Change
20
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Effect of Reflective Aerosols
SULFATE AEROSOL FORCINGS (1850-1997)
Temp RESPONSE
Precip RESPONSE
ROTSTAYN AND LOHMANN 02
NASA/GISS
13
Some Conclusions

• 20th Century drying of the Sahel is reproduced
  by almost all IPCC AR4 models ? it is (partly)
  externally forced. (But natural, internal
  variability is substantial.)
• The forcing was anthropogenic, with the most
  robust signal coming from the sulfate aerosol
  forcing.
• The response to GHG increase alone is
  inconsistent across models, which implies an
  uncertain outlook for the Sahel.

14
Precipitation Response in the Sahel
GFDL
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What are the possible causes of discrepancy?

• Given the role of SST in simulations of the
• 20th Century, is it SST?
• different SST anomalies?
• different sensitivity to same SST anomalies?

16
Relationship of Sahel rainfall SST
(pre-industrial, not forced)
Biasutti et al., 2007
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goodness of model
Linear Multi-Regressive Model from SST
(Indo-Pacific Atlantic Gradient) to Sahel
Rainfall
PI (training run)
XX
A1B
? interannual (detrended)
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goodness of model
Linear Multi-Regressive Model trained on
(detrended) PI from SST (Indo-Pacific
Atlantic Gradient) to Sahel Rainfall
PI
XX
nb same results if NTA STA are used (3
predictors) and/or if model is trained on XX.
? interannual
A1B
? interannual trend
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Simulated Predicted Sahel Rainfall
Linear Regression Coefficients
CM2
obs
miroc
North Atlantic
AM2
CM2
miroc
Uniform Warming
Held Lu, 2007
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Conclusions

• 30(?) of 20th Century drying of the Sahel was
  externally forced. The forcing was anthropogenic,
  with the most robust signal coming from the
  sulfate aerosol forcing.
• In the 21st Century, when GHG are the dominant
  forcing, the Sahel response is inconsistent
  across models.
• Global SST changes can explain the 20th Century
  trend, but, in most models, not the 21st Century
  one (at least not through the same mechanisms
  active in the past).
• A models good representation of the past is no
  indication of a trustworthy prediction of the
  future. How can we reduce the uncertainty of our
  climate outlook?