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Sahel Climate Change in the IPCC

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Sahel Climate Change in the IPCC Powered By Docstoc
					    Sahel Climate Change
   in the IPCC AR4 models




              Michela Biasutti
         biasutti@ldeo.columbia.edu

           in collaboration with :
Alessandra Giannini, Adam Sobel, Isaac Held
                         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?
            SST-forced Sahel drought: natural?

                                                                                        AMIP
                                                                                                                  coupled
                                                                                                                    CTL




                        Fig. 5. The 1950–99 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 atmosphere–ocean model
Hoerling et al., 2006   simulations. The observed trend value is indicated by the gray bar.
                  IPCC Simulations
    PI                 XX                      A1B




 Pre-Industrial      20th Century      Global Warming
 Control (PI)        Simulation (XX)   Scenario (A1B)
NASA/GISS                                      GCMs
                        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
                    Importance of Internal Variability

                                                1950-1985 Trend
                                                1950-1999 Trend
60 XX Simulations




                                                1930-1999 Trend




                                         1. reduced variability

                                         2. predominance of
                                               drying trend
Forced Signal: (1975-1999 mean) minus (PI mean)
XX-PI Rainfall Change
XX-PI SST Change
                         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?
                       Effect of GHG
                       4x(yrs50:70)-PI
                                  Surface Temperature
Mean Rainfall Change




  Robustness of
  Rainfall Change

            20
               Effect of Reflective Aerosols
            SULFATE AEROSOL FORCINGS (1850-1997)




      Temp RESPONSE                   Precip RESPONSE



                                             QuickTime™ an d a
                                         TIFF (LZW) decomp ressor
                                      are need ed to see this p icture .




                                               Quic kTime™ and a
                                           TIFF ( LZW) dec omp resso r
                                       ar e need ed to s ee this picture.



                                          ROTSTAYN AND LOHMANN „02
NASA/GISS
                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.
 Precipitation Response in the Sahel




GFDL
         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?
Relationship of Sahel rainfall & SST (pre-industrial, not forced)




Biasutti et al., 2007
        goodness of model
                             Linear Multi-Regressive
                                     Model:
                                        from SST
PI                          (Indo-Pacific & Atlantic Gradient)
(training run)
                                    to Sahel Rainfall



  XX




A1B
                               interannual (=detrended)
      goodness of model
                            Linear Multi-Regressive
                                       Model
                           trained on (detrended) PI:
PI                                   from SST
                          (Indo-Pacific & Atlantic Gradient)
                                 to Sahel Rainfall

 XX                                 nb: same results if
                                    NTA & STA are used
                                      (3 predictors) and/or if
                                    model is trained on XX.

                                interannual
A1B
                               interannual + trend
                                        Simulated &Predicted
                                           Sahel Rainfall
                 Linear Regression
                    Coefficients

                  obs                      CM2
North Atlantic




                                miroc


                 AM2
                              CM2        miroc


                 Uniform Warming


   Held & Lu, 2007
                   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 model’s 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?

				
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posted:11/7/2010
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