Haywood_ESSI_2011

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					Palaeo constraints on longer term climate
sensitivity
Alan Haywood (School of Earth and Environment)
The Team
Modellers Vs Geologists
Back to the Future…




                      John Carter Brown Library,
                          Brown University
In a Nutshell…




         5       3
Climate or Charney Sensitivity

“The change in global mean near-surface air temperature that would result from a
sustained doubling of the atmospheric CO2 concentration.”
“Likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very
unlikely to be less than 1.5°C.” IPCC AR4.


Given a CO2 change, can estimate global mean temperature response
Given a temperature change, can estimate CO2 change


                                                                 Probability density
                                                                 functions from past
                                                                 transient    climate
                                                                 change (Meehl et
                                                                 al. 2008).
Address within a pure modelling
framework
A pure data framework

“Paleoclimate data show that climate sensitivity is ~3°C for doubled
CO2, including only fast feedback processes.”

“[Earth System] sensitivity, including slower surface albedo
feedbacks, is ~6°C for doubled CO2” Hansen et al. (2009)
Combined Data and Model Approach



Find a time period in Earth history when:
(a) CO2 was higher than modern
(b) We have an idea of ice sheets, vegetation and palaeogeography


Run a climate model with the elevated CO2 and prescribed
  (from data) ice and vegetation.
Modelled annual global mean temperature will give the
 long-term climate sensitivity (Earth System Sensitivity)
 to this CO2 forcing.
The Pliocene & Future Climate
Change


                “The mid-Pliocene represents an accessible
                example of a world that is similar in many
                respects to what models estimate could be
                the Earth of the late 21st Century.

                Taken together, the average of the warmest
                times during the mid-Pliocene presents a
                view of the equilibrium state of a globally
                warmer world, in which atmospheric CO2
                concentrations were likely higher than pre-
                industrial values”.

                Jansen et al. (2007) Chapter 6, pages 440 to
                442.
The Last 5 Million Years
Wonderful Records: SST Variability Site
982




                           Lawrence et al. (2009)
Our Knowledge Base
A History of PRISM
The Pliocene Paradox…or not…



                               Seki et al. (2010). EPSL,
                                    292, 201-211.
 Pliocene Sea-Level
 Atlantic Coastal Plain




 Orangeburg Scarp, ACP


Dowsett, H.J. and Cronin, T.M., 1990. High eustatic sea level during the middle Pliocene: Evidence
           from the southeastern U.S. Atlantic Coastal Plain. Geology, 18(5): 435-438.
Range in Sea Level Estimates

     Meters ASL

        60

        35 ±17

        25

        40

        40

        20

        15 to 30

        30
Ice Sheet Extent/Distribution

                            Palaeoshorelines
                         (e.g. Dowsett & Cronin
                            1990) – PLIOMAX
                          project (Raymo et al.
                                  2009)



                            Climate/ice sheet
                           modelling (e.g. Hill,
                           2009; 2007; Pollard
                           and DeConto, 2009)
Pliocene Biome
Reconstruction
  Mid-Pliocene Conditions


Tundra BIOME nearly absent


                             Reduced deserts




  Poleward shift in
   most BIOMES
Orographic Differences


                         Fission track data - Parrish et
                                    al. (2007)
Boundary Conditions

Pre-industrial, CO2 = 280ppmv




  Modern vegetation + ice       Modern orography
Pliocene, CO2 = 400ppmv




  Pliocene vegetation + ice     Pliocene orography
Modelling Pliocene climate


                     CO2 = 400ppmv

                     ΔT = 3.3oC




    HadCM3


                     But this is not Earth System Sensitivity
                                                        orog   ice   veg     CO2
  (1) CO2
                   forcings            Plio             P      P     P       P

  (2) Orography                        Pliomodorog      M      P     P       P
                                       Pliomodorogice   M      M     P       P
  (3) Vegetation                       Modplioco2       M      M     M       P
                   feedbacks
                                       Mod              M      M     M       M
  (4) Ice


Total Pliocene temp                           Orography forcing
                                                                                   Ice feedbacks
change, relative to            Direct contribution      Vegetation
pre-industrial                 from CO2 (“climate       feedbacks
                               sensitivity”)




EARTH SYSTEM                                                               Feedbacks due to
SENSITIVTY                                                                 CO2 alone
ΔT = 3.3oC   1.6oC   0.7oC   0.7oC   0.4oC
CS versus ESS (400 ppmv)




         1.6oC             2.3oC
(1) Causes of mid-Pliocene warmth:
ΔT=3.3oC                 1.6oC   0.7oC   0.7oC   0.4oC




(2) Earth System Sensitivity (to CO2
   doubling):



4.4 oC     3.0oC
Assumptions

• Feedbacks are independent of the forcing
• Forcings add linearly
• Spatial distribution of forcing or response
Summary/Future Work


• Over long timescales this study suggests that the equilibrium climate
  response to a increase in CO2 is likely to be larger than previously
  thought
• Given uncertainties in the timescale for vegetation and ice sheet
  responses, estimates of the impacts of long term greenhouse gas
  stabilisation scenarios could focus on ESS rather than CS


• Explore uncertainty in boundary conditions and impact on ESS
• Assess relationship between CS and ESS with more than one set of
  model parameters
• Repeat this study with more complete and higher resolution models
  which include additional Earth system components

				
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posted:12/11/2011
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