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New estimates of Earth system sensitivity from the Cenozoic
Introduction: Earth system sensitivity
Case study 1: The mid Pliocene
Case study 2: The PETM and the early Eocene
Conclusions
Dan Lunt, Alan Haywood, Paul Valdes, Harry DowsettEoMIP: M. Heinemann, M. Huber, A. Legrande, A. Winguth, C. Loptson PlioMIP: W. Le-Chan, N. Rosenbloom, M. Chandler, F. Bragg
Common theme – model uncertainty and intercomparisons
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. “
Climate sensitivity
IPCC, AR4, 2007
Given a CO2 change, can estimate global mean temperature response
Given a temperature change, can estimate CO2 change
But…..
(1) ….official definition is century-scale (short-term feedback processes only).
(2) Even models that do include dynamic oceans neglect many other processes….
Therefore define EARTH SYSTEM SENSITIVITY – long-term response to sustained elevated CO2 concentrations, including all Earth system feedbacks.
Knutti and Hegerl, Nature Geoscience, 2008
….combined model - data approach….(has advantages vs. ‘pure-data’ or ‘pure-modelling’ approaches)
Find a palaeo time period when
(a)CO2 was higher than modern (i.e. not the LGM!)
(b) we have an idea of ice sheets and palaeogeographies and other boundary conditions….
Run a climate model with the elevated CO2 and prescribed (from data) ice and vegetation….compare with proxy data where available.
Modelled global mean temp will give the long-term climate sensitivity (Earth System Sensitivity) to this CO2 forcing. Model is an ‘interpolator’ of paleo data.
(1) CASE STUDY 1: The mid-Pliocene….
Boundary conditions from the PRISM2 project, USGS
CO2 prescribed as 400 ppmv
ΔT=3.3oC
CO2 = 400ppmv
Mid-Pliocene simulated temperatures compared with pre-industrial:
Mid-Pliocene simulated temperatures compared with data
‘Global-annual mean’ response is good, but….
Dowsett et al, PPP, 2011
Total mid-Pliocene temp change, relative to pre-industrial
Direct contribition from CO2 (“climate sensitivity”)
Vegetation feedbacks
Ice feedbacks
EARTH SYSTEM SENSITIVTY
Feedbacks due to CO2 alone
Orography forcing
Implications for Earth system sensitivity:
dTCO2 = 1.6oC
dTESS = 2.3oC
Lunt et al, Nature Geoscience, 2010
CO2 = 400ppmv…
Model dependence??
PlioMIPHaywood et al., GMD, (2011)
Vegetation:
Salzmann et al., GPC, (2008)
New boundary conditions (PRISM3)…
Hill et al (2007)
Ice:
Orography:
Based on Markwick (2007) : derived from lithologic, tectonic and fossil information, the lithologic databases of the Chicago Paleogeographic Atlas Project, and DSDP/ODP data.
Hadley 3.3 oC GISS 2.1 oC
MIROC 3.5 oC CCSM4 1.8 oC
PlioMIP ensemble:
Haywood et al, in prep
NOR-ESM 3.2 oC ECHAM 3.3 oC
CO2 ~ 400ppmv…
The mid-Pliocene: summary
Initial work suggested ESS ~ 4.6oC, ESS/CS ~ 1.5
PlioMIP with new boundary conditions suggests ESS ~ 3.6oC – 7oC, (ESS/CS ~ 2 for HadCM3)
Future – more Earth system components into the models, assessment of orbital contribution.
(1) CASE STUDY 2: The early Eocene ….
Eocene: boundary conditions
Palaeogeography (~55 - 50 Ma)
Eocene: boundary conditions
(2) CO2
1680ppmv = 6*
1120ppmv = 4*
560ppmv = 2*
Beerling and Royer, Nature Geoscience, (2011)
Runs at 1,2,4,6 * pre-industrial CO2. ~3500 years
Global mean temperature vs CO2 forcing
CS ~ 4.5 oC
CS ~ 3.3 oC
Lunt et al, Geology, 2010.
Global mean temperature vs CO2 forcing
Now include vegetation feedbacks (i.e. consider a closer approximation to Earth system sensitivity….)
CS ~ 3.3 oC
ESS ~ 5 oC
Loptson et al, in prep.
Model dependence??
EoMIP: HadCM3 (Lunt et al, 2011), CCSM (Huber and Caballero, 2011), GISS (Roberts et al, 2010), ECHAM (Heinemann et al, 2009), CCSM (WInguth et al, 2010)
terrestrial annual mean SST as a function of CO2 and model..
Model results compared with data (terrestrial)
Data points from Huber and Caballero, CP, 2011. Leaf-margin analysis and/or CLAMP, + isotopic, geochemical, and pollen estimates.
Lunt et al, in prep.
The early Eocene: summary
Vegetation feedbacks increase sensitivity by ~50%
EoMIP suggests models can produce good agreement with (most) data (but over a wide range of CO2).
Future – more coordinated modelling study.
Outlook
Don’t believe absolute numbers from a single model!! But…..
Real potential for the palaeo community to make a crucial contribution to future climate predictions, by quantifying and ‘scoring’ model performance under warm climate conditions….but….
Need a consistent and comprehensive analysis of uncertainty in both temperature and CO2 estimates.
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