New estimates of Earth system sensitivity from the Cenozoic Introduction: Earth system sensitivity Case study 1: The mid Pliocene Case study 2: The PETM.

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.