Climate Sensitivity Estimates From Paleoclimate (LGM) Data

Transcription

1 Climate Sensitivity Estimates From Paleoclimate (LGM) Data Andreas Schmittner Oregon State University College of Earth, Ocean, and Atmospheric Sciences May 22, 2013 Lamont Doherty Earth Observatory Columbia University

2 Climate Sensitivity Estimates From Paleoclimate (LGM) Data Andreas Schmittner Oregon State University College of Earth, Ocean, and Atmospheric Sciences With help from N. Urban, J. Shakun, N. Mahowald, P. Clark, A. Mix, P. Bartlein, A. Rosell-Mele Funded by NSF s Paleoclimate Program May 22, 2013 Lamont Doherty Earth Observatory Columbia University

3 Climate Sensitivity Estimates From Paleoclimate (LGM) Data Andreas Schmittner Oregon State University College of Earth, Ocean, and Atmospheric Sciences With help from N. Urban, J. Shakun, N. Mahowald, P. Clark, A. Mix, P. Bartlein, A. Rosell-Mele Funded by NSF s Paleoclimate Program May 22, 2013 Lamont Doherty Earth Observatory Columbia University Reconstructions from Sea Surface Temperatures Multiproxy: MARGO (2009) Land Surface Air Temperature Pollen: Bartlein et al. (2010) Additional data Shakun et al. (2012)

4 Climate Sensitivity Estimates From Paleoclimate (LGM) Data Andreas Schmittner Oregon State University College of Earth, Ocean, and Atmospheric Sciences With help from N. Urban, J. Shakun, N. Mahowald, P. Clark, A. Mix, P. Bartlein, A. Rosell-Mele ΔSSTLGM = 1.9±1.8 K Funded by NSF s Paleoclimate Program May 22, 2013 Lamont Doherty Earth Observatory Columbia University Reconstructions from Sea Surface Temperatures Multiproxy: MARGO (2009) Land Surface Air Temperature Pollen: Bartlein et al. (2010) Additional data Shakun et al. (2012)

5 Climate Sensitivity CS = ΔT/ΔF ΔT: global mean surface air temperature change [K] ΔF: forcing = instantaneous perturbation of radiative balance at the top of the atmosphere [W/m 2 ] ECS2xC = ΔT2xC ECS2x: Equilibrium Climate Sensitivity for a doubling of atmospheric CO2 ΔF2xC = 3.7 W/m 2 ±10%

6 Roe & Baker (2007) Science Knutti and Hegerl (2008) Nature Geoscience

7 Let s assume a high climate sensitivity. ΔT 2 xc = 8K Let s also assume climate sensitivity does not depend on the background climate. ΔT LGM ΔF LGM = ΔT 2 xc ΔF 2 xc ΔT LGM = ΔT 2 xc ΔF 2 xc ΔF LGM 8K 4Wm 2 6Wm 2 = 12K

8 Approach: 1.Use models with different climate sensitivities 2.Numerical experiments: 1. Control 2. 2xCO2 3. LGM 3.Compare with LGM reconstructions Schmittner et al. (2011) Science

9 University of Victoria Climate Model (Weaver et al. 2001) includes 2D (1 layer) energy moisture balance atmosphere, 3D ocean general circulation model, sea ice, vegetation (Model of Intermediate Complexity) Symbols: Observations Lines: Models with different ECS Only one model parameter varied! Schmittner et al. (2011) Science

10 LGM simulations include forcing due to insolation changes greenhouse gases ice sheet dust Mahowald et al., (2006) JGR

11 black line: reconstructions grey shadings ± 1, 2, 3 K Zonally Averaged Temperature Changes (LGM minus modern) colored lines: models ECS2xC Schmittner et al. (2011) Science

12 High climate sensitivity models simulate a completely snow and ice covered planet (Snowball Earth) Schmittner et al. (2011) Science

13 Comparison of best-fitting model with reconstructions ΔTLGM = 3±0.7 K ΔSSTLGM = 1.7±0.4 K r=0.5 Schmittner et al. (2011) Science

14 Bayesian Statistics (Nathan Urban): Probability Distribution for ECS2xC Best estimate (median) 2.3 K (may be biased) 66 % probability K (may be overconfident) 90 % K (may be overconfident) Schmittner et al. (2011) Science

15 Issues Reconstruction bias: e.g. low resolution cores Model bias: best fitting model underestimates polar amplification and land-sea contrast (perhaps related to atmospheric heat flux parameterization, see comment by Fyke and Eby (2012) Science) Results may be biased or overconfident since - Uncertainty due to cloud albedo changes not considered - Forcing uncertainty not considered - Relation between LGM and 2xC may be model dependent

16 Annan and Hargreaves (2013, CP): PMIP2 models scaled to match data ΔT LGM = (4 ± 0.8)K r=0.73

17 mean(ecs) = 2.3 K 5-95% = K Hargreaves et al. (2013) GRL

18 LGM constraints on climate sensitivity High values (>6 K) seem exceedingly unlikely Median ~ 2.5 K Future work will need to incorporate more uncertainties in a larger model ensemble

19 PMIP3 Models *Annan & Hargreaves Clim. Past. Disc. ** MARGO r=0.63

20 Thanks

21 Posterior Prior based on equally probable models individual model Hargreaves et al. (2013) GRL

22 Efficacy of LGM Forcing = CSLGM/CS2xC Crucifix (2006) Geophys. Res. Lett.

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