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Estimates of global ocean cooling at the Last Glacial Maximum based on sea-surface temperature and oxygen-isotope reconstructions

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The MARGO (Multi-proxy Approach for the Reconstruction of the Glacial Ocean Surface) sea-surface temperature (SST) reconstruction for the Last Glacial Maximum (LGM,~23,000-19,000 a before present) has been criticized to yield a low estimate of the fast-feedback climate sensi- tivity of less than 3 °C for a doubling of the atmospheric CO2 concentration.

Motivation

Conclusions

1.The estimated global surface change in δ18Ow of 0.9 ‰ ± 0.1 ‰ agrees with the global average change estimated by Labeyrie et al. (1987), Adkins et al. (2002) and Duplessy et al. (2002).

2.Thus the global mean cooling implied by the MARGO annual-mean SST anomaly, albeit uncertain, appears to be consistent with the MARGO oxygen isotope ratios measured on planktic foraminifera δ18Oc.

3.This gives support to the MARGO SST reconstruction, without necessarily implying a low climate sensitivity.

Combined oxygen isotope ratios measured on planktic foraminifera δ18Oc (Fig. 1, 2) with published MARGO SST anomaly for LGM (Fig. 3).

Used variational method “Data-Interpolating Variational Analysis” (DIVA – Troupin et al.) to estimate MARGO SST anomaly including error field.

Methods

Estimates of global ocean cooling at the Last Glacial Maximum based on sea-surface temperature and oxygen-isotope reconstructions

André Paul

1

, Stefan Mulitza

1

, Martin Losch

2

and MARGO Isotope Group

(1) MARUM - Center for Marine Environmental Sciences and Department of Geosciences, University of Bremen, Bremen, Germany (apaul@marum.de) (2) Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Fig. 3 MARGO summer (JAS) SST anomaly between LGM

and late Holocene.

EGU2012-9198

www.glacialoceanatlas.org

www.marum.de/en/OC5.htm

Fig. 1 MARGO isotope data.

Core tops

Last Glacial Maximum

Fig. 2 Species- specific

difference

Δδ18Oc between LGM and late

Holocene.

Results

Global change in analyzed SST 1 °C with large uncertainty and in surface δ18Ow at core locations 0.9 ‰ ± 0.1 ‰.

Regional patterns in Δδ18Oc (Fig. 2) similar to SST anomaly (Fig. 3), e.g. in the Mediterranean.

Individual error (1σ):

±0.1 ‰

Preliminary variational analysis using DIVA yields small global SST cooling with large uncertainty due to large regions void of data.

However, climate models consistent with the MARGO SST data (Hargreaves et al., 2011) still show a global cooling larger than 3 °C; those that simulated best the MARGO LGM SST reconstruction have a medium climate sensitivity between 2.8 °C and 3.3 °C for a doubling of the atmospheric CO2 concentration (Otto-Bliesner et al., 2009).

In contrast, the “adjoint method” applied to a simplified climate model gives a low climate sensitivity of 2.2 °C to 2.5 °C (Fig. 4 – cf. Paul and Losch, 2012).

Discussion

Fig. 4 Classical EBM fitted to data from GLAMAP 2000 project (Sarnthein et al., 2003). Control variables: radiative forcing

parameter ΔQ2xCO2 and diffusion coefficients.

5°x5° block averages

DIVA analyzed

field on 1°x1° grid

DIVA error field on

1°x1° grid

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