"Evidence of unusual late 20th century warming from an Australasian temperature reconstruction spanning the last millennium," J. Climate, Joëlle Gergis et al.

Journal of Climate (2012); doi: http://dx.doi.org/10.1175/JCLI-D-11-00649.1

Evidence of unusual late 20th century warming from an Australasian temperature reconstruction spanning the last millennium

Joëlle Gergis,¹ Raphael Neukom,¹ Steven J. Phipps,^2,3 Ailie J. E. Gallant,¹ David J. Karoly,¹ and PAGES Aus2K Project Members^†

¹ School of Earth Sciences, University of Melbourne, Australia

² Climate Change Research Centre, University of New South Wales, Sydney, Australia

³ ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, Australia

Abstract

This study presents the first multi-proxy warm season (September-February) temperature reconstruction for the combined land and oceanic region of Australasia (0° S - 50° S, 110° E - 180° E). We perform a 3000-member ensemble Principal Component Reconstruction (PCR) using 27 temperature proxies from the region. The proxy network explained 69% of the inter-annual variance in the HadCRUT3v SONDJF spatial mean temperature over the 1921-1990 calibration period. Applying eight stringent reconstruction ‘reliability’ metrics identified post A.D. 1430 as the highest quality section of the reconstruction, but also revealed a skillful reconstruction is possible over the full A.D. 1000-2001 period.


The average reconstructed temperature anomaly in Australasia during A.D. 1238-1267, the warmest 30-year pre-instrumental period, is 0.09 °C (±0.19 °C) below 1961-1990 levels. Following peak pre-industrial warmth, a cooling trend culminates in a temperature anomaly of 0.44 °C (±0.18 °C) below 1961-1990 levels between A.D. 1830-1859. A preliminary assessment of the roles of solar, volcanic, and anthropogenic forcings and natural ocean-atmosphere variability is performed using CSIRO Mk3L model simulations and independent palaeoclimate records. Solar and volcanic forcing does not have a marked influence on reconstructed Australasian temperature variations, which appear to be masked by internal variability.

In 94.5% of the 3,000-member reconstruction ensemble, there are no other warm periods in the past 1,000 years that match or exceed post-1950 warming observed in Australasia. The unusual 20th century warming cannot be explained by natural variability alone, suggesting a strong influence of anthropogenic forcing in the Australasian region

^†Aus2K project member data and other contributions from Kathryn Allen, Patrick Baker, Gretel Boswijk, Brendan Buckley, Matthew Brookhouse, Edward Cook, Louise Cullen, Mark Curran, Rosanne D’Arrigo, Pavla Fenwick, Anthony Fowler, Ian Goodwin, Pauline Grierson, Erica Hendy, Braddock Linsley, Janice Lough, Andrew Lorrey, Helen McGregor, Andrew Moy, Jonathan Palmer, Christopher Plummer, Chris Turney, Tessa Vance, Tas Van Ommen and Limin Xiong.

Corresponding author: Dr Joëlle Gergis, School of Earth Sciences, University of Melbourne, VIC 3010, Australia. Ph: +61 3 834 49868, Fax: +61 3 834 47761, Mobile: + 61 415 449 241. e-mail: jgergis@unimelb.edu.au