Fresh hockey sticks from the Southern Hemisphere

In the Northern Hemisphere, the late 20th / early 21st century has been the hottest time period in the last 400 years at very high confidence, and likely in the last 1000 – 2000 years (or more). It has been unclear whether this is also true in the Southern Hemisphere. Three studies out this week shed considerable new light on this question. This post provides just brief summaries; we’ll have more to say about these studies in the coming weeks.

First, a study by Gergis et al., in the Journal of Climate [Update: this paper has been put on hold – see comments] uses a proxy network from the Australasian region to reconstruct temperature over the last millennium, and finds what can only be described as an Australian hockey stick. They use an ensemble of 3000 different reconstructions, using different methods and different subsets of the proxy network. Worth noting is that while some tree rings are used (which can’t be avoided, as there simply aren’t any other data for some time periods), the reconstruction relies equally on coral records, which are not subject to the same potential (though often-overstated) issues at low frequencies. The conclusion reached is that summer temperatures in the post-1950 period were warmer than anything else in the last 1000 years at high confidence, and in the last ~400 years at very high confidence.

Gergis et al. Figure 4, showing Australian mean temperatures over the last millennium, with 95% confidence levels.

Second, Orsi et al., writing in Geophysical Research Letters, use borehole temperature measurements from the WAIS Divide site in central West Antarctica, a region where the magnitude of recent temperature trends has been subject of considerable controversy. The results show that the mean warming of the last 50 years has been 0.23°C/decade. This result is in essentially perfect agreement with that of Steig et al. (2009) and reasonable agreement with Monaghan (whose reconstruction for nearby Byrd Station was used in Schneider et al., 2012). The result is totally incompatible (at >95%>80% confidence) with that of O'Donnell et al. (2010).

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References

  1. J. Gergis, R. Neukom, S.J. Phipps, A.J.E. Gallant, and D.J. Karoly, "Evidence of unusual late 20th century warming from an Australasian temperature reconstruction spanning the last millennium", Journal of Climate, pp. 120518103842003, 2012. http://dx.doi.org/10.1175/JCLI-D-11-00649.1
  2. A.J. Orsi, B.D. Cornuelle, and J.P. Severinghaus, "Little Ice Age cold interval in West Antarctica: Evidence from borehole temperature at the West Antarctic Ice Sheet (WAIS) Divide", Geophysical Research Letters, vol. 39, pp. n/a-n/a, 2012. http://dx.doi.org/10.1029/2012GL051260
  3. E.J. Steig, D.P. Schneider, S.D. Rutherford, M.E. Mann, J.C. Comiso, and D.T. Shindell, "Warming of the Antarctic ice-sheet surface since the 1957 International Geophysical Year", Nature, vol. 457, pp. 459-462, 2009. http://dx.doi.org/10.1038/nature07669
  4. D.P. Schneider, C. Deser, and Y. Okumura, "An assessment and interpretation of the observed warming of West Antarctica in the austral spring", Clim Dyn, vol. 38, pp. 323-347, 2011. http://dx.doi.org/10.1007/s00382-010-0985-x
  5. R. O’Donnell, N. Lewis, S. McIntyre, and J. Condon, "Improved Methods for PCA-Based Reconstructions: Case Study Using the Steig et al. (2009) Antarctic Temperature Reconstruction", Journal of Climate, vol. 24, pp. 2099-2115, 2011. http://dx.doi.org/10.1175/2010JCLI3656.1