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Can Earth's Two Hemispheres Get Their Climatic Act Together?
Neukom, R., Gergis, J., Karoly, D.J., Wanner, H., Curran, M., Elbert, J., Gonzalez-Rouco, F., Linsley, B.K., Moy, A.D., Mundo, I., Raible, C.C., Steig, E.J., van Ommen, T., Vance, T., Villalba, R., Zinke, J. and Frank, D. 2014. Inter-hemispheric temperature variability over the past millennium. Nature Climate Change 4: 362-367.

The authors write that "earth's climate system is driven by a complex interplay of internal chaotic dynamics and natural and anthropogenic external forcing," and they note in this regard that "recent instrumental data have shown a remarkable degree of asynchronicity between Northern Hemisphere and Southern Hemisphere temperature fluctuations, thereby questioning the relative importance of internal versus external drivers of past as well as future climate variability," citing the work of Thompson et al. (2010), Deser et al. (2012) and Friedmann et al. (2013).

What was done
As they describe it, Neukom et al. "introduce a Southern Hemisphere temperature reconstruction ensemble and assess inter-hemispheric temperature variability over the past millennium in both empirical reconstructions and state-of-the-art climate model simulations," wherein they "use an extensive Southern Hemisphere palaeoclimate data network from more than 300 individual sites (Neukom and Gergis, 2012) yielding 111 temperature predictors." This proxy collection, as they continue, is such that it "nearly doubles the number of records considered in the most advanced previous reconstruction attempt," i.e., that of Mann et al. (2008), which they indicate now allows "the development of an annually-resolved and well-verified Southern Hemisphere temperature reconstruction for the past millennium that is insensitive to moderate changes in reconstruction methodology or proxy network composition."

What was learned
The seventeen scientists report that "in conjunction with an independent Northern Hemisphere temperature reconstruction ensemble (Frank et al., 2010), this record reveals an extended cold period (1594-1677) in both hemispheres but no globally coherent warm phase during the pre-industrial (1000-1850) era," noting that "the current (post-1974) warm phase is the only period of the past millennium where both hemispheres are likely to have experienced contemporaneous warm extremes."

What it means
As a result of these newest and most comprehensive data analyses, Neukom et al. conclude, in the final sentence of their paper's abstract, that "climate system predictability on decadal to century timescales may be lower than expected based on assessments of external climate forcing and Northern Hemisphere temperature variations (Frank et al., 2010; Hegerl et al., 2006) alone." And in the concluding paragraph of the body of their paper, they state that the strong inter-hemispheric coupling in the climate model simulations they conducted "suggests that models overestimate the strength of externally-forced relative to internal climate system variability, therefore implying more limited predictability not only on regional (Deser et al., 2012; Braconnot et al., 2012) but also hemispheric scales." In addition, they say that "the stronger coherence between the Northern Hemisphere temperature reconstructions and external forcings similarly implies that detection and attribution studies (Hegerl et al., 2007) and climate sensitivity estimates (Hegerl et al., 2006; Frank et al., 2010) based on Northern Hemisphere data alone may not be representative of the global climate system."

Braconnot, P., Harrison, S.P., Kageyama, M., Bartlein, P.J., Masson-Delmotte, V., Abe-Ouchi, A., Otto-Bliesner, B. and Zhao, Y. 2012. Evaluation of climate models using palaeoclimatic data. Nature Climate Change 2: 417-424.

Deser, C., Knutti, R., Solomon, S. and Phillips, A.S. 2012. Communication of the role of natural variability in future North American climate. Nature Climate Change 2: 775-779.

Frank, D.C., Esper, J., Raible, C.C., Büntgen, U., Trouet, V., Stocker, B. and Joos, F. 2010. Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate. Nature 463: 527-532.

Friedmann, A., Hwang, Y., Chiang, J. and Frierson, D. 2013. Inter-hemispheric temperature asymmetry over the 20th century and in future projections. Journal of Climate 26: 5419-5433.

Hegerl, G., Crowley, T.J., Allen, M., Hyde, W.T., Pollack, H.N., Smerdon, J. and Zorita, E. 2007. Detection of human influence on a new, validated 1500-year temperature reconstruction. Journal of Climate 20: 650-666.

Hegerl, G.C., Crowley, T.J., Hyde, W.T. and Frame, D.J. 2006. Climate sensitivity constrained by temperature reconstructions over the past seven centuries. Nature 440: 1029-1032.

Mann, M.E., Zhang, Z.H., Hughes, M.K., Bradley, R.S., Miller, S.K., Rutherford, S. and Ni, F. 2008. Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proceedings of the National Academy of Sciences USA 105: 13,252-13,257.

Neukom, R. and Gergis, J. 2012. Southern Hemisphere high-resolution palaeoclimate records of the last 2000 years. The Holocene 22: 501-524.

Thompson, D.W.J., Wallace, J.M., Kennedy, J.J. and Jones, P.D. 2010. An abrupt drop in Northern Hemisphere sea surface temperature around 1970. Nature 467: 444-447.

Reviewed 30 July 2014