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.
To further investigate this topic, 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."
In discussing their findings, 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."
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."
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