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Earth's Temperature: Deconstructing the Past to Predict the Future
Qian, W.-H. and Lu, B. 2010. Periodic oscillations in millennial global-mean temperature and their causes. Chinese Science Bulletin 55: 4052-4057.

The authors say that "to understand the causes of 20th century global warming, the contributions of natural variability and human activities need to be determined," and they state that "to make sound predictions for this century's climate, understanding past climate change is important."

What was done
In an attempt to gain that understanding, Qian and Lu began with the reconstructed global-mean temperature anomaly history of Mann et al. (2008), combined with HadCRUT3 data for 1000-2008, relative to 1961-1990 (Brohan et al., 2006); and after removing the mean temperatures of the Medieval Warm Period (MWP), the Little Ice Age (LIA) and what they denoted the Global Warming Period (GWP), they used a wavelet transform procedure to identify four oscillations in the millennial temperature time series with periods of 21.1, 62.5, 116.0 and 194.6 years. Next, they similarly examined a reconstructed 400-year solar radiation series based on 10Be data (Lean et al., 1995; Bard and Frank, 2006), using the results they obtained "to analyze their causality relationship" with the periodic oscillations they had detected in the reconstructed millennial global-mean temperature series.

What was learned
The two Chinese researchers determined that "the ~21-year, ~115-year and ~200-year periodic oscillations in global-mean temperature are forced by and lag behind solar radiation variability," and they report that the "relative warm spells in the 1940s and the beginning of the 21st century resulted from overlapping of warm phases in the ~21-year and other oscillations," noting that "between 1994 and 2002 all four periodic oscillations reached their peaks and resulted in a uniquely warm decadal period during the last 1000 years," which latter time interval represents the approximate temporal differential between the current Global Warming Period and the prior Medieval Warm Period.

What it means
Very interesting is the fact that Qian and Lu needed no help from greenhouse gas emissions to reconstruct the past thousand-year history of earth's global mean temperature; it was sufficient to merely employ known oscillations in solar radiation variability. As for the future, they predict that "global-mean temperature will decline to a renewed cooling period in the 2030s, and then rise to a new high-temperature period in the 2060s."

Bard, E. and Frank, M. 2006. Climate change and solar variability: What's new under the sun? Earth and Planetary Science Letters 248: 1-14.

Brohan, P., Kennedy, J.J., Harris, I., Tett, S.F.B. and Jones, P.D. 2006. Uncertainty estimates in regional and global observed temperature changes: A new dataset from 1850. Journal of Geophysical Research 111: 10.1029/2005JD006548.

Lean, J., Beer, J. and Bradley, R. 1995. Reconstruction of solar irradiance since 1610: Implications for climate change. Geophysical Research Letters 22: 3195-3198.

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.

Reviewed 18 May 2011