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Temperature Reconstructions of the Past Millennium
Reference
Esper, J., Wilson, R.J.S., Frank, D.C., Moberg, A., Wanner, H. and Luterbacher, J.  2005.  Climate: past ranges and future changes.  Quaternary Science Reviews 24: 2164-2166.

What was done
In this Viewpoint article, several authors who have previously published temperature reconstructions covering all or portions of the past millennium weigh in with their thoughts on why there are differences among such reconstructions and what it will take to reduce present uncertainties to gain a more complete and correct understanding of temperature changes over the past thousand years.

What was learned
According to the six scientists, we understand, for the most part, the shape of long-term climate fluctuations better than their amplitudes.  For instance, nearly all 1000-year temperature reconstructions capture the major climatic episodes of the Medieval Warm Period, Little Ice Age and Current Warm Period; but for various reasons they exhibit differences in the degree of climatic warming or cooling experienced in the transitions between them, which for decadal means may amount to as much as 0.4 to 1.0C.  Among the list of action items they believe will help to reduce these discrepancies are their suggestions to (1) reduce the calibration uncertainty among the proxies, (2) ensure the accurate preservation and assessment of low-to-high frequency variation in proxy data, (3) use appropriate frequency bands to best fit instrumental data, (4) avoid the use of regional tree-ring and other palaeo records in which long-term trends (low-frequency variations) are not preserved, (5) select instrumental data with which to compare proxy records to avoid incorrect alterations to the observational data that can result from homogeneity adjustments and methodological differences, and (6) obtain more proxy data that cover the full millennium and that represent the same spatial domain as the instrumental target data (e.g., hemisphere).

What it means
As stated by Esper et al., knowledge of the correct amplitude of the major climatic episodes of the past millennium is "critical for predicting future trends."  Why?  Because if it can be shown that the amplitudes of the major historical climate episodes were as large as, or even greater than, that of 20th-century global warming, there would be a "redistribution of weight towards the role of natural factors in forcing temperature changes, thereby relatively devaluing the impact of anthropogenic emissions and affecting future predicted scenarios."  And if that turns out to be the case, efforts to reduce greenhouse gas emissions via national or international agreements, such as the Kyoto Protocol, "would be less effective than thought."  In fact, they could even be counterproductive, in view of the significant boost to agricultural production that is provided by the aerial fertilization effect of atmospheric CO2 enrichment, which will likely be sorely needed to provide for the planet's growing population in the years and decades to come.

Reviewed 4 January 2006