Earth's Temperature History: How Well Is It Known?

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The primary empirical evidence (as opposed to the theoretical predictions derived from climate models) for believing that "business as usual," with respect to anthropogenic CO2 emissions, will produce unprecedented global warming and lead to a host of catastrophic consequences (which are typically claimed to be more serious than nuclear warfare and global terrorism) is the temperature history of the planet over the past millennium, which is typically depicted by climate alarmists as slowly declining for approximately nine hundred years and then rising dramatically to unprecedented levels over the course of the 20th century.  We have long criticized this representation of the planet's climatic history for ignoring the non-CO2-induced millennial-scale oscillation of climate that produced the Medieval Warm Period of a thousand years ago, the subsequent Little Ice Age and, last of all, the Modern Warm Period (which climate alarmists largely attribute to anthropogenic CO2 emissions).  A new paper by Esper et al. (2005) now adds another dimension to this important subject.

The team of two Swiss and two British scientists began by selecting four representations of earth's surface air temperature history over the past thousand years (Jones et al., 1998; Mann et al., 1999; Briffa, 2000; Esper et al., 2002) for inclusion in an analysis designed to reveal the significance of problems associated with some of those histories that rarely make their way into public discussions, noting that "these records were developed using tree ring data alone or using multi-proxy data, and are reported to represent different regions (e.g. Northern Hemisphere (NH) extra-tropics, or full NH)," thereby highlighting two of several factors (different types of data, different areas of applicability) that lead to different results.  Other sources of divergent results that Esper et al. (2005) investigated were methodological differences, including "using scaling or regression, the calibration time period, and smoothing data before calibration."  They also point out that different histories sometimes represent different seasons of the year, that they either include or exclude sea surface temperatures, that the available data are "more uncertain back in time," and that the average temperature as one travels back in time "becomes biased towards Europe, North America, and areas in Asia."

Reporting on the results of their analysis of the effects of various scaling and regression approaches applied in the recent scientific literature to proxy-based temperature records, Esper et al. (2005) say that "these various approaches alone [our italics] can result in differences in the reconstructed temperature amplitude ["measured from the coldest to warmest decades"] of about 0.5°C," which difference, in their words, is "equivalent to the mean annual temperature change for the Northern Hemisphere reported in the last IPCC report for the 1000-1998 period."  What is more, they remind us that "consideration of temporally changing spatial coverage and uncertainty in both the instrumental and proxy data, as expressed by confidence limits accompanying such records, would further increase the range of amplitude estimates over the past millennium."  Last of all, and on top of these problems, they say that "when linear regression is used for calibration, the variance of a proxy record remains below that of the target data, leaving the visual impression that the recent dynamics are substantially larger than the historic ones when splicing such records together," which is precisely the impression the world's climate alarmists hope to convey when they attach the modern instrumental record onto the end of calibrated proxy data, as in the case of the infamous hockeystick temperature record of Mann et al. (1999).

In view of the many problems associated with existing real-world temperature histories, the precautionary principle, so highly touted by climate alarmists, would seem to suggest that humanity proceed with extreme caution with respect to any proposed measures for dealing with the "problem" of anthropogenic CO2 emissions.  Until we have a better-defined record of the planet's temperature history, we cannot be confident there even is a climatic problem.  And if there isn't, cutting anthropogenic CO2 emissions will only deny us the many biological benefits of the aerial fertilization and water-conserving effects of atmospheric CO2 enrichment, which will be sorely needed to adequately feed the burgeoning population of the planet in the years and decades ahead.

Sherwood, Keith and Craig Idso

References
Briffa, K.R.  2000.  Annual climate variability in the Holocene: Interpreting the message of ancient trees.  Quaternary Science Reviews 19: 87-105.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Esper, J., Frank, D.C., Wilson, R.J.S. and Briffa, K.R.  2005.  Effect of scaling and regression on reconstructed temperature amplitude for the past millennium.  Geophysical Research Letters 32: 10.1029/2004GL021236.

Jones, P.D., Briffa, K.R., Barnett, T.P. and Tett, S.F.B.  1998.  High-resolution palaeoclimatic records for the last millennium: Integration, interpretation and comparison with general circulation model control run temperatures.  The Holocene 8: 455-471.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.