Huang and Pollack (1997) searched the large terrestrial heat flow database compiled by the International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior for measurements suitable for reconstructing a ground surface temperature history of the earth over the last 20,000 years.  Based on a total of 6,144 qualifying sets of heat flow measurements, which they obtained from every continent of the globe, they produced a climate history they describe as being "independent of other proxy interpretations [and] of any preconceptions or biases."  This "global climate history" derived from "worldwide observations" indicates that the Medieval Warm Period was indeed warmer than the last decades of the 20th century - by perhaps as much as 0.5°C, in fact - and that the Little Ice Age was as much as 0.7°C cooler than the latter part of the 20th century.

In another intriguing paper, Steig (1999) reviews what we know about climate change throughout the Holocene, focusing on the period from 7000 to 5000 years ago, when "land air temperatures appear to have declined across much of the globe."  During this period, there is evidence to suggest that the atmosphere's CO2 concentration may have increased by just over 10 ppm at a time when "the evidence from ice cores suggests that both hemispheres cooled," thereby providing an example of atmospheric CO2 concentration and near-surface global air temperature moving in opposite directions.

With respect to the Northern and Southern Hemispheres cooling in unison during the middle Holocene, Keigwin and Boyle (2000) note that the same has likely been true of the millennial-scale climatic oscillations that periodically produce Medieval Warm Period and Little Ice Age conditions.  They report, for example, that "mounting evidence indicates that the Little Ice Age was a global event, and that its onset was synchronous within a few years in both Greenland and Antarctica."  In addition, the work of Oppo et al. (1998) suggests that these millennial-scale climatic oscillations have been occurring throughout both glacial and interglacial periods alike for at least the past half million years.

Overpeck and Webb (2000) have also reviewed what we know about various aspects of climate change throughout the Holocene.  In the category of ENSO/Pacific Variability, they note that shifts in ENSO frequency occur at annual, interannual and multidecadal intervals, providing evidence that "ENSO may change in ways that we do not yet understand."  In fact, they report that data from corals seem to suggest that "interannual ENSO variability, as we now know it, was substantially reduced, or perhaps even absent," during the middle Holocene.

In the category of African-Asian Monsoon Variability, Overpeck and Webb indicate that large and abrupt changes in monsoon moisture availability have occurred multiple times throughout the past several thousand years, but that "a lack of research prevents precise reconstruction, explanation, or modeling of these changes."  Likewise, in the category of North American Drought Variability, they note that "droughts of the 20th century were relatively minor compared with those in the past," which they say "opens up the possibility that future droughts may be much greater as well."

Clearly, all of these large and dramatic climate changes of the current interglacial, i.e., the Holocene, have occurred without any help from the CO2 emissions of man; yet humanity is blamed for nearly everything that happens - or is only imagined to happen - with respect to current weather phenomena (see our Editorial of 31 January 2001: Sound the Alarm Bells!).  If the truth be told, however - as we try to do in reviewing the continual progress of scientific research in these areas - nature still rules in all aspects of climate change.

References
Huang, S. and Pollack, H.N.  1997.  Late Quaternary temperature changes seen in world-wide continental heat flow measurements.  Geophysical Research Letters 24: 1947-1950.

Keigwin, L.D. and Boyle, E.A.  2000.  Detecting Holocene changes in thermohaline circulation.  Proceedings of the National Academy of Sciences USA 97: 1343-1346.

Oppo, D.W., McManus, J.F. and Cullen, J.L.  1998.  Abrupt climate events 500,000 to 340,000 years ago: evidence from subpolar North Atlantic sediments.  Science 279: 1335-1338.

Overpeck, J. and Webb, R.  2000.  Nonglacial rapid climate events: Past and future.  Proceedings of the National Academy of Sciences USA 97: 1335-1338.

Steig, E.J.  1999.  Mid-Holocene climate change.  Science 286: 1485-1487.