The climate of the current interglacial, i.e., the Holocene, exhibits a millennial-scale oscillation that has resulted in several-hundred-year alternating warm and cold periods, the most recent of which is the Modern Warm Period that many people are telling us is due to the CO2-emitting activities of man. Before this claim is accepted, however, it would seem only prudent to determine if there is any evidence for similar warm periods in earth's recent history that were clearly not man-induced. If there were some such warm periods, for example, there would be no reason to accept the climate-alarmist claim that human activities have produced the one in which we currently live.
In an attempt to help resolve this issue, Karlen (1998) examined proxy climate data related to changes in the size of glaciers, the altitude of the alpine tree-limit, and the width of tree rings to derive a history of summer temperatures in Scandinavia over the last 10,000 years. The results revealed a number of long- and short-term temperature fluctuations, the warmer periods of which were "about 2°C warmer than at present." In addition, the data indicated the observed climate changes were likely driven by changes in solar radiation. It was also concluded that "the frequency and magnitude of changes in climate during the Holocene do not support the opinion that the climatic change of the last 100 years is unique." Indeed, the author of the report bluntly states "there is no evidence of a human influence so far."
In pursuing the same goal, Bianchi and McCave (1999) studied the grain sizes of North Atlantic deep-sea sediment cores that appeared to be related to the flow rate of the thermohaline circulation of the ocean in that part of the world. Covering the last 11,000 years, these data also showed several warm/cool oscillations that had a quasi-periodicity on the order of 1500 years. These oscillations, according to the scientists, were comparable to the Little Ice Age and the Medieval Warm Period and appear to be a "recurrent feature" of Holocene climatic history.
In another approach to the question, Hormes et al. (2001) determined the ages of sub-fossil wood and peat samples from six glacier forelands in the Central Swiss Alps. Their data revealed several periods of glacier recession during the Holocene beyond present glacier positions. Observational records also indicated that since the 19th century the glaciers under study have twice retreated and subsequently readvanced, around 1920 and 1980. Hence, the current terminus positions of the Central Swiss Alps glaciers fall well within their natural range of Holocene variability, indicating nothing unusual about the region's present climate.
Utilizing yet another technique, McDermott et al. (2001) derived a ð18O record from a stalagmite discovered in a cave in southwestern Ireland with a time resolution that they say is "approximately an order of magnitude better than in the North Atlantic cores that record evidence for quasi-periodic (1475 ± 500 year) ice rafting during the Holocene." Their data exhibited climatic variations that are "broadly consistent with a Medieval Warm Period at ~1000 ± 200 years ago and a two-stage Little Ice Age." Also evident in their data are the ð18O signatures of the earlier Roman Warm Period and the Dark Ages Cold Period that comprised the previous full climatic cycle of the region.
Finally, Allen et al. (1999) analyzed sediment cores from a lake in southern Italy and from the Mediterranean Sea, deriving high-resolution climate and vegetation data sets for the last 100,000 years in this region. Over the warmest portion of the record, i.e., the Holocene, the organic carbon content of the vegetation reached its highest level, which was more than twice as great as that experienced over the earlier glacial portion of the record. Other data indicated that the greater vegetation cover of the Holocene led to less soil erosion.
In summary, many records bear testimony to the reality of a millennial-scale oscillation in Holocene climate in Europe, the warm and cold extremes of which are similar to what we know as the Medieval Warm Period and Little Ice Age. Where comparisons are possible, the available data suggest that the Modern Warm Period in which we currently live is not much different from that of most earlier warm periods and, in fact, may actually not yet be as warm as some of them. The data of Allen et al. (1999) additionally suggest that the biosphere was a full participant in the various climatic fluctuations that occurred, both prior to and during the Holocene, "contrary to widely held views that vegetation is unable to change with such rapidity." And, of course, all indications are that warmer has always been better than colder in terms of plant productivity. In conclusion, therefore, there would appear to be no compelling reason to believe that any of the warming of the past century or so was necessarily produced by the CO2-emitting activities of man ... although even if some of it were, that would clearly not be bad!
References
Allen, J.R.M., Brandt, U., Brauer, A., Hubberten, H.-W., Huntley, B., Keller, J., Kraml, M., Mackensen, A., Mingram, J., Negendank, J.F.W., Nowaczyk, N.R., Oberhansli, H., Watts, W.A., Wulf, S. and Zolitschka, B. 1999. Rapid environmental changes in southern Europe during the last glacial period. Nature 400: 740-743.
Bianchi, G.G. and McCave, I.N. 1999. Holocene periodicity in North Atlantic climate and deep-ocean flow south of Iceland. Nature 397: 515-517.
Hormes, A., Muller, B.U. and Schluchter, C. 2001. The Alps with little ice: evidence for eight Holocene phases of reduced glacier extent in the Central Swill Alps. The Holocene 11: 255-265.
Karlen, W. 1998. Climate variations and the enhanced greenhouse effect. Ambio 27: 270-274.
McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem ð18O record from SW Ireland. Science 294: 1328-1331.