To determine if the Modern Warm Period was produced by the historical rise in the air's CO2 content, as climate alarmists typically claim it was, we need to determine if there were any prior periods of equivalent or greater warmth when the atmosphere's CO2 concentration was less than what it is today. If there were, for example, there would clearly be no compelling reason to ascribe today's warmth to today's higher CO2 levels, as so many people with a political axe to grind are trying to do. Hence, we here review some studies of this subject based on proxy climate data for the Holocene - when the air's CO2 content was much lower than it is today - that were obtained in North America and from nearby ocean sediments.
Dahl-Jensen et al. (1998) used temperature measurements from two Greenland Ice Sheet boreholes to reconstruct the temperature history of the region over the past 50,000 years. The Medieval Warm Period and subsequent Little Ice Age were both evident in the data, with temperatures 1°C warmer and 0.5-0.7°C cooler than at present, respectively. After the Little Ice Age, they report that "temperatures reached a maximum around 1930 A.D." and that "temperatures have decreased during the last decades."
Wagner and Melles (2001) analyzed a sediment core taken from a lake on an island situated just off the east coast of Greenland, looking for signs of the presence of seabirds. Their data revealed sharp increases in the values of the parameters they measured between about 1100 and 700 years before the present, indicative of the summer presence of significant numbers of seabirds during what they called that "medieval warm period," which had been preceded by a several-hundred-year period of little to no bird presence. Thereafter, their data suggested another absence of birds during "a subsequent Little Ice Age," which they note was "the coldest period since the early Holocene." Finally, there was evidence of a "resettlement of seabirds during the last 100 years." However, values of the most recent biogeochemical measurements revealed a smaller presence of seabirds than what was characteristic of the Medieval Warm Period.
Darby et al. (2001) derived a 10,000-year record of climatic change from cores of a thick sequence of post-glacial sediments located on the upper continental slope off the Chukchi Sea shelf in the Arctic Ocean. Their data revealed "previously unrecognized millennial-scale variability in Arctic Ocean circulation and climate," along with evidence that "in the recent past, the western Arctic Ocean was much warmer than it is today." Indeed, they say that "Holocene variability in the western Arctic is larger than any change observed in this area over the last century" and that "temperatures may have been 5°C warmer only a few thousand years ago." And, we hasten to note, there is no evidence the air's CO2 concentration was either higher or fluctuated wildly during this period. In addition, similar results were obtained by Levac (2001) for Canada's Atlantic region using a high-resolution palynological record from the Scotian Shelf.
In summary, we note that these several studies bear testimony to the fact that Holocene climate has fluctuated more or less continuously on a millennial timescale between alternating several-hundred-year warm and cold periods, independent of any changes in the air's CO2 content. These studies also recognize the presence of a Medieval Warm Period centered at approximately the end of the first millennium AD, as well as a subsequent Little Ice Age; and most of them suggest that the Medieval Warm Period was significantly warmer than the current Modern Warm Period.
We thus conclude there is nothing unusual about the global warming of the past century or so; it is merely the most recent upswing in a persistent climatic rhythm that has reverberated throughout all the glacials and interglacials we have been able to check for its presence. Hence, there is no compelling reason to conclude that any portion of our current warmth is derived from anthropogenic CO2 emissions. The planet would likely be just as warm as it is now if the internal combustion engine had never been invented.
References
Dahl-Jensen, D., Mosegaard, K., Gundestrup, N., Clow, G.D., Johnsen, S.J., Hansen, A.W. and Balling, N. 1998. Past temperatures directly from the Greenland Ice Sheet. Science 282: 268-271.
Darby, D., Bischof, J., Cutter, G., de Vernal, A., Hillaire-Marcel, C., Dwyer, G., McManus, J., Osterman, L., Polyak, L. and Poore, R. 2001. New record shows pronounced changes in Arctic Ocean circulation and climate. EOS, Transactions, American Geophysical Union 82: 601, 607.
Levac, E. 2001. High resolution Holocene palynological record from the Scotian Shelf. Marine Micropaleontology 43: 179-197.
Wagner, B. and Melles, M. 2001. A Holocene seabird record from Raffles So sediments, East Greenland, in response to climatic and oceanic changes. Boreas 30: 228-239.