In their quest to accomplish this objective, the three Swiss researchers examined glacier recessions in the Swiss Alps over the past ten thousand years based on radiocarbon-derived ages of materials found in proglacial fluvial sediments of subglacial origin, focusing on subfossil remains of wood and peat. Combining their results with earlier data of a similar nature, they then constructed a master chronology of Swiss glacier fluctuations over the course of the Holocene.

So what did they find?

First of all, Joerin et al. report discovering that "alpine glacier recessions occurred at least 12 times during the Holocene," once again demonstrating the reality of the millennial-scale oscillation of climate that has reverberated throughout glacial and interglacial periods alike as far back in time as scientists have searched for the phenomenon (see Climate Oscillations (Millennial Variability) in our Subject Index). As a result of this finding, it is clear that 20th-century global warming was not unusual. It is was merely the latest example of what has been the norm throughout hundreds of thousands of years.

Second, they determined that glacier recessions have been decreasing in frequency since approximately 7000 years ago, and especially since 3200 years ago, "culminating in the maximum glacier extent of the 'Little Ice Age'." Consequently, the significant warming of the 20th century cannot be considered strange, since it represents a climatic rebounding from the coldest period of the current interglacial, which interglacial just happens to be the coldest of the last five interglacials (Petit et al., 1999). And when the earth has been that cold for a few centuries, it is not unnatural to expect that, once started, warming would be rather significant.

Third, the last of the major glacier recessions in the Swiss Alps occurred between about 1400 and 1200 years ago, according to Joerin et al.'s data, but between 1200 and 800 years ago, according to the data of Holzhauser et al. (2005) for the Great Aletsch Glacier. Of this discrepancy, Joerin et al. say that given the uncertainty of the radiocarbon dates, the two records need not be considered inconsistent with each other. What is more, their presentation of the Great Aletsch Glacier data indicates that the glacier's length at about AD 1000 - when there was fully 100 ppm less CO2 in the air than there is today - was just slightly less than its length in 2002, suggesting that the peak temperature of the Medieval Warm Period likely was slightly higher than the peak temperature of the 20th century. Consequently, 20th-century warming has likely not been unprecedented over the past millennium.

In conclusion, it is our feeling that being neither unusual, unnatural nor unprecedented, there is no compelling reason to attribute 20th-century global warming to anthropogenic CO2 emissions. The temperature increase of the past hundred or so years has been simply a run-of-the-mill consequence of cyclically-recurring forces of nature that have manifested themselves again and again throughout earth's history at millennial-scale intervals.

Sherwood, Keith and Craig Idso

References
Holzhauser, H., Magny, M. and Zumbuhl, H.J. 2005. Glacier and lake-level variations in west-central Europe over the last 3500 years. The Holocene 15: 789-801.

Joerin, U.E., Stocker, T.F. and Schluchter, C. 2006. Multicentury glacier fluctuations in the Swiss Alps during the Holocene. The Holocene 16: 697-704.

Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davis, M.., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E. and Stievenard, M. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399: 429-436.