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The Medieval Warm Period and Little Ice Age: Their Untimely Demise and Welcome Resurrection
Volume 5, Number 13: 27 March 2002

The Medieval Warm Period and subsequent Little Ice Age - which followed hard on the heels of the Roman Warm Period and Dark Ages Cold Period (McDermott et al., 2001) - were long considered to be classic examples of the warm and cold phases of a millennial-scale climate oscillation that has reverberated seemingly endlessly throughout glacial and interglacial periods alike (Oppo et al., 1998; McManus et al., 1999), as well as across the early Pleistocene (Raymo et al., 1998).

In addition to their intrinsic historical value, the last of these warm and cold periods have particular relevance to the highly-charged global warming debate. If there was truly a period near the beginning of the past millennium when temperatures were as warm as they are presently, for example, but when the atmosphere's CO2 concentration was about 90 ppm lower than it is today, and if that period was followed by a several-centuries-long cold period with essentially no decline in the air's CO2 content, there would be little basis for invoking the 20th-century increase in atmospheric CO2 concentration as a reason for the planet's return to the degree of warmth it had experienced a millennium earlier, as Idso (1988) argued nearly 15 years ago and Broecker (1999, 2001) has reminded us more recently.

Faced with this dilemma, the political forces that view the theory of CO2-induced global warming as a mighty lever for moving the nations of the earth in the direction of global governance - via the establishment of an entity with power to regulate nearly all forms of human enterprise in the guise of protecting the planet from the climatic consequences of CO2-producing activities - realized they had a serious problem on their hands. To keep their political juggernaut alive, therefore, they had to "deep-six" the concept of both the Medieval Warm Period and the Little Ice Age, in order to imbue their program with a semblance of rationality; and they saw the perfect opportunity to do so in a pair of papers published by Mann et al. (1998, 1999).

These papers presented an entirely new perspective on earth's climatic history over the past thousand years, which was different from what had previously been accepted by even the Intergovernmental Panel on Climate Change (Houghton et al., 1990). Whereas IPCC documents up to at least 1995 had faithfully depicted the existence of both the Medieval Warm Period and Little Ice Age, the new history - derived from a few select proxy temperature records - showed, in the words of Esper et al. (2002), "an almost linear temperature decrease from the year 1000 to the late 19th century, followed by a dramatic and unprecedented temperature increase to the present time," which is now routinely described as "the warmest period of the past millennium."

Thus died the Medieval Warm Period; and with its passing, the Little Ice Age also succumbed. With not much else to block their progress, the political forces behind the Kyoto Protocol consequently began to press forward in a major way; and they would probably have quickly achieved their goals, but for the stubborn resolve of a U.S. president who refused to cooperate. Now, however, thanks to the meticulous and careful work of Esper et al., both of these unique climatic periods have been resurrected, and they stand as stronger and healthier witnesses than ever to the intellectual bankruptcy of the climate-alarmist claim that the warming of the past century is CO2-induced.

So what did Esper et al. do? In the simplest of terms, they employed an analysis technique that allows accurate long-term climatic trends to be derived from individual tree-ring series that are of much shorter duration than the potential climatic oscillation being studied; and they applied this technique to over 1200 tree-ring series derived from 14 different locations scattered over the extratropical region of the Northern Hemisphere.

Two separate chronologies were thus developed: one from trees that exhibited age trends that are weakly linear and one from trees with age trends that are more nonlinear. The results, in their words, were "two nearly independent tree-ring chronologies covering the years 800-1990," which were "very similar over the past ~1200 years." These tree-ring histories were then calibrated against Northern Hemispheric (0 to 90N) mean annual instrumental temperatures from the period 1856-1980 to make them compatible with the temperature reconstructions of Mann et al.

What do the results show? The biggest difference between the Esper et al. and Mann et al. temperature histories is the degree to which the coolness of the Little Ice Age is expressed. The Little Ice Age is much more evident in the record of Esper et al., and its significantly lower temperatures are what make the Medieval Warm Period stand out more dramatically in their temperature reconstruction. Also, they note that "the warmest period covers the interval 950-1045, with the peak occurring around 990." This finding, they say, "suggests that past comparisons of the Medieval Warm Period with the 20th-century warming back to the year 1000 have not included all of the Medieval Warm Period and, perhaps, not even its warmest interval."

In commenting on these findings in a companion "perspective" paper, Briffa and Osborn (2002) make several interesting and important points. First, they acknowledge that "the last millennium was much cooler than previously interpreted" and that "an early period of warmth in the late 10th and early 11th centuries is more pronounced than in previous large-scale reconstructions." In fact, the Esper et al. record makes it abundantly clear that the peak warmth of the Medieval Warm Period was fully equivalent to the warmth of the present.

This fact reaffirms the point raised by Idso (1988), i.e., that there is no need to invoke CO2-induced global warming as a cause of the planet's recovery from the global chill of the Little Ice Age. "Since something other than atmospheric CO2 variability was ... clearly responsible for bringing the planet into the Little Ice Age," as he phrased it, "something other than atmospheric CO2 variability may just as well have brought the planet out of it." And that something else, as suggested by Esper et al., is probably "the 1000- to 2000-year climate rhythm (1470 500 years) in the North Atlantic, which may be related to solar-forced changes in thermohaline circulation," as has recently been described in compelling detail by Bond et al. (2001) and which we heartily endorse.

Briffa and Osborn also note that Esper et al.'s record clearly shows that the warming of the 20th century was actually "a continuation of a trend that began at the start of the 19th century." In addition, the Esper et al. record indicates that the Northern Hemisphere warmed in a consistent near-linear fashion over this entire 200-year period, contrary to the climate-alarmist claim of unprecedented warming over only the last century. Hence, the new data do great damage to the claim that CO2-enhanced greenhouse warming is responsible for the temperature increase that brought us out of the Little Ice Age, since the increase in the atmosphere's CO2 concentration over this period was highly non-linear, rising by only 10 to 15 ppm over the 19th century, but by fully 70 to 75 ppm over the 20th century, with no analogous increase in the latter period's rate of warming.

Finally, Briffa and Osborn say that "we need to know why it was once so warm and then so cool, before we can say whether 21st-century warming is likely to be nearer to the top or the bottom of the latest IPCC [predicted temperature] range." Actually, we probably already know the answer to this question: the extremes of warmth and coolness to which they refer were likely caused by "solar-forced changes in thermohaline circulation," as suggested by Esper et al. and described by Bond et al. In any event, it is becoming ever more clear with each passing day that these significant climatic changes were not caused by changes in the air's CO2 content.

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

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