We begin with the study of Briffa et al. (1998), who used tree-ring wood density data from over 380 boreal forest locations in the Northern Hemisphere to reconstruct summertime temperatures going back to AD 1400, while looking for effects of large volcanic eruptions on climate. This work indicated that all major documented volcanic eruptions resulted in significant Northern Hemispheric cooling trends in the year following their occurrence. The largest of these temperature declines was 0.81°C in 1601, the year following the eruption of Huaynaputina in Peru, while of the six centuries the researchers examined, the seventeenth century experienced the greatest number of climatically significant eruptions, exhibiting a total of six such events. Strong temperature anomalies also suggest there were three other major eruptions during the late seventeenth century that have not been reported in historical accounts. And it is noteworthy that every Northern Hemispheric temperature drop of 0.3°C or more since 1400 (19 total events) followed on the heels of a major volcanic eruption. Altogether, this study thus demonstrated the strong linkage between volcanic activity and large-scale temperature variability and may help to explain the period of cold temperatures that prevailed during the Little Ice Age, illustrating how closely-spaced multiple eruptions can reduce hemispheric temperatures on decadal and multi-decadal time scales and how a lack of eruptions can result in periods of warmer global temperatures.

Three years later, Harris and Chapman (2001) analyzed 439 borehole temperature logs to determine the temperature history of the mid-latitude sector of the Northern Hemisphere (30°-60°N). As might have been expected, the data revealed a vast array of results for the many sites, with some even depicting cooling over the past two centuries. In the mean, however, as the two scientists described it, their analysis indicated "0.7 ± 0.1°C of ground warming between preindustrial time and the interval 1961-1990," thereby portraying a temperature increase well in line with what should have been expected as the earth recovered from the global chill of the Little Ice Age.

One year later, Esper et al. (2002) used an analysis technique that allows accurate long-term climatic trends to be derived from 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 locations scattered over the extratropical region of the Northern Hemisphere, developing two separate chronologies: one from trees that exhibited age trends that were weakly linear and one from trees with age trends that were more nonlinear. The results were two nearly independent -- but very similar -- tree-ring histories covering the years 800-1990, which they calibrated against Northern Hemispheric (0 to 90°N) mean annual instrumental temperatures for the period 1856-1980 in order to make them compatible with the temperature reconstruction of Mann et al. (1999), which had done away with both the Medieval Warm Period and the Little Ice Age.

So what did the results show? The biggest difference between the Esper et al. and Mann et al. temperature histories was the degree to which the coolness of the Little Ice Age was expressed. The Little Ice Age was much more evident in the record of Esper et al.; and its significantly lower temperatures were what made the Medieval Warm Period stand out more dramatically in their temperature reconstruction. Also, they noted that "the warmest period covers the interval 950-1045, with the peak occurring around 990." This finding, they remarked, "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) made several important points. First, they acknowledged 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 made it abundantly clear that the peak warmth of the Medieval Warm Period was fully equivalent to the warmth of the present.

This fact reaffirmed 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 had previously been described in compelling detail by Bond et al. (2001).

Briffa and Osborn also noted that Esper et al.'s record clearly showed 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 indicated 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 did great damage to the climate-alarmist claim that CO2-enhanced greenhouse warming was responsible for the temperature increase that brought the earth 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.

Two years later, in a study of lichens of the subspecies Rhizocarpon geographicum found on avalanche boulder tongues in the eastern part of the Massif des Ecrins of the French Alps, Jomelli and Pech (2004) made an important discovery that added to the growing body of evidence that demonstrated that what climate alarmists were calling the unprecedented and CO2-induced warming of the 20th century was neither unprecedented nor driven by rising CO2 concentrations. According to their findings, high-altitude avalanche activity during the Little Ice Age reached an early maximum prior to 1650, after which it decreased until about 1730, whereupon it increased once again, reaching what was likely its greatest maximum about 1830.

In further support of these findings, Jomelli and Pech noted that "a greater quantity of snow mobilized by avalanches during the Little Ice Age can be supported by the fact that the two periods, AD 1600-1650 and 1830, during which the run-out distances [of the avalanches] were maximum at high elevation sites, have corresponded overall to the periods of maximum glacial advances for these last 500 years," citing the work of Le Roy Ladurie (1983) and Reynaud (2001). In addition, they reported that "since 1850 most French Alpine glaciers have decreased," and that "the mass balance of these glaciers is directly correlated with summer temperature and spring precipitation," citing Vincent and Vallon (1997) and Vincent (2001, 2002).

The French Alps findings of Jomelli and Pech, plus those obtained from several other Northern Hemispheric sites by the other scientists they cited, all suggested that the "beginning of the end" of the Little Ice Age started somewhere in the early to mid-1800s. Moore et al. (2002), for example, had determined a similar start-time for the demise of the Little Ice Age based on temperature data gathered on Mount Logan in Canada, while further support for this conclusion came from studies of still other parameters, including deep soil temperatures (Gonzalez-Ruco et al., 2003), deep ocean temperatures (Lindzen, 2002), and dates of ice break-up of lakes and rivers (Yoo and D'Odorico, 2002). What is more, this was also the period of time during which the temperature record of Esper et al. (2002) had indicated that the entire Northern Hemisphere began its nearly-linear-with-time recovery from the depths of the Little Ice Age. And as Briffa and Osborn (2002) described it, Esper et al.'s record clearly showsed that the warming of the 20th century was actually "a continuation of a trend that began at the start of the 19th century."

In contrast to these observations, the temperature history of Mann et al. (1999) -- which is cited by climate-alarmists as justification for the great warming power they attribute to anthropogenic CO2 emissions -- suggests that post-Little Ice Age warming did not begin until about 1910; and, consequently, it can be appreciated that (1) perhaps half of the warming experienced by the earth in recovering from what was likely the coldest part of the Little Ice Age occurred well before the Mann et al. temperature history indicates any warming at all, that (2) an even greater part of the total warming occurred before the air's CO2 concentration began increasing in earnest (about 1930, which is actually close to the time when warming peaked in the United States and many other parts of the Northern Hemisphere), and that (3) the lion's share of the warming of the past nearly two centuries must therefore owe its existence to something other than the historical increase in the air's CO2 concentration.

Also in 2004, von Storch et al. used a coupled atmosphere-ocean model simulation of the climate of the past millennium as a surrogate climate to test the skill of the empirical reconstruction methods used by Mann et al. (1999) in deriving their thousand-year "hockeystick" temperature history of the Northern Hemisphere. This they did by (1) generating a number of pseudo-proxy temperature records by sampling a subset of the model's simulated grid-box temperatures representative of the spatial distribution of the real-world proxy temperature records used by Mann et al. in creating their hockeystick history, (2) degrading these pseudo-proxy records with statistical noise, (3) regressing the results against the measured temperatures of the historical record, and (4) using the relationships thus derived to construct a record they could compare against their original model-derived surrogate temperature history.

The result of performing these operations was that the centennial variability of the Northern Hemispheric temperature was underestimated by the regression-based methods von Storch et al. applied, suggesting, in their words, that past variations in real-world temperature "may have been at least a factor of two larger than indicated by empirical reconstructions." And the unfortunate consequences of this result are readily evident in the reduced degree of Little Ice Age cooling and Medieval Warm Period warming that result from the fault-prone techniques employed by Mann et al.

In an accompanying commentary on this analysis and its findings, Osborn and Briffa (2004) wrote that "if the true natural variability of Northern Hemispheric temperature is indeed greater than is currently accepted," which they appear to have suggested is likely the case, "the extent to which recent warming can be viewed as 'unusual' would need to be reassessed." And that this reassessment is indeed sorely needed is additionally suggested by the fact that what von Storch et al. refer to as "empirical methods that explicitly aim to preserve low-frequency variability (Esper et al., 2002)" clearly show much more extreme Medieval Warm Period warming and Little Ice Age cooling than do the reconstructions of Mann et al., which suffer from the problems elucidated in the study of von Storch et al.

In light of these observations, it is becoming ever more evident that the Northern Hemispheric temperature record of Esper et al. is likely to be much more representative of reality than is the IPCC-endorsed Northern Hemispheric temperature record of Mann et al., and that the lion's share of the warming experienced since the end of the Little Ice Age occurred well before mankind's CO2 emissions significantly perturbed the atmosphere, which indicates that the majority of post-Little Ice Age warming was due to something other than rising atmospheric CO2 concentrations, which in turn suggests that the lesser warming of the latter part of the 20th century may well have been due to something else as well.

Contemporaneously, Cook et al. (2004) felt it necessary to (1) carefully review what Esper et al. had done, and (2) conduct some further analyses of the data the latter researchers had employed in their reconstruction effort. After all they did in this regard, however, their conclusions were not much different. They concluded from their reanalysis of the Esper et al. reconstruction, for example, that (1) "its strongly expressed multi-centennial variability is highly robust over the AD 1200-1950 interval, with strongly expressed periods of 'Little Ice Age' cooling indicated prior to AD 1900," and that (2) "persistently above-average temperatures in the AD 960-1050 interval also suggest the large-scale occurrence of a 'Medieval Warm Period' in the Northern Hemisphere extra-tropics."

It is interesting to note, in this regard, that Cook et al. concluded the latter in spite of what they describe as strong criticism personally communicated to them by one of the Mann et al. authors (R.S. Bradley). And why is this point so hotly debated? Because if it was as warm as it is today a thousand or more years ago, when the air's CO2 concentration was fully 100 ppm less than it is today, there is no compelling reason for believing that the 100-ppm higher concentration of today has necessarily had anything to do with the global warming of the past century.

Another important finding of the Cook et al. study is that, like the reconstruction of Esper et al., their reanalysis of the data revealed that the "beginning of the end" of the Little Ice Age started nearly a century earlier than what the Mann et al. curve suggests, which is well before the lion's share of the historical increase in the air's CO2 content occurred, which clearly indicates that the bulk of the planet's recovery from the final cold spell of the Little Ice Age had to have been caused by something other than rising atmospheric CO2 concentrations.

Rounding out this Summary, Moberg et al. (2005) presented a new temperature history of the Northern Hemisphere that spans the past two millennia and improves significantly upon the highly controversial reconstruction of Mann and Jones (2003), which evolved from the earlier controversial studies of Mann et al. (1998, 1999). The new temperature history, which represents a major move in the right direction, was produced from two different sources of paleoclimatic data: tree-rings, which capture very high frequency climate information, and lake and ocean sediments, which, in the words of Moberg et al., "provide climate information at multi-centennial timescales that may not be captured by tree-ring data."

The new temperature history clearly revealed the existence of one full cycle of the roughly 1500-year climatic oscillation that reverberates throughout the Holocene and across prior glacial and interglacial periods alike (see Climate Oscillations (Millennial Variability) in our Subject Index). Its creators noted, for example, that "high temperatures -- similar to those observed in the twentieth century before 1990 -- occurred around AD 1000 to 1100, and minimum temperatures that are about 0.7°C below the average of 1961-90 occurred around AD 1600," while the 20th century has seen a return to a new period of relative warmth.

So where did the new low-frequency variability that was missing from the temperature reconstructions of Mann et al. originate? It came from a set of eleven non-tree-ring proxy climate records that covered most of the past two millennia, nine of which data sets had already been calibrated to local/regional temperatures by their developers. Speaking to the logic and straightforwardness of their new analysis, Moberg et al. wrote that simple averages of temperature proxy series, such as the ones they used, "can yield adequate estimates of Northern Hemisphere century-scale mean-temperature anomalies," citing the work of von Storch et al. (2004) as authority for this statement; and when this procedure (simple averaging) is all that is done, as noted in Moberg et al.'s Figure 2a, the Medieval Warm Period (MWP) is observed to peak just prior to AD 900 and is strongly expressed between about AD 600 to 1100, which is very possibly the most correct temperature reconstruction of all.

What does this result tell us about modern temperatures? Where the temperature history discussed above ends, it is at approximately the level at which the MWP begins, which makes the final 20th-century temperature of that record cooler than all of the temperatures of the entire 500-year time span of the MWP. Also, at the point where Moberg et al.'s full reconstruction (which includes tree-ring results) ends, it is cooler than the MWP temperatures they find "around AD 1000 to 1100." In fact, it is only when the directly-measured instrumental temperatures of the latter part of the 20th century are added to the new temperature history that the Swedish and Russian scientists observe an extremely recent ("post-1990") modern warming that "appears to be unprecedented" over the prior two millennia; and in carefully stating that this appears to be the case, Moberg et al. speak appropriately, for one cannot make a definitive comparative judgment on the matter when the two types of data involved are significantly different from each other. That is to say, one cannot compare real apples with reconstructed oranges, especially when the apples may have been contaminated by an environmental factor (the growing urban heat island effect) that likely had little to no influence on the oranges.

So where does all of this leave us? It leaves us with more and stronger evidence that the Northern Hemisphere some 900 to 1400 years ago -- when there was 100 ppm less CO2 in the atmosphere than there is currently -- was as warm as, or warmer than, it has been since that time. This observation is extremely important, for it means that the opposite trend of whatever change(s) in climate forcing factor(s) brought the earth down into the depths of the Little Ice Age may well have been what has restored the lost warmth of the Medieval Warm Period and established the Current Warm Period.

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