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Temperature Trends (Global) -- Summary
It has been claimed that the last decade of the 20th century was the warmest of the past hundred years and possibly the warmest of the entire past millennium (Mann et al., 1998, 1999). It has also been claimed that this observation is a cause for much concern, because the temperatures in question are supposedly so unprecedented. In fact, those who would have us believe that these supposedly high air temperatures are the result of anthropogenic CO2 emissions resulting from the burning of fossil fuels contend that the near-surface air temperature of the planet is currently so high that we must radically reformulate the energetic basis of the entire industrialized world in order to avoid a host of unwanted climatic consequences. But is this climatic characterization correct? ... and is its adherents' call to action truly prudent?

Fully cognizant of the seriousness of this situation, Petit et al. (1999) described their analysis of the deepest ice core ever recovered from the Russian Vostok drilling station in East Antarctica, from which they extracted a 420,000-year history of the earth's near-surface air temperature and atmospheric CO2 concentration. This record covered the current interglacial period in which we now live, i.e., the Holocene, as well as the preceding four such climatic intervals. And why is this fact so important? Simply for what it tells us about the uniqueness or non-uniqueness of the Holocene; and that is that the current interglacial is by far the coldest of the five most recent such periods. In fact, the four interglacials that preceded the Holocene were, on average, more than 2°C warmer than the one in which we currently live. Also, atmospheric CO2 concentrations during all four prior interglacials never rose above approximately 290 ppm, whereas the air's CO2 concentration today (mid-2013) stands at about 400 ppm. Therefore, if there was anything unusual, unnatural or unprecedented about late-20th-century air temperatures, it was that they were so low in the presence of such high CO2 concentrations.

Similar findings have been obtained from the Dome Fuji ice core, which was extracted from a site in an entirely different sector of East Antarctica that is separated from the Vostok ice-core site by 1500 km (Watanabe et al., 2003). Although of somewhat shorter duration and, therefore, covering only the last three glacial-interglacial periods (marine stages 5.5, 7.5 and 9.3), this independent proxy temperature record also reveals that the last three interglacials, in the words of the six scientists, "were much warmer than the most recent 1,000 years (~4.5°C for stage 5.5 and up to 6°C for stage 9.3)."

This prior interglacial warmth is also evident in a 550,000-year sea surface temperature (SST) data set derived by Herbert et al. (2001) from marine sediment cores obtained along the western coast of North America, from around 22°N latitude at the southern tip of the Baja Peninsula to around 42°N latitude off the coast of Oregon. According to this reconstructed SST data set, in the words of the nine researchers who developed it, "the previous interglacial (isotope stage 5e) produced surface waters several degrees warmer than today," such that "waters as warm as those now at Santa Barbara occurred along the Oregon margin." Furthermore, as can readily be seen from the SST data of their paper, SSTs for this region in the current interglacial have not reached the warm peaks witnessed in all four of the preceding interglacial periods, falling short by a margin of 1 to 4°C.

Consequently, over the past half-million years, and within the context of the most recent five full interglacials, it is clear that the average near-surface air temperature of the earth during the 1990s was not unusually warm, but unusually cool. And not even the much greater atmospheric CO2 concentration of the latter period was able to reverse this incongruity, which only exists, of course, in the minds of those who believe we understand how earth's climate system works well enough to have absolute confidence in the output of today's climate models.

Shifting our perspective from the past 500 thousand years to the past 20 thousand years, Huang and Pollack (1997) searched the large database of terrestrial heat flow measurements compiled by the International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior for measurements suitable for reconstructing an average ground surface temperature history of the earth over the last 20,000 years. Working with a total of 6,144 qualifying sets of heat flow measurements obtained from every continent, they produced a global climate reconstruction, which they said was "independent of other proxy interpretations [and] of any preconceptions or biases as to the nature of the actual climate history." And based on their climatic reconstruction, they found strong evidence that the Medieval Warm Period was indeed warmer than it was at the time of their analysis, by perhaps as much as 0.5°C, which is only 0.1°C less than the peak warmth of the mid-Holocene Thermal Maximum, while their data also suggested that the Little Ice Age was as much as 0.7°C cooler than it was at that time of their study.

Shifting our perspective closer still to the present (the past one thousand years), there are many other reasons to believe that between the 10th and 14th centuries AD, earth's average global temperature may well have been higher than it is today (Lamb, 1977, 1984, 1988; Grove, 1988). The existence of this time interval of elevated temperature, which has come to be known as the Medieval Warm Period, was initially deduced from historical weather records and proxy climate data from England and Northern Europe. Interestingly, the warmer conditions associated with this interval of time are known to have had a largely beneficial impact on earth's plant and animal life. In fact, the environmental conditions of this time period have been determined to have been so favorable that it is often referred to as the Little Climatic Optimum (Imbrie and Imbrie, 1979; Dean, 1994; Petersen, 1994; Serre-Bachet, 1994; Villalba, 1994).

The degree of warming associated with the Medieval Warm Period varied from region to region; and, hence, its consequences were manifested in a number of different ways (Dean, 1994). In Europe, temperatures reached some of the warmest levels of the last 4,000 years, allowing enough grapes to be successfully grown in England to sustain an indigenous wine industry (Le Roy Ladurie, 1971). Contemporaneously, horticulturists in China extended their cultivation of citrus trees and perennial herbs further and further northward, resulting in an expansion of their ranges that reached its maximum extent in the 13th century AD (De'er, 1994). And considering the climatic conditions required to successfully grow these species, it has been estimated that annual mean temperatures in the region must have been about 1.0 °C higher than at present, with extreme January minimum temperatures fully 3.5 °C warmer than they are today (De'er, 1994).

Likewise, in North America, tree-ring chronologies from the southern Canadian Rockies have provided evidence for higher treelines and wider ring-widths between AD 950 and 1100, which findings are suggestive of warmer temperatures and more favorable growing conditions during that time period (Luckman, 1994). Similar results have also been derived from tree-ring analyses of bristlecone pines in the White Mountains of California, where much greater growth was recorded in the 11th and 12th centuries (Leavitt, 1994), while other data document vast glacial retreats during the Medieval Warm Period in parts of South America, Scandinavia, New Zealand and Alaska (Grove and Switsur, 1994; Villalba, 1994). And the subsequent work of Keigwin (1996a,b) with ocean-bed cores suggests that large-scale sea surface temperatures were warmer then as well, at a time when atmospheric CO2 concentrations hovered around a mean value on the order of 285 ppm or less.

A few years later, Santer et al. (2000) searched for explanations for a discrepancy that existed between global-scale temperature trends at the earth's surface (as recorded by conventional thermometers) and throughout the lower troposphere (as monitored by satellites) between 1979 and 1998, because of the fact that natural climate variability, as simulated by three state-of-the-art coupled atmosphere-ocean climate models, could not completely explain the difference between the two trends. What is more, modeling the effects of estimated changes in greenhouse gases, sulfate aerosols, and tropospheric ozone was also insufficient to explain the observed difference in temperature trends. Thus, they were forced to admit that they could not determine "the precise cause or causes of recent observed surface-troposphere temperature trend differences;" and they therefore called for additional simulations of the climate of the past two decades with a variety of models that explored then-current uncertainties in key natural and anthropogenic forcings to help resolve the issue. And by so doing, they thereby tacitly acknowledged that the world's climate system is far more complex than that of the models then being used to simulate it.

Narrowing their temporal focus even more, Huang et al. (2000) used temperature data obtained from 616 boreholes on all continents except Antarctica to reconstruct a temperature history of the globe as a whole over the past five centuries. And if all of the assumptions they made that went into translating depth profiles of temperature into temporal temperature trends at the surface of the earth were correct, the mean surface air temperature of the globe rose by approximately 1°C over the past 500 years, with about half of that warming coming in the last century.

About this same time, Briffa (2000) wrote in a review paper that "tree-growth, as represented in various standardized tree-ring chronologies in various parts of the world, often seems anomalous in the 20th century as compared to earlier centuries." And he said that this widespread anomaly is extremely important, because "the recent high growth rates . . . provide major pieces of evidence being used to assemble a case for anomalous global warming, interpreted by many as evidence of anthropogenic activity," specifically mentioning Mann et al. (1998, 1999) in this regard. But as Briffa further noted, the empirically-derived regression equations upon which the temperature reconstructions were based may be compromised if the growth rates of earth's trees have been substantially enhanced over the past century or so by some global environmental influence other than warming that has increasingly manifested itself over the same time period.

What might this influence be? Briffa cited a number of possibilities, including the historical and still-ongoing rise in the atmosphere's CO2 concentration, along with several plant physiological processes that become increasingly more efficient in response to this phenomenon, which fact had been established several years earlier by LaMarche et al. (1984), Graybill and Idso (1993), Polley et al. (1993) and Cowling and Sykes (1999), who had demonstrated that the historical rise in the air's CO2 content could readily explain the anomalous 20th-century tree-ring rate of width expansion; and it is very likely, therefore, that the enhanced tree growth induced by the historical rise in the air's CO2 content - possibly augmented by enhanced nitrogen deposition (Idso, 1995) - has been increasing the growth rates of trees the world over for the past century or more. In addition, this growth enhancement has actually been accelerating over time (Phillips and Gentry, 1994); and it is this ever-intensifying biological phenomenon that some are using to bolster their claim that the earth is warming at an ever-increasing rate, when such may not really be the case.

Moving forward in time a few years, Loehle (2004) used a pair of 3,000-year-long proxy climate records that had minimal dating errors to characterize the pattern of climate change over the past three millennia in a paper that provides the necessary context for properly evaluating the cause or causes of 20th century global warming.

The first of the two temperature series was the sea surface temperature (SST) record of the Sargasso Sea, which was derived by Keigwin (1996b) from a study of the oxygen isotope ratios of foraminifera and other organisms contained in a sediment core retrieved from a deep-ocean drilling site on the Bermuda Rise. This record provided SST data for about every 67th year from 1125 BC to AD 1975. The second temperature series was the ground surface temperature record derived by Holmgren et al. (1999, 2001) from studies of color variations of stalagmites found in a cave in South Africa, which variations are caused by changes in the concentrations of humic materials entering the region's ground water that have been reliably correlated with regional near-surface air temperature.

But why did Loehle use these two specific records? And only these two records? By way of explanation, he wrote that "most other long-term records have large dating errors, are based on tree rings, which are not reliable for this purpose (Broecker, 2001), or are too short for estimating long-term cyclic components of climate." Also, in a repudiation of the approach employed by Mann et al. (1998, 1999) and Mann and Jones (2003), he stated that "synthetic series consisting of hemispheric or global mean temperatures are not suitable for such an analysis because of the inconsistent timescales in the various data sets," noting further, as a result of his own testing, that "when dating errors are present in a series, and several series are combined, the result is a smearing of the signal."

But can only two temperature series reveal the pattern of global temperature change? Feeling a need to reassure us on this matter, Loehle reported that "a comparison of the Sargasso and South Africa series shows some remarkable similarities of pattern, especially considering the distance separating the two locations," and he said that this fact "suggests that the climate signal reflects some global pattern rather than being a regional signal only." He also noted that a comparison of the mean record with the South Africa and Sargasso series from which it was derived "shows excellent agreement," and that "the patterns match closely," concluding that "this would not be the case if the two series were independent or random."

Proceeding with his plan of attack, which was to fit simple periodic models to the temperature data as functions of time, with no attempt to make the models functions of solar activity or any other physical variable, Loehle fit seven different time-series models to the two temperature series and to the average of the two series, using no data from the 20th century. In all seven cases, he reported that good to excellent fits were obtained. As an example, the three-cycle model he fit to the averaged temperature series had a simple correlation of 0.58 and an 83% correspondence of peaks when evaluated by a moving window count.

Comparing the forward projections of the seven models through the 20th century leads directly to the most important conclusions of Loehle's paper. He notes, first of all, that six of the models "show a warming trend over the 20th century similar in timing and magnitude to the Northern Hemisphere instrumental series," and that "one of the models passes right through the 20th century data." These results clearly suggest, in his words, that "20th century warming trends are plausibly a continuation of past climate patterns" and, therefore, that "anywhere from a major portion to all of the warming of the 20th century could plausibly result from natural causes."

As dramatic and important as these observations are, they are not the entire story of Loehle's insightful paper. His analyses also reveal a long-term linear cooling trend of 0.25°C per thousand years since the peak of the interglacial warm period that occurred some 7000 years ago, which result is essentially identical to the mean value of this trend that was derived from seven prior assessments of its magnitude and five prior climate reconstructions. In addition, Loehle's analyses reveal the existence of the Medieval Warm Period of AD 800-1200, which is shown to have been significantly warmer than the portion of the Modern Warm Period we have so far experienced, as well as the existence of the Little Ice Age of AD 1500-1850, which is shown to have been the coldest period of the entire 3000-year record.

As corroborating evidence for the global nature of these major warm and cold intervals, Loehle cites sixteen peer-reviewed scientific journal articles that document the existence of the Medieval Warm Period in all parts of the world, as well as eighteen other articles that document the worldwide occurrence of the Little Ice Age. And in one of the more intriguing aspects of his study - of which Loehle makes no mention, however - both the Sargasso Sea and South African temperature records reveal the existence of a major temperature spike that began sometime in the early 1400s. This abrupt warming pushed temperatures considerably above the peak warmth of the 20th century before falling back to pre-spike levels in the mid-1500s, providing support for the similar finding of higher-than-current temperatures in that time interval by McIntyre and McKitrick (2003) in their reanalysis of the data employed by Mann et al. in creating their controversial "hockeystick" temperature history, which gives no indication of the occurrence of this high-temperature regime.

In another accomplishment of note, the models developed by Loehle reveal the existence of three climate cycles previously identified by others. In his culminating seventh model, for example, there is a 2388-year cycle that he describes as comparing "quite favorably to a cycle variously estimated as 2200, 2300, and 2500 years (Denton and Karlen, 1973; Karlen and Kuylenstierna, 1996; Magny, 1993; Mayewski et al., 1997)." In addition, there is a 490-year cycle that likely "corresponds to a 500-year cycle found previously (e.g. Li et al., 1997; Magny, 1993; Mayewski et al., 1997)" and a 228-year cycle that "approximates the 210-year cycle found by Damon and Jirikowic (1992)."

The compatibility of these findings with those of several studies that have identified similar solar forcing signals caused Loehle to conclude that "solar forcing (and/or other natural cycles) is plausibly responsible for some portion of 20th century warming" or, as he indicates in his abstract, maybe even all of it.

Publishing concurrently, De Laat and Maurellis (2004) used a global data set developed by Van Aardenne et al. (2001) - which reveals the spatial distribution of various levels of industrial activity over the planet, as quantified by the intensity of anthropogenic CO2 emissions - to divide the surface of the earth into non-industrial and industrial sectors of various intensity levels, after which they plotted the 1979-2001 temperature trends (°C/decade) of the different sectors using data from both the surface and the lower and middle troposphere. This work revealed that "measurements of surface and lower tropospheric temperature change give a very different picture from climate model predictions and show strong observational evidence that the degree of industrialization is correlated with surface temperature increases as well as lower tropospheric temperature changes." Specifically, they found that the surface and lower tropospheric warming trends of all industrial regions were greater than the mean warming trend of the earth's non-industrial regions, and that the difference in warming rate between the two types of land-use had grown ever larger as the degree of industrialization grew ever larger.

In contemplating the significance of their findings, the two researchers wrote that "areas with larger temperature trends (corresponding to higher CO2 emissions) cover a considerable part of the globe," which implies that "the 'real' global mean surface temperature trend is very likely to be considerably smaller than the temperature trend in the CRU [Hadley Center/Climate Research Unit] data," since the temperature measurements that comprise that data base "are often conducted in the vicinity of human (industrial) activity." And these observations, in their words, "suggest a hitherto-overlooked driver of local surface temperature increases, which is linked to the degree of industrialization" and which "lends strong support to other indications that surface processes (possibly changes in land-use or the urban heat effect) are crucial players in observed surface temperature changes," citing Kalnay and Cai (2003) and Gallo et al. (1996, 1999). Hence, they concluded that "the observed surface temperature changes might be a result of local surface heating processes and not related to radiative greenhouse gas forcing."

Contemporaneously, Bengtsson et al. (2004) used the European Centre for Medium-Range Weather Forecasts' 40-year reanalysis (ERA40) data set, which is described by Simmons and Gibson (2000), to calculate the temperature of the lower troposphere (TLT) for the period 1979-2001, in order to compare the result with the lower tropospheric temperature record obtained from the Microwave Sounding Unit (MSU) observations of Christy et al. (2003) over the same time period. Although they had reason to believe that their final result was somewhat "on the high side," they reported that the ERA40 TLT trend of +0.11°C/decade "is in agreement with Christy et al. (2003) but much smaller than the recent findings by Vinnikov and Grody (2003), who suggest that the TLT trend is +0.22 to +0.26°C/decade and thus closer to the IPCC results for the near surface temperature trend."

Controversy thus continued to swirl about the true rate of lower tropospheric and near-surface air temperature change over the past quarter-century, with multiple major studies both agreeing with and differing from the standard climate-alarmist contention that the earth had experienced truly unprecedented warming over that time period. And thus it was that Bengtsson et al. - stating that their study "stresses the difficulties in detecting long term trends in the atmosphere" - concluded that additional "major efforts along the lines indicated [in their study] are urgently needed" to help resolve the issue, which at times (such as the time of their writing) appeared to be almost intractable.

In another intriguing study from this same time period, Damon and Laut (2004) reported what they describe as errors made by Friis-Christensen and Lassen (1991), Svensmark and Friis-Christensen (1997), Svensmark (1998) and Lassen and Friis-Christensen (2000) in their presentation of solar activity data, which they correlated with terrestrial temperature data in a number of papers that appeared to explain most of the temperature variability of the earth over the past 140 years as arising from solar variability. The Danish scientists' error, in the words of Damon and Laut, of "adding to a heavily smoothed ('filtered') curve, four additional points covering the period of global warming, which were only partially filtered or not filtered at all," led to an apparent dramatic increase in solar activity over the last quarter of the 20th century that closely matched the equally dramatic rise in temperature manifest by the Northern Hemispheric temperature reconstruction of Mann et al. (1998, 1999) over the same period. With the acquisition of additional solar activity data in subsequent years, however, and with what Damon and Laut called the proper handling of the numbers, the late 20th-century dramatic increase in solar activity totally disappears.

The new result, to quote the two scientists, meant that "the sensational agreement with the recent global warming, which drew worldwide attention, has totally disappeared." In truth, however, it was only the agreement with the last quarter-century of the Mann et al. hockeystick temperature history that disappeared; and this new disagreement is most welcome, because the Mann et al. temperature reconstruction is likely vastly in error over this stretch of time, due to problems discussed earlier in this summary. Indeed, there are numerous studies that have found the late 1930s and early 1940s to have been the warmest period of the past century; and it is these temperature reconstructions that now take the place of the Mann et al. curve in displaying "sensational agreement" with the new-and-improved solar activity history produced by Damon and Laut, which also peaks in the late 1930s and early 1940s.

One year later, Ruddiman et al. (2005) put forth a whole new perspective on the global warming debate by noting that "ice-core evidence from previous interglaciations indicates that forcing by orbital-scale changes in solar radiation and greenhouse-gas concentrations should have driven earth's climate significantly toward glacial conditions during the last several thousand years," and that "the hypothesized reason most of this cooling did not occur is that humans intervened in the natural operation of the climate system by adding significant amounts of CO2 and CH4 to the atmosphere, thereby offsetting most of the natural cooling [that otherwise would have occurred] and fortuitously producing the climatic stability of the last several thousand years." So if true, how did humans do it?

Ruddiman et al. attributed the anomalous increase in atmospheric CO2 to the massive early deforestation of Eurasia, while they linked the anomalous CH4 increase to the introduction of irrigation for rice farming in southeast Asia, as well as to increases in biomass burning and the development of animal husbandry. Based on the periodicities and phases of the natural cycles of CO2 and CH4 that are revealed in the 400,000-year Vostok ice core, they first determined that the atmosphere's CO2 concentration should have fallen to 240-245 ppm, whereas it gradually rose to a level of 280-285 ppm, just before the start of the Industrial Revolution, while the air's CH4 concentration rose to approximately 700 ppb when it should have fallen to about 450 ppb. Then, based on the IPCC sensitivity estimate of a 2.5°C temperature increase for a doubling of the air's CO2 content, they calculated that the supposedly anthropogenic-induced CO2 and CH4 anomalies should have produced an equilibrium warming of approximately 0.8°C on a global basis and 2°C in earth's polar regions.

On the basis of these calculations, Ruddiman et al. concluded that "without any anthropogenic warming, earth's climate would no longer be in a full-interglacial state but well on its way toward the colder temperatures typical of glaciations," and that "an ice sheet would now be present in northeast Canada, had humans not interfered with the climate system."

If correct, the overdue-glaciation hypothesis suggests that in the absence of anthropogenic contributions of CO2 and CH4, the climate today would be, in their words, "roughly one third of the way toward full-glacial temperatures," which also suggests that the extra CO2 mankind is currently releasing to the atmosphere via the burning of fossil fuels may well be what's keeping us from going the rest of the way. Hence, even if the IPCC is correct in their analysis of climate sensitivity, the bottom line for the preservation of civilization and much of the biosphere is that governments ought not interfere with the normal progression of fossil fuel usage, for without more CO2 in the atmosphere, we could shortly resume the downward spiral to full-fledged ice-age conditions.

In another intriguing paper published in the same year, Oerlemans (2005) described a new glacier-based temperature history that spanned a four-century-long period of time (1600-2000). This history indicated that the Little Ice Age was reasonably globally-synchronous, with 96 of the records upon which it was based coming from the European Alps, 27 from Northwest America, nine from Central Asia, eight from Scandinavia, eight from the Caucasus, six from Patagonia, five from Tropical Africa, four from Iceland, three from Svalbard, two from Irian Jaya, two from New Zealand, one from South Greenland, and one from Jan Mayen. And another important observation of Oerlemans was that the glacial-based dataset "suggests that the Little Ice Age was at its maximum around 1800 rather than at the end of the 19th century as seen in some other temperature proxies (Mann and Jones, 2003)." In fact, whereas the warming that led to the development of the Current Warm Period begins about 1855 in the glacial-based record, the Mann et al. (1998, 1999) records do not show it starting until 1910!

Of most interest of all, however, is what happened over the final two decades of the 20th century. Mann et al. terminated their largely tree-ring-based reconstruction and continued their history with the instrumental temperature record. This tactic enabled them to utilize temperatures that may well have been significantly inflated by urban heat island effects, as suggested by the work of Kalnay and Cai (2003); and it enabled them to essentially ignore tree-ring-based temperature reconstructions that did not indicate nearly as much warming as the instrumental record did over this period. So what did the glacial-based temperature history reveal?

Unfortunately, the number of glaciers with records extending all the way to the year 2000 was but a small fraction of the number that provided data over the bulk of the 20th century; and Oerlemans thus terminated his temperature history at about 1990. However, if one looks at the stacked records of all glacier length data that do extend to the end of the 20th century, which Oerlemans presents in his Figure 2B, one sees that for both the plots of all glaciers and Alps excluded, mean glacial retreat ceases and is actually replaced by glacial advance over the last few years of the 1990s, so that mean glacial length ends up at a value that is equivalent to what it was between about 1975 and 1985, where glaciers exhibited little to no trend for about ten years. In fact, over that same period of time (1975-1985), the mean temperature of Orelemans' glacial-based reconstruction was actually less than what it was in 1940!

In an important study published the following year, Soule and Knapp (2006) sought "to determine if gradually increasing levels of atmospheric CO2, as opposed to 'step' increases commonly employed in controlled studies, have a positive impact on radial growth of ponderosa pine (Pinus ponderosa) in natural environments." Working in the interior U.S. Pacific Northwest in pursuit of this goal, they "developed a series of tree-ring chronologies from minimally disturbed sites across a spectrum of environmental conditions," after which "a series of difference of means tests was used to compare radial growth post-1950, when the impacts of rising atmospheric CO2 are best expressed, with that pre-1950." Breaking the 20th century into these two equal parts thus allowed them to see if the 3.5 times greater increase in the air's CO2 concentration over its last half (~58.5 ppm) compared to its first half (~16.7 ppm) led to greater rates of radial tree growth post-1950 compared to pre-1950.

This work revealed, in the words of the paper's authors, that (1) "significant increases in radial growth rates occurred post-1950, especially during drought years, with the greatest increases generally found at the most water-limited sites," and that (2) "site harshness [was] positively related to enhanced radial growth rates." More specifically, they found that when comparing the growth responses of trees during matched dry and wet years pre- and post-1950, the relative change in growth was upward at seven of their eight sites, "ranging from 11 to 133%, with responses during matched wet years less pronounced." In addition, they "found similar results when analyzing the data by Palmer Drought Severity Index category, with the greatest absolute and relative increases in radial growth post-1950 occurring during the years when soil moisture was most limiting."

In concluding, the two researchers say that their results, "showing that radial growth has increased in the post-1950s period at all sites, and significantly at 50% of the sites, while climatic conditions have generally been unchanged, suggest that non-climatic driving forces are operative," and that "these findings suggest that elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine."

Much the same thing had earlier been demonstrated by the two North Carolina scientists for another tree species. As they describe it, "our earlier work in the interior Pacific Northwest examining possible atmospheric CO2 fertilization (Knapp and Soule, 1996, 1998; Knapp et al., 2001a,b; Soule et al., 2003, 2004) suggests that the effect is operative for the tree species western juniper (Juniperus occidentalis var. occidentalis)." More specifically, it suggested that "the 25% increase in atmospheric CO2 during the past century, most of which occurred since 1950, contributed to both ecotonal expansion into drier areas and enhanced radial growth of western juniper."

In studying the careful and persistent work of Knapp and Soule, it is clear that it has provided some of the best real-world evidence to date that the historical increase in the atmosphere's CO2 concentration has enhanced the growth rates of two species of long-lived woody plants and, by implication, the growth rates of other such plants as well. This finding has two very important ramifications. First, it strengthens the CO2-induced Greening of the Earth hypothesis put forth by Idso (1986). Second, it weakens the "hockeystick" representation of earth's temperature history over the past millennium that was put forth by Mann et al. (1998, 1999), as it indicates that the large 20th-century warming they deduced from tree-ring studies likely contains a component that is CO2- (and not temperature-) driven.

One year later, Zhen-Shan and Xian (2007) used a novel multi-timescale analysis method known as Empirical Mode Decomposition to diagnose the variation of the annual mean temperature data of the globe, the Northern Hemisphere and China from 1881 to 2002. This work revealed that the temperature histories they studied "can be completely decomposed into four timescale quasi-periodic oscillations including an ENSO-like mode, a 6-8-year signal, a 20-year signal and a 60-year signal, as well as a trend." This latter residual, which they determined could account for no more than 40% of the global temperature variation, was attributed by them to the historical increase in the atmosphere's CO2 concentration; but it is clear that some unknown portion of it could well have been due to a different factor or set of factors. In addition, they found that the temperature variation in China precedes that of the globe and Northern Hemisphere, thereby providing what they called "a denotation for global climate changes." Consequently, by projecting the four oscillatory modes of temperature change they identified into the future, together with the residual temperature trend, they came to the conclusion that "global climate will be cooling down in the next 20 years."

In light of their findings and what those findings imply, the two Chinese researchers say that "although the CO2 greenhouse effect on global climate change is unsuspicious, it could have been excessively exaggerated." Consequently, they concluded that "it is high time to reconsider the trend of global climate change," which warning is especially appropriate in light of Zhen-Shan and Xian's demonstration of CO2's less-than-dominant role in the global warming of the last hundred and twenty years (which may itself be inflated), plus their conclusion that if the atmosphere's CO2 content were to be suddenly stabilized, "the CO2 greenhouse effect will be deficient in counterchecking the natural cooling of global climate in the following 20 years," much as Ruddiman et al. (2005) had warned.

Publishing in the same year as Zhen-Shan and Xian, McKitrick and Michaels (2007) noted that "the standard interpretation of global climate data is that extraneous effects, such as urbanization and other land surface effects, and data quality problems due to inhomogeneities in the temperature series, are removed by adjustment algorithms, and therefore do not bias the large-scale trends." Not inclined to believe this presumption, however, they worked with data from all available land-based grid cells around the world, evaluating this widely-accepted but largely-unverified assumption by testing "the null hypothesis that the spatial pattern of temperature trends in a widely used gridded climate data set is independent of socioeconomic determinants of surface processes and data inhomogeneities."

And what did they find? In the words of the two researchers, this hypothesis "is strongly rejected, indicating that extraneous (non-climatic) signals contaminate gridded climate data." In addition, they discovered that "the patterns of contamination are detectable in both rich and poor countries and are relatively stronger in countries where real income is growing." Last of all, they report that using a regression model to filter out the extraneous non-climatic effects revealed by their analysis "reduces the estimated 1980-2002 global average temperature trend over land by about half." And so they concluded that "trends in gridded climate data are, in part, driven by the varying socioeconomic characteristics of the regions of origin, implying a residual contamination remains even after adjustment algorithms have been applied."

Forging ahead another year, in a study somewhat reminiscent of that of Soule and Knapp (2006), Phillips et al. (2008) wrote that there is "a long held view," as they described it, that "old trees exhibit little potential for growth." And so they further said that "it may seem reasonable to conclude that old trees are not responsive to increased CO2," as many climate alarmists claim; but they then went on to demonstrate that this view is far from the truth.

The three researchers began their analysis of the subject by stating that "hydraulic constraints in tall trees," such as those of great age, "constitute a fundamental form of water limitation ... that is indistinguishable from soil water limitations," citing the work of Koch et al. (2004) and Woodruff et al. (2004). They also reported that "recent research indicates that tree size and its hydraulic correlates, rather than age per se, controls carbon gain in old trees," as indicated by the study of Mencuccini et al. (2005). And these findings imply, as they described it, that "factors that alleviate internal or external resource constraints on old trees could improve physiological function and ultimately growth," which is something elevated CO2 does quite well by increasing plant water use efficiency. In fact, they listed several phenomena that suggest "a fundamental potential for old growth trees to show greater photosynthesis and growth under industrial age increases in CO2 than they would under constant, pre-industrial CO2 levels."

Drawing from their own work, Phillips et al. thus recounted how "500- and 20-year-old Douglas-fir trees both show high sensitivity of photosynthesis to atmospheric CO2," presenting data which clearly demonstrated, as they phrased it, that "under optimal conditions there exists the potential for an approximately 30% increase in photosynthetic rate with an increase in CO2 from pre-industrial to current levels [i.e., from 280 to 385 ppm] in old trees." And they went on to note that "the phenomenon of twentieth-century ring-width increase," which could thus be expected to accompany the 20th-century increase in the air's CO2 concentration, had in fact been detected in several other studies, including those of LaMarche et al., (1984), Jacoby (1986), Graybill (1987), Kienast and Luxmoore (1988), Graumlich (1991), Knapp et al. (2001), Bunn et al. (2005), and Soule and Knapp (2006), to which can also be added the study of Graybill and Idso (1993).

In further commenting on the significance of the findings of these several studies, the three researchers wrote that the results of LaMarche et al. (1984) "could not be explained by temperature or precipitation variation over this time period, but were consistent with, and attributed to, the rise in atmospheric CO2," which was also the case with the results of Graybill and Idso (1993). Although these data, in their words, "appear to represent compelling circumstantial evidence for carbon fertilization of old growth trees," they noted that "this possibility has been discounted and climate change has instead been implicated for the observed responses in subsequent research." And it is that invalid discounting that has led to the erroneous climate-alarmist claim that 20th-century global warming was unprecedented over the past two millennia or more. In reality, however, it is becoming ever more clear that a goodly portion of the 20th-century increase in tree growth, which climate alarmists attribute almost solely to rising temperature, was actually a consequence of the concomitant growth-promoting and water-use-efficiency-enhancing increase in the air's CO2 content.

About this same time, and "to distinguish between simultaneous natural and anthropogenic impacts on surface temperature, regionally as well as globally," Lean and Rind (2008) performed "a robust multivariate analysis using the best available estimates of each together with the observed surface temperature record from 1889 to 2006." This work revealed that "contrary to recent assessments based on theoretical models (IPCC, 2007), the anthropogenic warming estimated directly from the historical observations is more pronounced between 45°S and 50°N than at higher latitudes," which finding, in their words, "is the approximate inverse of the model-simulated anthropogenic plus natural temperature trends ... which have minimum values in the tropics and increase steadily from 30 to 70°N." Furthermore, as they continue, "the empirically-derived zonal mean anthropogenic changes have approximate hemispheric symmetry whereas the mid-to-high latitude modeled changes are larger in the Northern Hemisphere." And because of what their analysis revealed, the two researchers concluded that "climate models may therefore lack - or incorrectly parameterize - fundamental processes by which surface temperatures respond to radiative forcings."

Creeping ahead one more year, Chylek et al. (2009) wrote that "one of the robust features of the AOGCMs [Atmosphere-Ocean General Circulation Models] is the finding that the temperature increase in the Arctic is larger than the global average, which is attributed in part to the ice/snow-albedo temperature feedback." More specifically, they said "the surface air temperature change in the Arctic is predicted to be about two to three times the global mean," citing the IPCC (2007). Therefore, utilizing Arctic surface air temperature data from 37 meteorological stations north of 64°N, Chylek et al. explored the latitudinal variability in Arctic temperatures within two latitude belts - the low Arctic (64°N-70°N) and the high Arctic (70°N-90°N) - comparing the results with the mean measured air temperatures of these two regions over three sequential periods: 1910-1940 (warming), 1940-1970 (cooling) and 1970-2008 (warming).

In initial apparent harmony with state-of-the-art AOGCM simulations, the five researchers report that "the Arctic has indeed warmed during the 1970-2008 period by a factor of two to three faster than the global mean." More precisely, the Arctic amplification factor was 2.0 for the low Arctic and 2.9 for the high Arctic. But that was the end of the real world's climate-change agreement with theory. During the 1910-1940 warming, for example, the low Arctic warmed 5.4 times faster than the global mean, while the high Arctic warmed 6.9 times faster. Even more out of line with climate model simulations were the real-world Arctic amplification factors for the 1940-1970 cooling: 9.0 for the low Arctic and 12.5 for the high Arctic.

These findings constitute yet another important example of the principle recently described (and proven to be correct) by Reifen and Toumi (2009), i.e., that a model that performs well in one time period will not necessarily perform well in another time period. And this apparently now-incontrovertible fact further suggests that since AOGCMs suffer from this shortcoming, they ought not be considered adequate justification for imposing dramatic cuts in anthropogenic CO2 emissions, as their simulations of future temperature trends may well be far different from what will actually transpire.

Contemporaneously, Perlwitz et al. (2009) recounted some interesting climatic facts, including the fact that there was, in their words, "a precipitous drop in North American temperature in 2008, commingled with a decade-long fall in global mean temperatures." They began their narrative by noting that there had been "a decade-long decline (1998-2007) in globally averaged temperatures from the record heat of 1998," citing Easterling and Wehner (2009). And in further describing this phenomenon, they said that U.S. temperatures in 2008 "not only declined from near-record warmth of prior years, but were in fact colder than the official 30-year reference climatology (-0.2°C versus the 1971-2000 mean) and further were the coldest since at least 1996." And with respect to the origin of this "natural cooling," as they described it, the five researchers pointed to "a widespread coolness of the tropical-wide oceans and the northeastern Pacific," focusing on the Niño 4 region, where they report that "anomalies of about -1.1°C suggest a condition colder than any in the instrumental record since 1871."

Shortly thereafter, Akasofu (2010) reported on a subject that had been initially broached by Idso (1988) in a brief paper wherein he had come to the conclusion that "a comparative analysis of long-term (several-hundred-year) temperature and carbon dioxide trends suggests that the global warming of the past century is not due to the widely accepted CO2 greenhouse effect but rather to the natural recovery of the earth from the global chill of the Little Ice Age, which was both initiated and ended by some unrelated [to CO2] phenomenon."

Akasofu addressed the same subject in a similar manner, but with the benefit of nearly a quarter-century of additional temperature and CO2 data, plus a greater variety of other information, employing "openly available data on sea level changes, glacier retreat, freezing/break-up dates of rivers, sea ice retreat, tree-ring observations, ice cores and changes of the cosmic-ray intensity, from the year 1000 to the present." And in doing so, the founding director of the International Arctic Research Center of the University of Alaska Fairbanks (USA) was able to demonstrate that earth's recovery from the Little Ice Age "has proceeded continuously, roughly in a linear manner, from 1800-1850 to the present," with the rate of recovery being about 0.5°C/century. Thus, he suggests that the earth is "still in the process of recovery from the LIA," which is being brought about by whatever was responsible for the mean linear warming of the 20th century, as modulated by a "multi-decadal oscillation of a period of 50 to 60 years" that is superimposed upon it and which "peaked in 1940 and 2000, causing the halting of warming temporarily after 2000." And extending these two phenomena into the future, Akasofu predicts the non-CO2-induced temperature increase over the 21st century to be 0.5°C ± 0.2°C, rather than the much greater 4°C ± 2°C that is predicted by the Intergovernmental Panel on Climate Change.

Working with 2249 globally-distributed monthly temperature records covering the period 1906-2005, which they obtained from NASA's Goddard Institute for Space Studies, Ludecke et al. (2011) evaluated "to what extent the temperature rise in the past 100 years was a trend or a natural fluctuation." They began by reporting that "the mean of all stations shows 0.58°C global warming from 1906 to 2005," but they say that "if we consider only those stations with a population of under 1000 and below 800 meters above sea level, this figure drops to 0.41°C." In addition, they note that "about a quarter of all records show falling temperatures," which in itself, in their words, "is an indication that the observed temperature series are predominantly natural fluctuations," where the word natural means that "we do not have within a defined confidence interval a definitely positive anthropogenic contribution." And continuing to explore this aspect of their analysis, they evaluated -- a with a confidence interval of 95% -- the probability that the observed global warming from 1906 to 2005 was a natural fluctuation, finding that probability to lie "between 40% and 70%, depending on the station's characteristics," while "for the period 1906 to 1955 the probabilities are arranged between 80% and 90% and for 1956 to 2005 between 60% and 70%."

Based on their several findings, the scientific trio wrote, in the final sentence of their paper's abstract, that the strongest statement they could make on the subject was that "only a marginal anthropogenic contribution cannot be excluded," which would seem to suggest that if there is a CO2-induced warming signal hidden somewhere in the global temperature data of the Goddard Institute for Space Studies, it must be relatively small, as is also suggested by the observation-based analyses of Idso (1998), Lindzen and Choi (2009, 2011) and Scafetta (2012).

In bringing this discussion to a close, the most recent contribution to the study of global temperature trends would appear to be that of the members of the PAGES 2k Consortium (2013), who "reconstructed past temperatures for seven continental-scale regions during the past one to two millennia," and who concluded that "there were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age," and who additionally concluded that "during the period AD 1971-2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years." However, Steve McIntyre is currently in the process of reviewing their methodology; and he is identifying numerous deadly errors that would seem to disqualify what the PAGES 2k Consortium has concluded, which is clearly at odds with the findings of the bulk of the studies reviewed in this summary.

Akasofu, S.-I. 2010. On the recovery from the Little Ice Age. Natural Science 2: 1211-1224.

Bengtsson, L., Hagemann, S. and Hodges, K.E. 2004. Can climate trends be calculated from reanalysis data? Journal of Geophysical Research 109: 10.1029/2004JD004536.

Briffa, K.R. 2000. Annual climate variability in the Holocene: Interpreting the message of ancient trees. Quaternary Science Reviews 19: 87-105.

Broecker, W.S. 2001. Was the Medieval Warm Period global? Science 291: 1497-1499.

Bunn, A.G., Graumlich, L.J. and Urban, D.L. 2005. Trends in twentieth-century tree growth at high elevations in the Sierra Nevada and White Mountains, USA. The Holocene 15: 481-488.

Christy, J.R., Spencer, R.W., Norris, W.B., Braswell, W. and Parker, D.E. 2003. Error estimates of version 5.0 of MSU-AMSU bulk atmospheric temperatures. Journal of Atmospheric and Oceanic Technology 20: 613-129.

Chylek, P., Folland, C.K., Lesins, G., Dubey, M.K. and Wang, M. 2009. Arctic air temperature change amplification and the Atlantic Multidecadal Oscillation. Geophysical Research Letters 36: 10.1029/2009GL038777.

Damon, P.E. and Jirikowic, J.L. 1992. Solar forcing of global climate change? In: Taylor, R.E., Long A. and Kra, R.S. (Eds.), Radiocarbon After Four Decades. Springer-Verlag, Berlin, Germany, pp. 117-129.

Damon, P.E. and Laut, P. 2004. Pattern of strange errors plagues solar activity and terrestrial climatic data. EOS, Transactions, American Geophysical Union 85: 370, 374.

Dean, J.S. 1994. The medieval warm period on the southern Colorado Plateau. Climatic Change 26: 225-241.

De'er, Z. 1994. Evidence for the existence of the medieval warm period in China. Climatic Change 26: 289-297.

De Laat, A.T.J. and Maurellis, A.N. 2004. Industrial CO2 emissions as a proxy for anthropogenic influence on lower tropospheric temperature trends. Geophysical Research Letters 31: 10.1029/2003GL019024.

Denton, G.H. and Karlen, W. 1973. Holocene climate variations - their pattern and possible cause. Quaternary Research 3: 155-205.

Easterling, D.R. and Wehner, M.F. 2009. Is the climate warming or cooling? Geophysical Research Letters 36: 10.1029/2009GL037810.

Friis-Christensen, E. and Lassen, K. 1991. Length of the solar cycle: An indicator of solar activity closely associated with climate. Science 254: 698-700.

Gallo, K.P., Easterling, D.R. and Peterson, T.C. 1996. The influence of land use/land cover on climatological values of the diurnal temperature range. Journal of Climate 9: 2941-2944.

Gallo, K.P., Owen, T.W., Easterling, D.R. and Jameson, P.F. 1999. Temperature trends of the historical climatology network based on satellite-designated land use/land cover. Journal of Climate 12: 1344-1348.

Graumlich, L.J. 1991. Subalpine tree growth, climate, and increasing CO2: an assessment of recent growth trends. Ecology 72: 1-11.

Graybill, D.A. 1987. A network of high elevation conifers in the western US for detection of tree-ring growth response to increasing atmospheric carbon dioxide. In: Jacoby, G.C. and Hornbeck, J.W., Eds. Proceedings of the International Symposium on Ecological Aspects of Tree-Ring Analysis. U.S. Department of Energy Conference Report DOE/CONF8608144, pp. 463-474.

Graybill, D.A. and Idso, S.B. 1993. Detecting the aerial fertilization effect of atmospheric CO2 enrichment in tree-ring chronologies. Global Biogeochemical Cycles 7: 81-95.

Grove, J.M. 1988. The Little Ice Age. Cambridge University Press, Cambridge, UK.

Grove, J.M. and Switsur, R. 1994. Glacial geological evidence for the medieval warm period. Climatic Change 26: 143-169.

Herbert, T.D., Schuffert, J.D., Andreasen, D., Heusser, L., Lyle, M., Mix, A., Ravelo, A.C., Stott, L.D. and Herguera, J.C. 2001. Collapse of the California Current during glacial maxima linked to climate change on land. Science 293: 71-76.

Holmgren, K., Karlen, W., Lauritzen, S.E., Lee-Thorp, J.A., Partridge, T.C., Piketh, S., Repinski, P., Stevenson, C., Svanered, O. and Tyson, P.D. 1999. A 3000-year high-resolution stalagmite-based record of paleoclimate for northeastern South Africa. The Holocene 9: 295-309.

Holmgren, K., Tyson, P.D., Moberg, A. and Svanered, O. 2001. A preliminary 3000-year regional temperature reconstruction for South Africa. South African Journal of Science 99: 49-51.

Huang, S., Pollack, H.N. and Shen, P.-Y. 2000. Temperature trends over the past five centuries reconstructed from borehole temperatures. Nature 403: 756-758.

Idso, S.B. 1986. Industrial age leading to the greening of the Earth? Nature 320: 22.

Idso, S.B. 1988. Greenhouse warming or Little Ice Age demise: A critical problem for climatology. Theoretical and Applied Climatology 39: 54-56.

Idso, S.B. 1995. CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution. Third Annual Kuehnast Lecture. Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN.

Idso, S.B. 1998. CO2-induced global warming: a skeptic's view of potential climate change. Climate Research 10: 69-82.

Imbrie, J. and Imbrie, K.P. 1979. Ice Ages. Enslow Publishers, Short Hills, New Jersey, USA.

Intergovernmental Panel on Climate Change. 2007. Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S. et al. (Eds.). Cambridge University Press, Cambridge, United Kingdom.

Jacoby G.C. 1986. Long-term temperature trends and a positive departure from the climate-growth response since the 1950s in high elevation lodgepole pine from California. In: Rosenzweig, C. and Dickinson, R. Eds. Proceedings of the NASA Conference on Climate-Vegetation Interactions. Office for Interdisciplinary Earth Studies (OIES), University Corporation for Atmospheric Research (UCAR), Boulder, Colorado, USA, pp. 81-83.

Kalnay, E. and Cai, M. 2003. Impact of urbanization and land-use change on climate. Nature 423: 528-531.

Karlen, W. and Kuylenstierna, J. 1996. On solar forcing of Holocene climate: evidence from Scandinavia. The Holocene 6: 359-365.

Keigwin, L.D. 1996a. Sedimentary record yields several centuries of data. Oceanus 39 (2): 16-18.

Keigwin, L.D. 1996b. The little ice age and the medieval warm period in the Sargasso Sea. Science 274: 1504-1508.

Kienast, F. and Luxmoore, R.J. 1998. Tree-ring analysis and conifer growth responses to increased atmospheric CO2 levels. Oecologia 76: 487-495.

Knapp, P.A. and Soule, P.T. 1996. Vegetation change and the role of atmospheric CO2 enrichment on a relict site in central Oregon: 1960-1994. Annals of the Association of American Geographers 86: 387-411.

Knapp, P.A. and Soule, P.T. 1998. Recent Juniperus occidentalis (Western Juniper) expansion on a protected site in central Oregon. Global Change Biology 4: 347-357.

Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001a. Detecting the potential regional effects of increased atmospheric CO2 on growth rates of western juniper. Global Change Biology 7: 903-917.

Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001b. Post-drought growth responses of western juniper (Juniperus occidentalis var. occidentalis) in central Oregon. Geophysical Research Letters 28: 2657-2660.

Koch, G.W., Sillett, S.C., Jennings, G.M. and Davis, S.D. 2004. The limits to tree height. Nature 428: 851-854.

LaMarche Jr., V.C., Graybill, D.A., Fritts, H.C. and Rose, M.R. 1984. Increasing atmospheric carbon dioxide: Tree ring evidence for growth enhancement in natural vegetation. Science 225: 1019-1021.

Lamb, H.H. 1977. Climate History and the Future. Methuen, London, United Kingdom.

Lamb, H.H. 1984. Climate in the last thousand years: natural climatic fluctuations and change. In: Flohn, H. and Fantechi, R. (Eds.), The Climate of Europe: Past, Present and Future. D. Reidel, Dordrecht, The Netherlands, pp. 25-64.

Lamb, H.H. 1988. Weather, Climate and Human Affairs. Routledge, London, United Kingdom.

Lassen, K. and Friis-Christensen, E. 2000. Reply to "Solar cycle lengths and climate: A reference revisited" by P. Laut and J. Gundermann. Journal of Geophysical Research 105: 27,493-27,495.

Le Roy Ladurie, E. 1971. Times of Feast, Times of Famine: A History of Climate Since the Year 1000. Doubleday, New York, New York, USA.

Lean, J.L. and Rind, D.H. 2008. How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. Geophysical Research Letters 35: 10.1029/2008GL034864.

Leavitt, S.W. 1994. Major wet interval in White Mountains medieval warm period evidenced in d13C of bristlecone pine tree rings. Climatic Change 26: 299-307.

Li, H., Ku, T.-L., Wenji, C. and Tungsheng, L. 1997. Isotope studies of Shihua Cave; Part 3, Reconstruction of paleoclimate and paleoenvironment of Beijing during the last 3000 years from delta and 13C records in stalagmite. Dizhen Dizhi 19: 77-86.

Lindzen, R.S. and Choi, Y.-S. 2009. On the determination of climate feedbacks from ERBE data. Geophysical Research Letters 36: 10.1029/2009GL039628.

Lindzen, R.S. and Choi, Y.-S. 2011. On the observational determination of climate sensitivity and its implications. Asia-Pacific Journal of Atmospheric Sciences 47: 377-390.

Loehle, C. 2004. Climate change: detection and attribution of trends from long-term geologic data. Ecological Modelling 171: 433-450.

Luckman, B.H. 1994. Evidence for climatic conditions between ca. 900-1300 A.D. in the southern Canadian Rockies. Climatic Change 26: 171-182.

Ludecke, H.-J., Link, R. and Ewert, F.-K. 2011. How natural is the recent centennial warming? An analysis of 2249 surface temperature records. International Journal of Modern Physics C 22: 10.1142/S0129183111016798.

Magny, M. 1993. Solar influences on Holocene climatic changes illustrated by correlations between past lake-level fluctuations and the atmospheric 14C record. Quaternary Research 40: 1-9.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Mayewski, P.A., Meeker, L.D., Twickler, M.S., Whitlow, S., Yang, Q., Lyons, W.B. and Prentice, M. 1997. Major features and forcing of high-latitude northern hemisphere atmospheric circulation using a 110,000-year-long glaciochemical series. Journal of Geophysical Research 102: 26,345-26,366.

McIntyre, S. and McKitrick, R. 2003. Corrections to the Mann et al. (1998) proxy data base and Northern Hemispheric average temperature series. Energy and Environment 14: 751-771.

McKitrick, R.R. and Michaels, P.J. 2007. Quantifying the influence of anthropogenic surface processes and inhomogeneities on gridded global climate data. Journal of Geophysical Research 112: 10.1029/2007JD008465.

Mencuccini, M., Martinez-Vilalta, J., Vanderklein, D., Hamid, H.A., Korakaki, E., Lee, S. and Michiels, B. 2005. Size-mediated ageing reduces vigour in trees. Ecology Letters 8: 1183-1190.

PAGES 2k Consortium. 2013. Continental-scale temperature variability during the past two millennia. Nature Geoscience 6: 339-345.

Perlwitz, J., Hoerling, M., Eischeid, J., Xu, T. and Kumar, A. 2009. A strong bout of natural cooling in 2008. Geophysical Research Letters 36: 10.1029/2009GL041188.

Petersen, K.L. 1994. A warm and wet little climatic optimum and a cold and dry little ice age in the southern Rocky Mountains, U.S.A. Climatic Change 26: 243-269.

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.

Phillips, N.G., Buckley, T.N. and Tissue, D.T. 2008. Capacity of old trees to respond to environmental change. Journal of Integrative Plant Biology 50: 1355-1364.

Phillips, O.L. and Gentry, A.H. 1994. Increasing turnover through time in tropical forests. Science 263: 954-958.

Polley, H.W., Johnson, H.B., Marino, B.D. and Mayeux, H.S. 1993. Increases in C3 plant water-use efficiency and biomass over glacial to present CO2 concentrations. Nature 361: 61-64.

Reifen, C. and Toumi, R. 2009. Climate projections: Past performance no guarantee of future skill? Geophysical Research Letters 36: 10.1029/2009GL038082.

Ruddiman, W.F., Vavrus, S.J. and Kutzbach, J.E. 2005. A test of the overdue-glaciation hypothesis. Quaternary Science Reviews 24: 1-10.

Santer, B.D., Wigley, T.M.L., Gaffen, D.J., Bengtsson, L., Doutriaux, C., Boyle, J.S., Esch, M., Hnilo, J.J., Jones, P.D., Meehl, G.A., Roeckner, E., Taylor, K.E. and Wehner, M.F. 2000. Interpreting differential temperature trends at the surface and in the lower troposphere. Science 287: 1227-1232.

Scafetta, N. 2012. Testing an astronomically based decadal-scale empirical harmonic climate model versus the IPCC (2007) general circulation climate models. Journal of Atmospheric and Solar-Terrestrial Physics: 10.1016/j.jastp.2011.12.005.

Serre-Bachet, F. 1994. Middle Ages temperature reconstructions in Europe, a focus on Northeastern Italy. Climatic Change 26: 213-224.

Simmons, A.J. and Gibson, J.K. 2000. The ERA-40 Project Plan, ERA-40 Project Rep. Ser., 1. European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom.

Soule, P.T. and Knapp, P.A. 2006. Radial growth rate increases in naturally occurring ponderosa pine trees: a late-20th century CO2 fertilization effect? New Phytologist 171: 379-390.

Soule, P.T., Knapp, P.A. and Grissino-Mayer, H.D. 2003. Human agency, environmental drivers, and western juniper establishment during the late Holocene. Ecological Applications 14: 96-112.

Soule, P.T., Knapp, P.A. and Grissino-Mayer, H.D. 2004. Comparative rates of western juniper afforestation in south-central Oregon and the role of anthropogenic disturbance. Professional Geographer 55: 43-55.

Svensmark, H. 1998. Influence of cosmic rays on Earth's climate. Physical Review Letters 22: 5027-5030.

Svensmark, H. and Friis-Christensen, E. 1997. Variation of cosmic ray flux and global cloud coverage - A missing link in solar-climate relationships. Journal of Atmospheric and Solar-Terrestrial Physics 59: 1225-1232.

Van Aardenne, J.A., Dentener, F.J., Olivier, J.G.J., Klein Goldewijk, C.G.M. and Lelieveld, J. 2001. A 1° x 1° resolution data set of historical anthropogenic trace gas emissions for the period 1890-1990. Global Biogeochemical Cycles 15: 909-928.

Villalba, R. 1994. Tree-ring and glacial evidence for the medieval warm epoch and the little ice age in southern South America. Climatic Change 26: 183-197.

Vinnikov, K.Y. and Grody, N.C. 2003. Global warming trend of mean tropospheric temperature observed by satellites. Science 302: 269-272.

Watanabe, O., Jouzel, J., Johnsen, S., Parrenin, F., Shoji, H. and Yoshida, N. 2003. Homogeneous climate variability across East Antarctica over the past three glacial cycles. Nature 422: 509-512.

Woodruff, D.R., Bond, J.B. and Meinzer, F.C. 2004. Does turgor limit growth in tall trees? Plant, Cell and Environment 27: 229-236.

Last updated 25 December 2013