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Temperature Trends (Potential Inaccuracies) -- Summary
Over the years, a large number of potential problems associated with various ways of assessing the temperatures of diverse types of land cover have been identified; and several of them are briefly described, with an emphasis on demonstrating the great difficulty of obtaining various surface-specific sets of annual near-surface air temperature which - when combined for the earth as a whole - yield a set of yearly mean values that accurately represents the mean near-surface thermal history of the planet well enough to determine whether or not the earth may be warming at the rate that is typically predicted by climate model assessments of the greenhouse effect of the CO2 emitted to the atmosphere as a consequence of mankind's utilization of fossil fuels, such as coal, gas and oil.

First of all, there are a number of mundane difficulties that are encountered in the course of obtaining numerous histories of accurate surface air temperature measurements and assembling them into an aggregate history of global climate change. These problems include (1) temporal changes in microclimate surrounding temperature measurement sites, such as urbanization, which often go unrecognized or for which insufficient adjustments are made, (2) long-term degradation of the shelters that house the temperature-measuring equipment, such as the shelters' white paint becoming less reflective and their louvers partially obstructed, (3) changes in what is actually being measured, such as true daily maximum and minimum temperatures or temperatures at specified times of day, (4) changes in measurement devices and ways of accessing the data, such as changing from having to open the shelter door to read the temperature, as was done in earlier days, to not having to do so, due to the automatic recording of the data, as has become commonplace in more recent times, (5) general station degradation and many station closures over time, (6) the changing and uneven geographical representation of the surface temperature network, (7) poor attention to careful acquisition of data in many parts of the world, and (8) a number of problems associated with obtaining a correct and geographically complete record of surface air temperature over the 70% of the globe that is covered by oceans. In this summary, however, a group of more esoteric problems will be discussed.

Correia and Safanda (1999) reviewed a set of twenty temperature logs derived from boreholes located at fourteen different sites in mainland Portugal in an attempt to reconstruct a five-century surface air temperature history for that part of the world; but little did they realize how difficult the task would be. For starters, seven of the borehole temperature logs were too "noisy" to use; while six displayed evidence of groundwater perturbations and were thus not usable for that reason. Of the remaining seven logs, all depicted little temperature change over the first three centuries of record. Thereafter, however, four of them exhibited warming trends that began about 1800 and peaked around 1940, one showed a warming that peaked in the mid-1800s, another was constant across the entire five centuries, and one actually revealed cooling over the last century. And at this point in time, the two researchers concluded that "the single inversions cannot be interpreted individually."

Not inclined to admit defeat, however, Correia and Safanda next performed a joint analysis of the seven borehole records, obtaining a warming of 0.5-0.6°C since the second half of the 18th century, which was followed by a cooling of 0.2°C. They then compared the joint borehole record with the surface air temperature record directly measured at a meteorological station in Lisbon, about 150-200 km to the northwest. And the result? From the beginning of the surface air temperature record in 1856 until 1949, the Lisbon data yielded a warming of 0.8°C; while the borehole record displayed a warming of only 0.3-0.4°C. So which was correct?

It is possible that the two temperature histories could both be right; for they are separated from each other by a significant distance. But they could also both be wrong. The Lisbon record, for example, could suffer from a number of urban heat island-type problems; while the joint borehole record was only obtained after several individual records were rejected for various reasons and a number of simplifying assumptions were invoked in the analyses of the remaining records. The authors reluctantly recognized these problems, noting that the issue was not yet resolved and that much more work needed to be done to arrive at a satisfactory conclusion.

Publishing concurrently, Changnon (1999) described how he used a series of soil temperature measurements obtained in a totally rural setting in central Illinois between 1889 and 1952 and a contemporary series of air temperature measurements made in an adjacent growing community, as well as similar data obtained from other nearby small towns, to evaluate the magnitude of unsuspected heat island effects that may be present in small towns and cities that are typically assumed by the IPCC to be free of urban-induced warming. And in this situation, the soil temperatures obtained in the totally rural setting revealed the existence of a temperature increase from the decade of 1901-1910 to that of 1941-1950 that amounted to 0.4°C, which warming was 0.2°C less than the 0.6°C warming determined for the same time period from data of the U.S. Historical Climate Network, which is supposedly corrected for urban heating effects. It was also 0.2°C less than the 0.6°C warming determined for this time period by eleven benchmark stations in Illinois with the highest quality long-term temperature data, all of which are located in communities with populations of less than 6,000 people as of 1990. And it was 0.17°C less than the 0.57°C warming derived from data obtained from the three benchmark stations closest to the site of the soil temperature measurements and with populations of less than 2,000 people.

As for the significance of his findings, the world-renowned climate specialist said they suggest that "both sets of surface air temperature data for Illinois believed to have the best data quality with little or no urban effects may contain urban influences causing increases of 0.2°C from 1901 to 1950." And he further remarked - in a grand understatement - that "this could be significant because the IPCC (1995) indicated that the global mean temperature increased 0.3°C from 1890 to 1950."

Contemporaneously, Cowling and Sykes (1999) noted that many tried-and-(supposedly)-true methods of palaeoclimate reconstruction "are built upon the assumption that plant-climate interactions remain the same through time or that these interactions are independent of changes in atmospheric CO2." This assumption had been challenged nearly a dozen years earlier by Idso (1989) and a few years later by Polley et al. (1993). And since that time, a number of other scientists had also come to the same conclusion, such that there was a sufficient volume of published research on the topic to conduct a review of it, which is precisely what Cowling and Sykes did. And their conclusion? "A growing number of physiological and palaeoecological studies indicate that plant-climate interactions are likely not the same through time because of sensitivity to atmospheric CO2."

So why aren't they the same? Cowling and Sykes describe three major reasons. First, they note that "C3-plant physiological research shows that the processes that determine growth optima in plants (photosynthesis, mitochondrial respiration, photorespiration) are all highly CO2-dependent." Second, they say "the ratio of carbon assimilation per unit transpiration (called water-use efficiency) is sensitive to changes in atmospheric CO2." And third, they report that "leaf gas-exchange experiments indicate that the response of plants to carbon-depleting environmental stresses are strengthened under low CO2 relative to today."

All of these phenomena combine to produce dramatic increases in plant growth and water use efficiency with increases in the air's CO2 content; and such increases in vegetative vigor had - to the date of their writing - been interpreted, almost exclusively, not in terms of atmospheric CO2 variations, but in terms of changes in air temperature and/or precipitation. Clearly, therefore, all such plant-based climate reconstructions that had been made for a period of time over which the air's CO2 content had experienced a significant change must have been in error to some degree, unless they accounted for the growth-enhancing effects of the CO2 increase, which none had yet done, excepting LaMarche et al. (1984) and Graybill and Idso (1993).

This point is extremely important, because many people had been using long-term tree-ring chronologies - specifically, Mann et al. (1998, 1999) - to create a climatic history of the earth that exhibits dramatic late-20th century warming that was likely far too great. As Briffa (2000) explains, the recent high growth rates of the trees in these chronologies "provide major pieces of evidence being used to assemble a case for anomalous global warming, interpreted by many as evidence of anthropogenic activity." Yet, as he continued, "the empirically derived regression equations upon which our reconstructions are based may be compromised if the balance between photosynthesis and respiration is changed" by anything other than air temperature. And as Cowling and Sykes concluded as a consequence of their literature review, "implicit in the assumption that plant-climate relationships remain the same through time is the notion that temperature-plant interactions are independent of changes in atmospheric CO2, which is not supported by physiological data."

Unfortunately, therefore, the flawed studies of Mann et al. fast became the centerpiece (Crowley, 2000; Mann, 2000) of the IPCC's misguided effort to rewrite earth's climatic history in an attempt to prod national governments to adopt Kyoto-type measures to combat what they professed to be CO2-induced global warming.

One year later, and changing gears a bit, Darling et al. (2000) reported on their examination of the genetic variation in the small subunit ribosomal RNA gene of three morpho-species of planktonic foraminifera from Arctic and Antarctic subpolar waters, which led to the discovery that foraminiferal morphospecies can consist of a complex of genetic types. In fact, they found that each "species," as these entities had previously been designated, was composed of three to five distinct genetic varieties that could possibly be classified as individual species themselves. And whereas the morphological, chemical and stable-isotope differences associated with the calcitic shells of the three morphospecies studied by them are used extensively by palaeoceanographers for purposes of climate reconstruction, based on the assumption that each morphospecies represents a genetically continuous species with a single environmental/habitat preference, they subsequently warned that "if this is not the case - as indicated by their study and others - stable-isotope and geochemical analyses of planktonic foraminiferal shells, and census-based transfer-function techniques derived from such pooled data, must include significant noise, if not error." Thus, as more is learned about the diversity of these biotic climate indicators, it is likely that certain palaeoclimatic histories may have to be revised.

Four years later, while exploring land cover change as a potential cause of the decline in rainfall and associated warming in southwest Western Australia in the middle of the past century, Pitman et al. (2004) used three high-resolution mesoscale models to simulate five different July climates for both natural and current land cover conditions. This effort revealed that the calculated changes in precipitation caused by the historical land cover change were similar to the observed precipitation changes in both magnitude and pattern. Hence, they reasoned that since the simulated precipitation changes were consistent for each of the three models for each of the five July climatic conditions studied, "it is extremely unlikely that this simulated pattern matches the observed pattern by coincidence." And they therefore concluded that "since we did not vary sea surface temperatures, carbon dioxide or the lateral boundary conditions between the simulations using current and natural land cover, the changes we simulate cannot be explained in these terms." But with respect to what did matter, they reported that "a change in just the roughness length and the zero plane displacement height [resulting from the removal of forest trees] could explain the rainfall changes." More specifically, they stated that the "current land cover reduces frictional drag and leads to increased horizontal winds that advect moisture being carried onshore from the Indian Ocean further inland," where strong convergence leads to increased vertical velocities and "the combination of more moisture and increased vertical velocities increases precipitation," which is exactly what observations showed to be the case. Thus, the four scientists suggested that" attributing the warming in this region to the enhanced greenhouse effect is premature."

Subsequent to Pitman et al.'s study, D'Arrigo et al. (2005) developed a tree-ring-based reconstruction of the December-May North Pacific Index (NPI) - which is a measure of the atmospheric circulation related to the Aleutian low pressure cell - for the period 1600-1983, based on data that they derived from 18 tree-ring chronologies, which they selected from a total of 67 candidate chronologies obtained from sites surrounding the North Pacific rim that calibrated "significantly at or above the 90% significance level" against winter/spring monthly values of the NPI derived from 20th-century instrumental data, after which they employed an intervention analysis to the NPI reconstruction "to identify significant shifts in the series."

This work revealed, as the nine researchers describe it, that "the NPI reconstruction successfully tracks the known regime shifts (1924/25, 1946/47, and 1976/77) seen in the instrumental NPI during the twentieth century," and they also state that "prior to the instrumental period there are decadal-scale variations that may also represent regime shifts," noting that "significant 'shifts' (at the 90% confidence limit) are identified in 1627, 1695, 1762, 1806, 1833, 1853, and 1891." And they thus concluded that their analysis suggests that "the 1976 transition was not unique in terms of magnitude." In addition, the recurring nature of the climate regime shifts suggests that they are natural non-anthropogenic-forced phenomena that have nothing to do with the historical increase in the air's CO2 content. And this conclusion is particularly noteworthy in light of the fact that the study of Seidel and Lanzante (2004) had suggested, in their words, that "it is reasonable to consider most of the warming during 1958-2001 to have occurred at the time of the climate 'regime shift,' modeled here at the start of 1977." Consequently, the complementary findings of these two studies do much to relieve anthropogenic CO2 emissions of responsibility for the global warming of the prior fifty or more years.

Returning to boreholes for a moment, Bodri and Cermak (2005) noted that temperature profiles derived from them had been increasingly used to obtain proxy climate signals at various locations across the surface of the earth. However, they cautioned that the techniques employed in reconstructing pre-observational (pre-instrumental era) mean temperatures from borehole data suffer from certain limitations, one of the major ones being the presence of underground fluids that can distort the true climatic signal (Lewis and Wang, 1992). In their study, therefore, they developed a corrective measure that accounts for vertical conductive and advective heat transport in a 1-D horizontally-layered stratum - as opposed to the then-popular purely conductive approach - which they proceeded to apply to four borehole records drilled near Tachlovice in the Czech Republic.

The duo's analyses of the four borehole temperature logs revealed that the new conductive/advective approach was far superior to the purely conductive approach, explaining 83-95% of the temperature signal where the purely conductive model could explain no more than 27-58%. In addition, the purely conductive approach was found to significantly underestimate the pre-observational mean temperature by 0.3 to 0.5°C. And this underestimate produces a significant overestimate of the degree of warming experienced from the pre-observational period to the present. What is more, the two scientists report that both of the pre-observational mean temperature values for 18th-century Bohemia (the one derived from the conductive/advective approach and the one derived from the purely conductive approach) "exceed the annual temperatures characteristic for the 19th/20th centuries," which "may indicate that the warming has still not achieved its earlier (late 18th century) level."

Also hard at work in the same year as Bodri and Cermak was the team of Zhang et al. (2005), who utilized the approach of Kalnay and Cai (2003) to determine the impacts of land-use changes on surface air temperature throughout eastern China (east of 110°E), where rapid urbanization, deforestation, desertification and other changes in land use occurred over the last quarter-century, focusing on daily mean, maximum and minimum air temperatures from 259 stations over the period 1960 to 1999. This work revealed that changes in land-use had little to no influence on daily maximum temperatures, but that they explained about 18% of the observed daily mean temperature increase and 29% of the observed daily minimum temperature increase in this region over the past 40 years, yielding decadal warming trends of about 0.12°C and 0.20°C for these two parameters, respectively, and convincingly demonstrating that changes in land-use can have significant impacts on near-surface air temperatures, which consequence raises some serious questions about the magnitude of late 20th-century global warming.

Contemporaneously, and in a more esoteric approach to the subject, Cohn and Lins (2005) analyzed statistical trend tests of hydroclimatological data such as discharge and air temperature in the presence of long-term persistence (LTP), in order to determine what LTP, if present, implied about the significance of observed trends. In doing so, they determined that "the presence of LTP in a stochastic process can induce a significant trend result when no trend is present, if an inappropriate trend test is used." Also, they say that "given the LTP-like patterns we see in longer hydroclimatological records ... such as the periods of multidecadal drought that occurred during the past millennium and our planet's geologic history of ice ages and sea level changes, it might be prudent to assume that hydroclimatological processes could possess LTP." And they add, in this regard, that "nearly every assessment of trend significance in geophysical variables published during the past few decades has failed to account properly for long-term persistence."

So what is their take-home message? With respect to temperature data, they note there is overwhelming evidence that the planet has warmed during the past century. However, as they ask, "could this warming be due to natural dynamics?" Answering their own question, they say that "given what we know about the complexity, long-term persistence, and non-linearity of the climate system, it seems the answer might be yes." Therefore, the bottom line with respect to the implications of their work is that although reported temperature trends may be real, they may also be insignificant, which leads, in the words of Cohn and Lins, to "a worrisome possibility," i.e., that "natural climatic excursions may be much larger than we imagine ... so large, perhaps, that they render insignificant the changes, human-induced or otherwise, observed during the past century."

Jumping ahead a couple more years, McKitrick and Michaels (2007) stated 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." Unimpressed by this position, however, and working with data from all available land-based grid cells around the world, they evaluated 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 learn? In the words of the two researchers, this hypothesis "is strongly rejected, indicating that extraneous (nonclimatic) 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." Finally, 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."

One year later, D'Arrigo et al. (2008) dissected the divergence problem, which they describe as "an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings," which problem has been detected in tree-ring width records from many circumpolar northern latitude sites since around the middle of the 20th century. This they did by reviewing the literature that had been published on the subject to that point in time, and by assessing the possible causes and implications of the phenomenon.

With respect to the cause or causes of the divergence problem, the four researchers listed the following possibilities: (1) moisture stress, (2) non-linear or threshold responses to warming, (3) local pollution, (4) delayed snowmelt, (5) changes in seasonality, (6) differential responses to maximum and minimum temperatures, (7) global dimming, (8) methodological issues related to "end effects," (9) biases in instrumental target data, (10) the modeling of such data, (11) declining stratospheric ozone concentrations, (12) increased UV-B radiation at ground level, and (unlucky number 13) "an upward bias in surface thermometer temperature measurements in recent years related to heat island effects."

In discussing their findings, D'Arrigo et al. note that one of the major difficulties resulting from the existence of the divergence problem is that "reconstructions based on northern tree-ring data impacted by divergence cannot be used to directly compare past natural warm periods (notably, the Medieval Warm Period) with recent 20th century warming, making it more difficult to state unequivocally that the recent warming is unprecedented." And with respect to the resolution of the issue, they say that their review "did not yield any consistent pattern that could shed light on whether one possible cause of divergence might be more likely than others," leading them to conclude that "a combination of reasons may be involved that vary with location, species or other factors, and that clear identification of a sole cause for the divergence is probably unlikely." Therefore, one can validly conclude, from their point of view, that there are a number of reasons why it is premature to be claiming that recent warming is unprecedented over the past millennium or more, particularly on the basis of tree-ring width data.

Tackling the same problem, Gonzales et al. (2008) wrote that (1) "there has been a growing awareness that past variations in CO2 may have globally influenced plant physiology, vegetation composition, and vegetation structure (Idso, 1989a; Cowling, 1999; Cowling and Sykes, 1999; Wu et al., 2007a,b)," and that (2) "this information has spurred a debate over the relative importance of CO2 versus climate as drivers of Quaternary vegetation change." Likewise, Idso (1989b) wrote that "since atmospheric CO2 has varied so dramatically in the past, it would seem to be an almost unavoidable conclusion that the effects of atmospheric CO2 on plant water use efficiency would significantly influence the floristic (i.e., species) composition of plant communities, as well as their distributions in space and time, and that this phenomenon, largely disregarded in the reconstruction of past climates, may be introducing errors into interpretations of several paleoclimatic indicators."

Thinking along these same lines, Gonzales et al. compared simulated and pollen-inferred leaf area index (LAI) values with regional vegetation histories of northern and eastern North America "to assess both data and model accuracy and to examine the relative influences of CO2 and climate on vegetation structure over the past 21,000 years." In doing so, they made use of BIOME4, "a biogeochemistry-biogeography equilibrium vegetation model (Kaplan, 2001)" that "was designed in part for paleo-vegetation applications, and has been widely used to simulate vegetation responses to late-Quaternary CO2 and climates." Concurrently, and "to provide paleo-climate scenarios for the BIOME4 simulations," the three researchers say they "used surface temperature, precipitation and cloudiness values from a series of Hadley Centre Unified Model simulations."

In discussing their results, Gonzales et al. noted that this was the first study "to use both BIOME4 simulations and pollen-based reconstructions to develop detailed Quaternary LAI histories for North America," and they report that their "BIOME4 sensitivity experiments indicated that climate was the primary driver of late-Quaternary changes in LAI in northern and eastern North America, with CO2 a secondary factor."

Even more important than this specific conclusion, however - which could well be modified somewhat as subsequent related studies are conducted - is Gonzales et al.'s observation that their work "emphasizes the need for models to incorporate the effects of both CO2 and climate on [the reconstruction of] late-Quaternary vegetation dynamics and structure." In like manner, their work also emphasizes the need for vegetation-based climate reconstructions to incorporate the biological effects of changes in atmospheric CO2 concentration, especially when attempting to compare late 20th-century reconstructed temperatures with reconstructed temperatures of the Roman and Medieval Warm Periods and the Holocene Climatic Optimum; for until this deficiency is corrected, truly valid comparisons between these earlier times and the present cannot be made based on tree-ring width data, for without properly adjusting for the growth- and water use efficiency-enhancing effects of the historical increase in the air's CO2 content on tree biomass production, reconstructed 20th-century temperatures -- which must be used in place of actual measured values when making comparisons with earlier reconstructed temperatures - will be artificially inflated.

Working concurrently, Knapp and Soule (2008) examined recent radial growth increases in western juniper trees (Juniperus occidentalis var. occidentalis Hook.) based on their analysis of a master tree-ring chronology dating from AD 1000-2006, which they developed from eleven semi-arid sites in the interior U.S. Pacific Northwest that had experienced minimal anthropogenic influence, other than that provided by the historical increase in the air's CO2 content that is everywhere present.

Their first step after developing the chronology was to use measured climate data for the period 1907-2006 to determine to which climatic parameter tree radial growth was most responsive: temperature, precipitation or drought severity, as represented by the Palmer Drought Severity Index (PDSI) for the month of June. This exercise revealed June PDSI to be the most important factor, explaining fully 54% of annual radial growth variability; and when they added CO2 as a second predictive factor, they found it "accounted for a 14% increase in explanatory power." In addition, they report that "use of the PDSI-only regression model produced almost exclusively positive residuals since 1977," but that "the +CO2 model has a greater balance of positive (53%) and negative residuals over the same period." And as a result of these findings, plus those of other tests they performed with the data, they concluded that "climatic reconstructions based on pre-1980 data would not be significantly influenced by rising CO2 levels," but that reconstructions produced after that time would be.

These observations suggest that the late-20th-century/early-21st-century radial growth of the western juniper trees - which was 27% greater than the long-term (AD 1000-2006) average during the period 1977-2006 that Knapp and Soule describe as being "unlike any other period during the last millennium," i.e., truly unprecedented - was likely due to the increase in the air's CO2 content over the latter period. It for sure could not have been due to any increase in air temperature; for they report that "western juniper responds negatively to temperature, negating any linkages to regional warming." Neither could the anomalous growth increase have been due to anomalous nitrogen fertilization, for the two researchers note that "the eleven chronology sites do not fall under any of the criteria used to identify ecosystems significantly impacted by N-deposition," citing the work of Fenn et al. (2003).

Consequently, Knapp and Soule were left with about the only remaining alternative explanation, i.e., that the truly unprecedented increase in the radial growth rates of western juniper trees throughout the interior U.S. Pacific Northwest over the last few decades of the 20th century, as well as throughout the initial years of the 21st century, was likely the result of the positive impact of the recent large increase in the air's CO2 concentration on the water use efficiency of the trees. Furthermore, since the water use efficiency of nearly all trees exhibits a significant positive response to atmospheric CO2 enrichment, it follows that nearly all radial growth chronologies likely possess a recent positive component that was not caused by rising temperatures or anomalous nitrogen fertilization, but by the sizable concomitant increase in the air's CO2 content. And this likelihood must be factored into all analyses designed to reconstruct temperature histories from long-term tree-ring chronologies, especially those that are developed for the purpose of comparing the degree of recent warmth with that of the Medieval Warm Period of a thousand years ago, when the air's CO2 concentration was more than a hundred parts per million less than it is today.

Making a final four-year leap forward, while noting that "deforestation exerts a number of regional and local climate effects," including "a decrease in water vapor mixing ratio (Sen et al., 2004), reduced precipitation (Werth and Avissar, 2005), and a change in the water cycle (Houghton, 1990)," along with "an increase in near-surface air temperature (Sampaio et al., 2007)," Gao and Liu (2012) studied the effect of the deforestation of portions of Heilongjiang Province in Northeast China, which has an annual temperature that ranges from -4°C to +4°C, with its winters being "long and frigid" and its summers "short and cool." This they did over the period 1958 to 1980, when forest cover was reduced from 238,335 km2 to 216,009 km2, and from 1980 to 2000, when forest cover was further reduced to 207,629 km2. This work revealed that over the entire period that the two researchers analyzed (from 1958-2000), there was a nation-wide warming of 0.99°C, while the annual temperature of Heilongjiang Province rose by 1.68°C, which suggests a concomitant deforestation-induced warming of 0.69°C. Thus, in response to the 13% reduction in forest cover over the 42-year interval that Gao and Liu analyzed, the mean annual temperature of Heilongjiang Province rose by 0.69°C, which is a truly substantial amount, considering that the temperature of the globe had only risen by an average of 0.6°C since the start of the Industrial Revolution. Perhaps, therefore, it is not so farfetched to think that a goodly portion of that global warming may have been due to one or more factors that have not yet been incorporated into the climate models that currently attribute the bulk of the post-Little Ice Age temperature increase to the direct climatic effect of anthropogenic CO2 emissions.

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Last updated 1 January 2014