Climate alarmists hotly contend that the degree of global warmth experienced over the latter part of the 20th century was greater than that experienced at any other time over the past two millennia. Why? Because this contention bolsters their claim that the so-called "unprecedented" temperatures of the past few decades were caused by the cumulative impact of anthropogenic CO2 emissions associated with the Industrial Revolution and its aftermath. As a result, most climate alarmists are loath to admit that temperatures of the Medieval Warm Period of a thousand years ago and the Roman Warm Period of two thousand years ago may have rivaled -- or even exceeded -- those of the recent past, since the atmospheric CO2 concentrations of those two earlier eras were much lower than today's CO2 concentrations. And if it was as warm as, or warmer than, it is today when there was much less CO2 in the air -- like more than 100 ppm less -- it is very possible, even likely, in fact, that the warmth of today may not be due to the atmosphere's higher CO2 concentration. In this Summary, therefore, we examine some of the evidence for the occurrence of the clearly non-CO2-induced Roman Warm Period, focusing our attention on North America.
Starting at the top of the continent, Norgaard-Pedersen and Mikkelsen (2009) worked with a sediment core they retrieved in August 2006 from the deepest basin of Narsaq Sound in southern Greenland, measuring and analyzing several properties of the materials thus obtained, from which they were able to infer what they describe as "glacio-marine environmental and climatic changes," which had occurred there over the prior 8,000 years. In doing so, they discovered that their coarse fraction and magnetic susceptibility data, along with their ice-rafted debris data, revealed the existence of two periods (2.3-1.5 ka and 1.2-0.8 ka) that "appear to coincide roughly with the 'Medieval Warm Period' and 'Roman Warm Period'." In addition, they identified the colder period that followed the Medieval Warm Period as the Little Ice Age and the colder period that preceded it as the Dark Ages Cold Period. And citing the works of Dahl-Jensen et al. (1998), Andresen et al. (2004), Jensen et al. (2004) and Lassen et al. (2004) -- who also studied the climatic history of Greenland -- the Danish scientists reported that the cold and warm periods identified in those earlier studies "appear to be more or less synchronous to the inferred cold and warm periods observed in the Narsaq Sound record," further testifying to the occurrence of both the Roman and Medieval Warm Periods in Greenland.
In another important study that appeared in the Proceedings of the U.S. National Academy of Sciences, Hu et al. (2001), as they describe it, "conducted multiproxy geochemical analyses of a sediment core from Farewell Lake in the northwestern foothills of the Alaska Range," obtaining thereby "the first high-resolution quantitative record of Alaskan climate variations that spans the last two millennia." These data, in their words, "suggest that at Farewell Lake SWT [surface water temperature] was as warm as the present at AD 0-300, after which it decreased steadily by ~3.5°C to reach a minimum at AD 600." From that point in time, they say "SWT increased by ~3.0°C during the period AD 600-850 and then exhibited fluctuations of 0.5-1.0°C until AD 1200." Completing their narrative, they say that "between AD 1200-1700, SWT decreased gradually by 1.25°C, and from AD 1700 to the present, SWT increased by 1.75C," leading to the establishment of the Current Warm Period.
In discussing their findings, Hu et al. remark that "the warmth before AD 300 at Farewell Lake coincides with a warm episode extensively documented in northern Europe," i.e., the Roman Warm Period, "whereas the AD 600 cooling is coeval with the European 'Dark Ages'." They also say that "the relatively warm climate AD 850-1200 at Farewell Lake corresponds to the Medieval Climatic Anomaly," which they describe as "a time of marked climatic departure over much of the planet." And they say that "these concurrent changes suggest large-scale teleconnections in natural climatic variability during the last two millennia, likely driven by atmospheric controls."
Noting that "20th-century climate is a major societal concern in the context of greenhouse warming," Hu et al. conclude by reiterating that their record "reveals three time intervals of comparable warmth: AD 0-300, 850-1200, and post-1800," and they say that "these data agree with tree-ring evidence from Fennoscandia, indicating that the recent warmth is not atypical of the past 1000 years," in unmistakable contradiction of those who claim that it is.
On the other side of the continent, Carbotte et al. (2004) located fossil oyster beds within the Tappan Zee area of the Hudson River estuary (New York, USA) via sub-bottom and side-scan sonar surveys, after which they retrieved sediment cores from the sites that provided shells for radiocarbon dating. This work revealed that oysters flourished there during the mid-Holocene warm period, when "summertime temperatures were 2-4°C warmer than today (e.g., Webb et al., 1993; Ganopolski et al., 1998)." Thereafter, they found that the oysters "disappeared with the onset of cooler climate at 4,000-5,000 cal. years BP," but that they "returned during warmer conditions of the late Holocene," which they identified as the Roman and Medieval Warm Periods as manifest in the studies of Keigwin (1996) and McDermott et al. (2001), explicitly stating that "these warmer periods coincide with the return of oysters in the Tappan Zee." Unfortunately, they report that their shell dates suggest a final "major demise," which they say is "consistent with the onset of the Little Ice Age." And they note that "similar aged fluctuations in oyster presence are observed within shell middens elsewhere along the Atlantic seaboard," citing results obtained all the way from Maine to Florida.
Also working within this particular stretch of coastline -- analyzing data obtained from four different sediment cores retrieved from the Chesapeake Bay between 1996 and 2000 -- were Cronin et al. (2003), who created a 2200-year record of spring sea surface temperature, using the Mg/Ca ratio as a paleothermometer, as per Chivas et al. (1986). For most of this period, which began at 200 BC, their sampling yielded one data point every eight years; but from AD 1700 to the end of the record, one data point was obtained every 1-3 years. And these data revealed that mean 20th-century temperatures were not any warmer than the mean temperatures of the first stage of the Medieval Warm Period. In addition, Cronin et al. determined there were similar periods of equivalent warmth during the Roman Warm Period, which was most strongly expressed between approximately 100 BC and AD 200.
In addition to its high temperatures, the Roman Warm Period in North America was often accompanied by drier conditions. Willard et al. (2003), for example, examined the late Holocene (since 2300 yr BP) record of Chesapeake Bay and its watershed through the study of fossil dinoflagellate cysts and pollen obtained from sediment cores. And in doing so, they found that "several dry periods ranging from decades to centuries in duration are evident in Chesapeake Bay records."
The first of these periods of lower precipitation (200 BC-AD 300) occurred during the Roman Warm Period, while the next such period (~AD 800-1200), according to Willard et al., "corresponds to the 'Medieval Warm Period,' which has been documented as drier than average by tree-ring (Stahle and Cleaveland, 1994) and pollen (Willard et al., 2001) records from the southeastern USA." Willard et al. additionally note that droughts "in the 'Medieval Warm Period' and between ~AD 50 and AD 350 [the Roman Warm Period] spanning a century or more have been indicated by Great Plains tree-ring (Stahle et al., 1985; Stahle and Cleaveland, 1994), lacustrine diatom and ostracode (Fritz et al., 2000; Laird et al., 1996a, 1996b) and detrital clastic records (Dean, 1997)."
Much the same has been found to be the case for Alberta, Canada, where Campbell (2002) analyzed the grain sizes of sediment cores obtained from Pine Lake (52°N, 113.5°W) to provide a high-resolution record of climate variability for this part of the continent over the past 4000 years. This effort revealed periods of both increasing and decreasing grain size (moisture availability) throughout the record at decadal, centennial and millennial time scales. The most prominent departures included several-centuries-long epochs that corresponded to the Little Ice Age (about AD 1500-1900), Medieval Warm Period (about AD 700-1300), Dark Ages Cold Period (about BC 100 to AD 700) and Roman Warm Period (about BC 900-100). In addition, a standardized median grain-size history revealed that the highest rates of stream discharge during the past 4000 years occurred during the Little Ice Age at about 300-350 years ago, when grain sizes were about 2.5 standard deviations above the 4000-year mean. In contrast, the lowest rates of streamflow were observed around AD 1100, when median grain sizes were nearly 2 standard deviations below the 4000-year mean.
Back in Alaska, Wiles et al. (2008) used comparisons of temperature sensitive climate proxy records with tree-ring, lichen and radiocarbon dated histories from land-terminating, non-surging glaciers for the last two millennia from southern Alaska in examining the role of summer temperature as a primary driver of glacial expansions, based on field and laboratory work over the past decade that yielded five new or updated glacier histories, one each for Bear Glacier (Kenai Mountains), Marathon Mountain Cirque (Kenai Mountains), Amherst Glacier (Chugach Mountains), Crescent Glacier (Chugach Mountains) and Yakutat Glacier (St. Elias Mountains), all located just above the Gulf of Alaska (about 60°N) between approximately 140 and 150°W. This work suggested the presence of the Roman Warm Period near the beginning of their 2000-year record, because of subsequent "general glacier expansions during the First Millennium AD" that experienced their "strongest advance" at AD 600, which latter cold interval -- with ice extent "as extensive as [the] subsequent Little Ice Age" -- is typically known as the Dark Ages Cold Period. This latter cold interval was followed by the Medieval Warm Period (MWP), the evidence for which "consists of soil formation and forest growth on many forefields in areas that today are only just emerging from beneath retreating termini," which suggests that the MWP was likely both warmer and longer-lived than what we have so far experienced of the Current Warm Period. They also report, in this regard, that at the Sheridan, Tebenkof and Princeton glaciers, "tree-ring chronologies show that forest growth on these forefields was continuous between the 900s and 1200s."
Noting that the alternating warm-cold-warm-cold-warm sequence of the past 2000-plus years "is consistent with millennial-scale records of ice-rafted debris flux in the North Atlantic and Northern Hemisphere temperature reconstructions," and that "variable Holocene solar irradiance has been proposed as a potential forcing mechanism for millennial-scale climate change," the four researchers concluded that this scenario "is supported by the Southern Alaskan glacial record." And this conclusion implies that the warming of the past century that led to the establishment of the Current Warm Period may well have been similarly orchestrated and have had essentially nothing to do with the concomitant increase in the air's CO2 content.
Dropping down to the "lower 48," Persico and Meyer (2009) used beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park, which feat was accomplished by comparing the distribution of beaver-pond deposit ages to paleoclimatic proxy records in the Yellowstone region. In doing so, they found that "gaps in the beaver-pond deposit record from 2200-1800 and 700-1000 cal yr BP are contemporaneous with increased charcoal accumulation rates in Yellowstone lakes and peaks in fire-related debris-flow activity, inferred to reflect severe drought and warmer temperatures (Meyer et al., 1995)." In addition, they note that "the lack of evidence for beaver activity 700-1000 cal yr BP is concurrent with the Medieval Climatic Anomaly, a time of widespread multi-decadal droughts and high climatic variability in Yellowstone National Park (Meyer et al., 1995) and the western USA (Cook et al., 2004; Stine, 1998; Whitlock et al., 2003)," which leads us to note that the lack of evidence for beaver activity 2200-1800 cal yr BP is concurrent with the Roman Warm Period. Furthermore, in both of these instances, the two researchers concluded that the severe droughts of these periods "likely caused low to ephemeral discharges in smaller streams, as in modern severe drought," which implies that climatic conditions during the Medieval and Roman Warm Periods were likely just as dry and as warm as they are today.
Ranging throughout the mixed and shortgrass prairies of the U.S. Great Plains, Nordt et al. (2008) developed a time series of C4 vs. C3 plant dynamics for the past 12,000 years, based on isotopic soil carbon measurements made on 24 modern soils and 30 buried soils; and because, as they describe it, the percent soil carbon derived from C4 plants "corresponds strongly with summer temperatures as reflected in the soil carbon pool (Nordt et al., 2007; von Fischer et al., 2008)," they were able to devise a history of the relative warmth of the region over this protracted time period. And this history suggests that the region of study was slightly warmer than it has been in modern times during parts of both the Medieval and Roman Warm Periods, and that it was significantly warmer during a sizeable portion the mid-Holocene Thermal Maximum or Climatic Optimum. Consequently, for a broad swath of the midsection of the United States, stretching from the center of Texas all the way to the U.S. border with Canada, the supposedly unprecedented warming of the 20th century (according to claims of the world's climate alarmists) was not unprecedented at all, having likely been surpassed one thousand, two thousand and four to five thousand years ago, when there was much less CO2 in the air than there is currently; and this fact makes one wonder just what was the cause of those earlier warmer-than-present periods. The answer of Nordt et al. is that "these warm intervals ... exhibit a strong correlation to increases in solar irradiance," as per the irradiance reconstruction of Perry and Hsu (2000).
Last of all, working with sediment cores extracted from three sites on the eastern slope of the Gulf of California, Barron and Bukry (2007) derived high-resolution records of diatoms and silicoflagellate assemblages that spanned the past 2000 years, finding the relative abundance of Azpeitia nodulifera -- a tropical diatom whose presence is indicative of high sea surface temperatures -- to have been far greater during the Medieval Warm Period than at any other time over the 2000-year period, while during the Current Warm Period its relative abundance was actually lower than the 2000-year mean, also in all three of the sediment cores. What is more, the first of the cores exhibited elevated A. nodulifera abundances from the start of the record to about AD 350, during the latter part of the Roman Warm Period; and by analyzing radiocarbon production data, they determined that the changes in climate they identified likely were driven by variable solar forcing.
In conclusion, in reviewing the results of the several studies described above, it is clear that the Roman Warm Period was a very real phenomenon throughout much, if not most, of North America, manifesting itself in the form of both warmer temperatures and -- in certain locations -- drier moisture conditions, providing thereby ever more evidence for the reality of this naturally-occurring and non-CO2-induced millennial-scale oscillation of climate (of which the Roman Warm Period represents a very significant warm node) that now been identified throughout the world.
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