Climate alarmists contend that the degree of global warmth over latter part of the 20th century was greater than it has been at any other time over the past one to two millennia. Why? Because this contention helps them sell their claim that the "unprecedented" temperatures of the past few decades were CO2-induced. Hence, they cannot stomach the thought that the Medieval Warm Period of a thousand years ago could have been just as warm as, or even warmer than, it has been recently, especially since there was so much less CO2 in the air a thousand years ago than there is now. Likewise, they are equally loath to admit that the temperatures of the Roman Warm Period of two thousand years ago may also have rivaled, or exceeded, those of the recent past, since atmospheric CO2 concentrations at that still earlier time were also much lower than they are today. As a result, climate alarmists rarely even speak of the Roman Warm Period, as they are happy to let sleeping dogs lie. In addition, they refuse to accept the possibility that these two prior warm periods were global in extent, claiming instead, with respect to the Medieval Warm Period, that it was a purely local phenomenon restricted to lands that surround the North Atlantic Ocean. In another part of our Subject Index we explore these contentions as they apply to the Medieval Warm Period. In this Summary, we explore them as they pertain to the Roman Warm Period, beginning with the part of the planet where climate alarmists are willing to acknowledge the Medieval Warm Period's existence, but not its magnitude, i.e., Europe.
We begin this discussion by noting that the studies of Olafsdottir et al. (2001) and Jiang et al. (2002) document the existence of relatively benign weather conditions in Iceland and its oceanic environs up to about 2500 ± 200 years ago (the "beginning of the end" of the Roman Warm Period), after which their data depict the region gradually descending into what has come to be known as the Dark Ages Cold Period. The first of these research teams also describes the concurrent long-term cooling-induced decline in vegetative productivity on Iceland, which was actually reversed for about four centuries during the Medieval Warm Period but then declined to what they describe as "an unprecedented low" during the Little Ice Age that lasted from about AD 1300 to 1900. In like manner, Jiang et al.'s data, obtained from the seabed of the north Icelandic shelf, depict a similar post-Roman Warm Period decline in summer sea surface temperature, which exhibited a dramatic increase that peaked around AD 1150 after having risen more than 1°C above the line describing the long-term downward trend. Thereafter, however, the temperature fell rapidly, by approximately 2.2°C, as the depths of the Little Ice Age were encountered, after which modern warming overcomes some of the dramatic cooling but cannot return the region to the pinnacle of Roman Warm Period warmth.
Further east in Ireland, McDermott et al. (2001) derived a similar picture of post-Roman Warm Period cooling based on ð18O data derived from a stalagmite. Here, however, the initial climatic deterioration did not begin until about 2000 years ago. Then, Berglund (2003) documented what he called a great "retreat of agriculture" throughout northwest continental Europe that was coincident with the declining temperature, based on assessments of "insolation, glacier activity, lake and sea levels, bog growth, tree line, and tree growth."
Contemporaneously, in northern Swedish Lapland, Grudd et al. (2002) developed a 7400-year history of summer mean temperature based on tree-ring widths obtained from 880 living, dead and subfossil northern Swedish pines. The most dependable portion of the record, based upon the number of trees that were sampled, consists of the last two millennia, which the researchers say "display features of century-timescale climatic variation known from other proxy and historical sources, including a warm 'Roman' period in the first centuries AD and a generally cold 'Dark Ages' climate from about AD 500 to about AD 900." They also note that "the warm period around AD 1000 may correspond to a so-called 'Mediaeval Warm Period', known from a variety of historical sources and other proxy records." Lastly, they note that "the climatic deterioration in the twelfth century can be regarded as the starting point of a prolonged cold period that continued to the first decade of the twentieth century," which "Little Ice Age," in their words, is also "known from instrumental, historical and proxy records."
Dropping down to northwest Germany, Niggemann et al. (2003) employed petrographical and geochemical techniques to develop a climatic history of the last seventeen millennia from a set of three stalagmites. This history closely matches the one derived by McDermott et al., with Niggemann et al. explicitly noting that it provides evidence for the existence of the Little Ice Age, the Medieval Warm Period and the Roman Warm Period, which also implies the existence of the Dark Ages Cold Period that separated the Medieval and Roman Warm Periods, as well as the cold period that preceded the Roman Warm Period.
Continuing south, Desprat et al. (2003) studied the climatic variability of the last three millennia in northwest Iberia via a high-resolution pollen analysis of a sediment core retrieved from the central axis of the Ria de Vigo in the south of Galicia. There they detected "an alternation of three relatively cold periods with three relatively warm episodes." In order of their occurrence, these periods are described by them as the "first cold phase of the Subatlantic period (975-250 BC)," which was "followed by the Roman Warm Period (250 BC-450 AD)," which was followed by "a successive cold period (450-950 AD), the Dark Ages," which "was terminated by the onset of the Medieval Warm Period (950-1400 AD)," which was followed by "the Little Ice Age (1400-1850 AD), including the Maunder Minimum (at around 1700 AD)," which "was succeeded by the recent warming (1850 AD to the present)."
In light of these findings, Desprat et al. conclude that "a millennial-scale climatic cyclicity over the last 3000 years is detected for the first time in NW Iberia paralleling global climatic changes recorded in North Atlantic marine records (Bond et al., 1997; Bianchi and McCave, 1999; Chapman and Shackelton, 2000)." Considering that the same findings are reported by the other studies described above, the establishment of the Modern Warm Period in Europe over the course of the past century or so is seen to be nothing more than the most recent manifestation of the warming phase of this ever-recurring cycle of climate, which is totally unrelated to the coincidental historical increase in the air's CO2 content.
References
Berglund, B.E. 2003. Human impact and climate changes - synchronous events and a causal link? Quaternary International 105: 7-12.
Bianchi, G.G. and McCave, I.N. 1999. Holocene periodicity in North Atlantic climate and deep-ocean flow south of Iceland. Nature 397: 515-517.
Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., de Menocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates. Science 278: 1257-1266.
Chapman, M.R. and Shackelton, N.L. 2000. Evidence of 550-year and 1000-year cyclicities in North Atlantic circulation patterns during the Holocene. The Holocene 10: 287-291.
Desprat, S., Goñi, M.F.S. and Loutre, M.-F. 2003. Revealing climatic variability of the last three millennia in northwestern Iberia using pollen influx data. Earth and Planetary Science Letters 213: 63-78.
Grudd, H., Briffa, K.R., Karlen, W., Bartholin, T.S., Jones, P.D. and Kromer, B. 2002. A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. The Holocene 12: 657-665.
Jiang, H., Seidenkrantz, M-S., Knudsen, K.L. and Eiriksson, J. 2002. Late-Holocene summer sea-surface temperatures based on a diatom record from the north Icelandic shelf. The Holocene 12: 137-147.
McDermott, F., Mattey, D.P. and Hawkesworth, C. 2001. Centennial-scale Holocene climate variability revealed by a high-resolution speleothem ð18O record from SW Ireland. Science 294: 1328-1331.
Niggemann, S., Mangini, A., Richter, D.K. and Wurth, G. 2003. A paleoclimate record of the last 17,600 years in stalagmites from the B7 cave, Sauerland, Germany. Quaternary Science Reviews 22: 555-567.
Olafsdottir, R., Schlyter, P. and Haraldsson, H.V. 2001. Simulating Icelandic vegetation cover during the Holocene: Implications for long-term land degradation. Geografiska Annaler 83 A:203-215.