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Little Ice Age (Global) -- Summary
In light of the desperate attempts of certain scientists and politicians to convince the nations of the world that our current warmth is so great as to be unprecedented over the past millennium or more, and that our current high temperatures are therefore the result of the rise in the air's CO2 content that has been driven by the burning of fossil fuels associated with the development of the Industrial Revolution and our expanding global population, we routinely review scientific journal articles that reveal the worldwide existence of the even warmer Medieval Warm Period of a thousand years ago, as well as the subsequent global impact of the Little Ice Age, demonstrating thereby that there is nothing unusual or "CO2-induced" about what we could call the Modern Warm Period, it being but the most recent manifestation of the pervasive warm node of a millennial-scale oscillation of climate that reverberates throughout glacial and interglacial periods alike [see Climate Oscillations (Millennial Variability) in our Subject Index].  As part of this endeavor, we also list and discuss -- under the heading of Little Ice Age in our Subject Index -- numerous scientific papers that document the existence of this latest several-century cool phase of the 1300-1500-year climatic cycle that affects all parts of the planet.  In addition, we have a subheading under which we file Editorials and Journal Reviews of papers that individually provide evidence for the global extent of this relatively cold period of earth's climatic history, as well as its likely cause; and in this Summary we describe the results of this group of globally-oriented papers.

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 planet over the last 20,000 years.  Based on a total of 6,144 qualifying sets of heat flow measurements obtained from every continent, they produced a truly global climate reconstruction, which they describe as being "independent of other proxy interpretations [and] of any preconceptions or biases as to the nature of the actual climate history."  And what did they find?  They found strong evidence that the Medieval Warm Period was indeed warmer than it is now, by perhaps as much as 0.5C, and that the Little Ice Age was as much as 0.7C cooler than it is currently worldwide.

Broecker (2001a) began his analysis of the subject by noting that glacial evidence for the Little Ice Age can be found all the way from the Swiss Alps in the Northern Hemisphere to the Southern Alps of New Zealand's South Island, stating that "the Little Ice Age cooled not just Europe but the world."  He too notes the existence of alternating warm and cold periods that have occurred, in his words, "virtually unchanged, in both amplitude and duration," with a "nearly regular, 1,500-year cycle" that monotonously repeats itself as far back in time as we have been able to discern.  Hence, as he continues, "we cannot assume that in the absence of human intervention, earth's temperatures would have remained stable."  Like us, therefore, he concludes there is plenty of "room for maneuvering ... for those who doubt that the buildup of carbon dioxide and other greenhouse gases constitutes a substantial threat."

Another major step in the right direction was provided by Soon and Baliunas (2003) and Soon et al. (2003), who reviewed an immense body of evidence pertaining to the climatic history of the earth over the last millennium.  And what did they find?  As Soon and Baliunas describe it, "the assemblage of local representations of climate establishes both the Little Ice Age and Medieval Warm Period as climatic anomalies with worldwide imprints, extending earlier results by Bryson et al. (1963), Lamb (1965), and numerous intervening research efforts."  In addition, they say that "across the world, many records reveal that the 20th century is probably not the warmest nor a uniquely extreme climatic period of the last millennium."

Krenke and Chernavskaya (2002) also present an impressive review of what is known about the Medieval Warm Period and Little Ice Age throughout the world, based upon glaciological, hydrologic and written historical evidence, as well as dendrological, archaeological and palynological data, concluding that "from the 9th century to, apparently, the mid-15th century, the climatic conditions were warmer than during most of the subsequent five centuries, including the early 20th century."  In some places, in fact, they report it was even warmer during this Medieval Warm Period than it was during the latter part of the 20th century.  For example, they note that "the northern margin of boreal forests in Canada was shifted [north] by 55 km during the Medieval Warm Period, and the tree line in the Rocky Mountains in the southern United States and in the Krkonose Mountains was higher by 100-200 m than that observed at the present time."

Concentrating on data from within Russia, the two members of the Russian Academy of Sciences report large differences in a number of climate variables between the Little Ice Age (LIA) and the preceding Medieval Warm Period (MWP).  With respect to the annual mean temperature of northern Eurasia, they note an MWP to LIA drop on the order of 1.5C.  They also say that "the frequency of severe winters reported was increased from once in 33 years in the early period of time, which corresponds to the MWP, to once in 20 years in the LIA," additionally noting that "the abnormally severe winters [of the LIA] were associated with the spread of Arctic air masses over the entire Russian Plain."  Finally, they remark that the data they used to draw these conclusions were "not used in the reconstructions performed by Mann et al.," which perhaps explains why the Mann et al. (1998, 1999) temperature history of the past millennium does not reproduce the Little Ice Age nearly as well as does the more appropriately derived temperature history of Esper et al. (2001).  Hence, and in complete contradiction of the contentions of Mann et al., the Russian Academicians unequivocally state that "the Medieval Warm Period and the Little Ice Age existed globally."

Yet another study to provide a proper perspective of the Little Ice Age and the preceding Medieval Warm Period was that of Loehle (2004), who used a pair of well-dated and lengthy proxy climate records to characterize the pattern of temperature change over the past three millennia.  The first of these records was derived by Keigwin (1996) from a study of the oxygen isotope ratios of foraminifera and other organisms contained in a Sargasso Sea sediment core retrieved from a deep-ocean drilling site on the Bermuda Rise.  This record provides sea surface temperature data for about every 67th year from 1125 BC to 1975 AD.  The second temperature series was 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.

So why does Loehle use these two specific records and only these two records?  By way of explanation, he says that "most other long-term records have large dating errors, are based on tree rings, which are not reliable for this purpose (Broecker, 2001b), or are too short for estimating long-term cyclic components of climate."  Also, and in repudiation of the approach employed by Mann et al. (1998, 1999) and Mann and Jones (2003), he reports 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 reports 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 says that this fact "suggests that the climate signal reflects some global pattern rather than being a regional signal only."  He also notes 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 reports 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."  In addition, Loehle's analyses reveal the existence of the Medieval Warm Period of 800-1200 AD, 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 1500-1850 AD, which is shown to have been the coldest period of the entire 3000-year record.

So what is the cause of the reliable and well-defined millennial-scale climatic oscillation that has alternately produced the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and, last of all, the Modern Warm Period?  This is the question Bond et al. (2001) set out to answer in a study of ice-rafted debris found in North Atlantic deep-sea sediment cores and its temporal correlation with concentrations of the cosmogenic nuclides 10Be and 14C, which can be used as proxies for solar activity, as noted long ago by Lal and Peters (1967).  In this particular case, they utilized measurements of 10Be sequestered in the Greenland ice cap and 14C contained within Northern Hemispheric tree rings, demonstrating, in their words, that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum."  Hence, they concluded that the underlying climatic cycle is solar-induced, and that "a solar influence on climate of the magnitude and consistency implied by our evidence could not have been confined to the North Atlantic."

In support of their contention that the phenomenon is global in scope, Bond et al. cite a number of studies that confirm the presence of the millennial-scale climatic oscillation in Scandinavia, Greenland, the Netherlands, the Faroe Islands, Oman, the Sargasso Sea, coastal West Africa, the Cariaco Basin, equatorial East Africa, and the Yucatan Peninsula, demonstrating thereby that "the footprint of the solar impact on climate we have documented extend[s] from polar to tropical latitudes."  They also note that "the solar-climate links implied by our record are so dominant over the last 12,000 years it seems almost certain that the well-documented connection between the Maunder solar minimum and the coldest decades of the LIA could not have been a coincidence," and that both the Little Ice Age and Medieval Warm Period "may have been partly or entirely linked to changes in solar irradiance."

In a similar approach to the subject, Bard et al. (2000) created a 1200-year history of cosmonuclide production in earth's atmosphere from 10Be measurements of South Pole ice (Raisbeck et al., 1990) and atmospheric 14C measured in tree rings (Bard et al., 1997), which they converted to Total Solar Irradiance (TSI) values by "applying a linear scaling using the TSI values published previously for the Maunder Minimum."  This approach resulted in an extended TSI record that suggests, in their words, that "solar output was significantly reduced between 1450 and 1850 AD, but slightly higher or similar to the present value during a period centered around 1200 AD."  Hence, they say "it could thus be argued that irradiance variations may have contributed to the so-called 'little ice age' and 'medieval warm period'," additionally noting that TSI variations "would tend to force global effects."

Noting that "the most direct mechanism for climate change would be a decrease or increase in the total amount of radiant energy reaching the earth," Perry and Hsu (2000) developed a simple solar-luminosity model by summing the amplitude of solar radiation variance for fundamental harmonics of the eleven-year sunspot cycle throughout an entire 90,000-year glacial cycle.  Their model output was well correlated with the amount of 14C in well-dated tree rings gong back to the time of the Medieval Warm Period (about A.D. 1100), as well the sea-level curve developed by Ters (1987); and present in both of these records over the entire expanse of the Holocene was a "little ice age"/"little warm period" cycle with a period of approximately 1300 years.  As a result, they too concluded that the idea of "the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable."  And at the other end of the cause-and-effect spectrum, Hunt (1998) used a global climate model to demonstrate that most of the observed features in the climatic record of the earth "can be reproduced by processes associated with internal mechanisms of the climatic system," concluding on this basis also that "the modern temperature increase being caused solely by an increase in CO2 concentrations appears questionable.

In light of the results of these several studies, there would appear to be no question but what the Little Ice Age was global in scope [for even more evidence, see Little Ice Age (Regional - Africa, Antarctica, Arctic, Asia, Australia/New Zealand, Europe, North America, Oceans, South America, Tropics, as was the Medieval Warm Period.  There is also considerable evidence that suggests these two climatic states were produced by a millennial-scale climatic oscillation driven by similar variations in solar activity, or possibly by no external forcing at all.  Hence, there is no compelling reason to believe, as climate alarmists claim, that the historical increase in the air's CO2 content is responsible for the development of the Modern Warm Period.  Earth's current high (but not unprecedented) temperatures could possibly have developed without any forcing at all, but are more likely the result of cyclical variations in solar activity.

Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J.  1997.  Solar modulation of cosmogenic nuclide production over the last millennium: comparison between 14C and 10Be records.  Earth and Planetary Science Letters 150: 453-462.

Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J.  2000.  Solar irradiance during the last 1200 years based on cosmogenic nuclides.  Tellus 52B: 985-992.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.

Broecker, W.S.  2001a.  Glaciers That Speak in Tongues and other tales of global warming.  Natural History 110 (8): 60-69.

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

Bryson, R.A., Arakawa, H., Aschmann, H.H. and Baerris, D.A. plus 36 others. 1963. NCAR Technical Note. In: Bryson R.A., and Julian P.R.  (Eds.) Proceedings of the Conference on Climate of the 11th and 16th Centuries, Aspen CO, June 16-24 1962, National Center for Atmospheric Research Technical Notes 63-1, Boulder, CO.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

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. and Pollack, H.N.  1997.  Late Quaternary temperature changes seen in world-wide continental heat flow measurements.  Geophysical Research Letters 24: 1947-1950.

Hunt, B.G.  1998.  Natural climate variability as an explanation for historical climate fluctuations.  Climatic Change 38: 133-157.

Keigwin, L.D.  1996.  The Little Ice Age and Medieval Warm Period in the Sargasso Sea.  Science 274: 1504-1508.

Krenke, A.N. and Chernavskaya, M.M.  2002.  Climate changes in the preinstrumental period of the last millennium and their manifestations over the Russian Plain.  Isvestiya, Atmospheric and Oceanic Physics 38: S59-S79.

Lal, D. and Peters, B.  1967.  Cosmic ray produced radio-activity on the Earth.  In: Handbuch der Physik, XLVI/2.  Springer, Berlin, Germany, pp. 551-612.

Lamb, H.H.  1965.  The early medieval warm epoch and its sequel.  Palaeogeography, Palaeoclimatology, Palaeoecology 1: 13-37.

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

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.

Perry, C.A. and Hsu, K.J.  2000.  Geophysical, archaeological, and historical evidence support a solar-output model for climate change.  Proceedings of the National Academy of Sciences USA 97: 12433-12438.

Raisbeck, G.M., Yiou, F., Jouzel, J. and Petit, J.-R.  1990.  10Be and 2H in polar ice cores as a probe of the solar variability's influence on climate.  Philosophical Transactions of the Royal Society of London A300: 463-470.

Soon, W. and Baliunas, S.  2003.  Proxy climatic and environmental changes of the past 1000 years.  Climate Research 23: 89-110.

Soon, W., Baliunas, S., Idso, C.D., Idso, S.B. and Legates, D.R.  2003.  Reconstructing climatic and environmental changes of the past 1000 years: A reappraisal.  Energy & Environment 14: 233-296.

Ters, M.  1987.  Variations in Holocene sea level on the French Atlantic coast and their climatic significance.  In: Rampino, M.R., Sanders, J.E., Newman, W.S. and Konigsson, L.K. (Eds.) Climate: History, Periodicity, and Predictability.  Van Nostrand Reinhold, New York, NY, pp. 204-236.