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Little Ice Age (Regional - Asia: India/Pakistan) -- Summary
The short- and long-handle hockeystick temperature histories of Mann et al. (1999) and Mann and Jones (2003) fail to depict the occurrence of the Little Ice Age and Medieval Warm Period; and, hence, they make the putative dramatic warming of the latter part of the 20th century appear highly unusual, which they equate with anthropogenic-induced, which they associate with the historical rise in the air's CO2 content, which gives them a pretense to call for dramatic reductions in the use of fossil fuels, which we believe to be unwarranted.  Hence, we continually search the emerging scientific literature for new evidence that the Little Ice Age and Medieval Warm Period were truly significant global events.  This brief review thus summarizes what we have learned over the past few years about the Little Ice Age in the India/Pakistan region of Asia.

Kar et al. (2002) explored the nature of historical climate change in the Uttarkashi district of Uttranchal in the Western Himalayas at Gangotri Glacier, which is the source of the Holy Ganga.  There, based on pollen analyses of a 1.25-meter sediment profile of an outwash plain located some 2-3 km from the glacier's snout, they developed a 2000-year record of regional climate.  Between 1700 and 850 years ago, their analysis indicates there was an "amelioration of climate," as the region recovered from the depressed temperatures of the Dark Ages Cold Period to experience the milder conditions of the Medieval Warm Period, after which the climate became "much cooler," in their words, indicative of its transition to Little Ice Age conditions.  In fact, between 300 and 200 years ago, when the Little Ice Age was visiting much of the world with temperatures that were lower than any previously experienced over the entire Holocene, Kar et al.'s data indicate that the long-term retreat of the Gangotri Glacier actually "ceased," and that there was possibly "some minor advancement."  Then, over the final 200 years of the record - when the study of Esper et al. (2002a) indicates that the Northern Hemisphere warmed rather steadily - the glacier's snout retreated by about 2 km.

In another study from the western Himalaya region of India, Yadav and Singh (2002) developed a spring temperature history based on twelve tree-ring chronologies of Himalayan cedar.  Noting that "spring temperature is significantly correlated with mean annual temperature," they say that their temperature history to some extent reflects the general variation of temperature over the Himalayan region.  That said, the "most conspicuous feature" of this history, in their words, is "the long-term cooling trend since the late 17th century that ended early in the 20th century," which represents both the long-term development and the beginning of the end of the Little Ice Age.

Moving to the Northwest Karakorum of Pakistan and the Southern Tien Shan of Kirghizia, Esper et al. (2002b) employed more than 200,000 ring-width measurements from 384 trees obtained from 20 individual sites ranging from the lower to upper timberline to reconstruct regional patterns of climate variations in Western Central Asia since AD 618, noting that these high-elevation sites are "exceptionally sensitive to climatic variations" and that "conspicuous interactions exist between [their] ecosystems and climate."  This record provides an important perspective on both the Medieval Warm Period and Little Ice Age.  Esper et al. note, for example, that early in the seventh century the Medieval Warm Period was already firmly established and growing even warmer.  In fact, between AD 900 and 1000 tree growth was exceptionally rapid, at rates that they say "cannot be observed during any other period of the last millennium."  Between AD 1000 and 1200, however, growing conditions deteriorated; and at about AD 1500, minimum tree ring-widths were reached that persisted well into the seventeenth century, as the Little Ice Age held sway over the region and much of the rest of the world.  Towards the end of the twentieth century, however, ring-widths once again increased; but Esper et al. say that "the twentieth-century trend does not approach the AD 1000 maximum."  In fact, there is almost no comparison between the two periods, with the Medieval Warm Period being far more conducive to good tree growth than the Modern Warm Period.  As Esper et al. describe the situation, "growing conditions in the twentieth century exceed the long-term average, but the amplitude of this trend is not comparable to the conditions around AD 1000."

In a study published the following year, Esper et al. (2003) processed several extremely long juniper ring-width chronologies for the Alai Range of the western Tien Shan in Kirghizia in such a way as to preserve multi-centennial growth trends that are typically "lost during the processes of tree ring data standardization and chronology building."  In doing so, they used two techniques that maintain low frequency signals - long-term mean standardization (LTM) and regional curve standardization (RCS) - as well as the more conventional spline standardization (SPL) that obscures long-term trends.  Carried back in time a full thousand years, the SPL chronologies depict significant inter-decadal variations but no longer-term trends.  The LTM and RCS chronologies, on the other hand, show long-term decreasing trends until about AD 1600, broad minima from 1600 to 1800, and long-term increasing trends after about 1800.  As a result, in the words of Esper et al., "the main feature of the LTM and RCS Alai Range chronologies is a multi-centennial wave with high values towards both ends."  And sandwiched between them, of course, is the Little Ice Age.

This grand result has essentially the same form as the Northern Hemisphere extratropic temperature history of Esper et al. (2002a), which is vastly different from the notorious hockeystick temperature histories of Mann et al. (1999) and Mann and Jones (2003), in that it depicts the existence of both the Little Ice Age and the preceding Medieval Warm Period, which are nowhere to be found in the Mann and Company reconstructions.  In addition, the new result - especially the LTM chronology, which has a much smaller variance than the RCS chronology - depicts several periods in the first half of the last millennium that were warmer than any part of the last century.  Consequently, there is no need to invoke the historical increase in the air's CO2 content as the cause of 20th-century warming; the temperature increase of that period was merely the natural recovery of the planet from the cold of the Little Ice Age, which also came sans the help of humanity, just like the Medieval Warm Period.

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

Esper, J., Schweingruber, F.H. and Winiger, M.  2002b.  1300 years of climatic history for Western Central Asia inferred from tree-rings.  The Holocene 12: 267-277.

Esper, J., Shiyatov, S.G., Mazepa, V.S., Wilson, R.J.S., Graybill, D.A. and Funkhouser, G.  2003.  Temperature-sensitive Tien Shan tree ring chronologies show multi-centennial growth trends.  Climate Dynamics 21: 699-706.

Kar, R., Ranhotra, P.S., Bhattacharyya, A. and Sekar B.  Vegetation vis--vis climate and glacial fluctuations of the Gangotri Glacier since the last 2000 years.  Current Science 82: 347-351.

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.

Yadav, R.R. and Singh, J.  2002.  Tree-ring-based spring temperature patterns over the past four centuries in western Himalaya.  Quaternary Research 57: 299-305.

Last updated 26 October 2005