Tree rings can be counted to date the time of an event, and their summertime width is greater under good growing conditions (warmth, rainfall) than during poor growing seasons (cold, dry). They are limited by the distance back in time researchers can find live trees, dead trees, or buried wood from an earlier time which can be accurately dated to its growth period.
In mountainous northwestern Pakistan, more than 200,000 tree-ring measurements were assembled from 384 long-lived trees that grew on more than twenty individual sites. The 1,300-year temperature proxy shows the warmest decades occurred between 800 and 1000, and the coldest periods between 1500 and 1700.128
Mountain tree line elevations are another sensitive and highly accurate proxy for temperature change. A number of studies of European tree lines testify to the fact that tree lines, farming, and villages moved upslope during the Medieval Warming and back with the Little Ice Age.
A recent study of tree line dynamics in Western Siberia showed that advances in tree lines during the warmer weather of the 20th century were “part of a long-term reforestation of tundra environments.” Two Swiss scientists, Jan Esper and Fritz-Hans Schweingruber, note that “stumps and logs of Larix sibirica can be preserved for hundreds of years” and that “above the tree line in the Polar Urals such relict material from large, upright trees were sampled and dated, confirming the existence, around A.D. 1000, of a forest tree line 30 meters above the late 20th century limit.” They also note, “this previous forest limit receded around 1350, perhaps caused by a general cooling trend.” Thus, the Siberian tree lines testify to the Medieval Warming and the Little Ice Age well outside of Europe.129
Lisa J. Graumlich of Montana State University combined both tree rings and tree lines to assess past climate changes in California’s Sierra Nevada. The trees in the mountains’ upper tree lines are preserved in place, living and dead, for up to 3,000 years. Graumlich says:
A relatively dense forest grew above the current tree line from the beginning of our records to around 100 B.C., and again from A.D. 400 to 1000, when temperatures were warm. Abundance of trees and elevation of tree line declined very rapidly from A.D. 1000 to 1400, the period of severe, multi-decadal droughts. Tree lines declined more slowly from 1500 to 1900 under the cool temperatures of the Little Ice Age, reaching current elevations around 1900.130
Graumlich’s tree evidence confirms both of the last two 1,500-year cycles: the Roman Warming/Dark Ages climate cycle and the Medieval Warming/Little Ice Age. Severe drought, which has been documented in California during the latter part of the Medieval Warming, obscured the timing of the shift from the Medieval Warming to the Little Ice Age. However, both events were clearly evident.
Cave stalagmite cores confirm the global nature of the 1,500-year cycle found in ice cores, seabed sediments, and trees. Their carbon and oxygen isotopes and their trace element content vary with temperature. Moreover, the stalagmites go back further in time than the tree evidence. One German stalagmite goes back more than 17,000 years. Cave stalagmites have been found in Ireland, Germany, Oman, and South Africa whose layers all show the Little Ice Age, the Medieval Warming, the Dark Ages, and the Roman Warming.131 A number of the stalagmites also show the unnamed cold period that preceded the Roman Warming.
In southern Ontario, pollen shows that the warmth-loving beech trees of the Medieval Warming gradually gave way to cold tolerant oaks as the Little Ice Age came on—and then the forest became dominated by pine trees. The oak trees have been making a comeback in Ontario since 1850 and the beech trees can be expected to resurge as the Modern Warming continues in the centuries ahead.132
Remains of prehistoric villages in Argentina were analyzed by Marcela A. Cioccale of the National University of Cordoba to determine where Argentina’s native peoples lived over the past 1,400 years. Using carbon-14 dating, she found that the inhabitants clustered in the lower valleys during the Dark Ages period, and then moved higher up the slopes as the Medieval Warming brought “a marked increase of environmental suitability, under a relatively homogeneous climate.”133 Habitation moved up as high as 4,300 meters in the Central Peruvian Andes around 1000 as the Medieval Warming not only raised temperatures but created more stable conditions for farming. After 1320, people migrated back down the slopes as the colder, less stable climate of the Little Ice Age set in.
Yang Bao of the Chinese Academy of Sciences reconstructed China’s temperature history for the last 2,000 years from ice cores, lake sediments, peat bogs, tree rings, and the historic documents that date back farther in China than in any other country. He found China had its highest temperature during the second and third centuries, toward the end of the Roman Warming. China’s climate was also warm from 800 to 1400, cold from 1400 to 1920, and then began to warm again after 1920.134 (See Figure 4.1.)
Figure 4.1: 2,000 Years of Chinese Temperature History
Source [for above figure]: Y. T. Hong et al., “Response of Climate to Solar Forcing Recorded in a 6,000-Year Time-Series of Chinese Peat Cellulose,” The Holocene 10 (2000): 1-7.
 J. Esper et al., “1,300 Years of Climate History for Western Central Asia Inferred from Tree Rings,” The Holocene 12 (2002): 267-77.
 J. Esper and F. H. Schweingruber, “Large-Scale Tree Line Changes Recorded in Siberia,” Geophysical Research Letters 31 (2004): 10.1029/2003GLO019178.
 L. J. Graumlich, “Global Change in Wilderness Areas: Disentangling Natural and Anthropogenic Changes,” U. S. Department of Agriculture Forest Service Proceedings RMRS-P-15-Vol. 3, 2000
 F. McDermott et al., “Centennial-Scale Holocene Climate Variability Revealed by a High-Resolution Speleothem 018 Record from SW Ireland,” Science 294 (2001): 1328-331; S. Niggemann et al., “A Paleoclimate Record of the Last 17,600 Years in Stalagmites from the B7 Cave, Sauerland, Germany,” Quaternary Science Reviews 22 (2003): 555-67; U. Neff et. al., “Strong Coherence between Solar Variability and the Monsoon in Oman between 9 and 6 kyr ago,” Nature 411 (2001): 290-93; and Tyson et al., “The Little Ice Age and Medieval Warming in South Africa,” South African Journal of Science 96, no. 3 (2000).
 I. D. Campbell and J. H. McAndrews, “Forest Disequilibrium Caused by Rapid Little Ice Age Cooling,” Nature 366 (1993): 336-38
 M. A. Cioccale, “Climatic fluctuations in the Central Region of Argentina in the last 1000 years,” Quaternary International 62, (1999): 35-47.
 Yang Bao et al., “General Characteristics of Temperature Variation in China during the Last Two Millennia,” Geophysical Research Letters 10 (2002): 1029/2001GLO014485.
- S. Fred Singer and Dennis T. Avery, Unstoppable Global Warming: Every 1,500 Years (Blue Ridge Summit, PA: Rowman & Littlefield, 2007), 63-66.