Office of Public Information and Communications
Columbia University
New York, N.Y.  10027
(212) 854-5573

Fred Knubel, Director of Public Information
Contact: Laurence Lippsett
Lamont-Doherty Earth Observatory

Tree Rings Show Rising Earth Temperatures

(Note to editors: Color images, showing scientists coring trees in Mongolia and cross sections of tree rings, are available by calling Laurence Lippsett, above, or via the World Wide Web at

Scientists at Columbia University's Lamont-Doherty Earth Observatory today reported new evidence from tree rings that temperatures on Earth have risen steadily since the late 1800s to unusual heights.

The tree rings, from remote Mongolian forests, show that "the 20th century has been a time of unusual warmth relative to the past several centuries," the scientists said in the Aug. 9 issue of Science.

The research was conducted by Gordon Jacoby and Rosanne D'Arrigo, dendrochronologists at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y., and Tsevegyn Davaajamts, a professor at the Institute of Botany at the Mongolian Academy of Sciences in Ulaanbaatar.

Sampling 300-to-500-year-old Siberian pine trees in the Tarvagatay Mountains of western central Mongolia, the scientists analyzed annual growth rings, which generally grow wider during warm periods and narrower in colder times in trees at the timber line. They developed a tree-ring record reflecting annual temperatures in the region dating back to 1550. The Mongolian tree rings show temperature changes that are strikingly similar to records from tree rings in North America, Europe and western Russia.

The new Mongolian tree-ring record fills in a large gap of climate data from a remote and previously unsampled region of the globe, the scientists said. The network of data, now spanning most of the northern tree line, has combined to create an emerging picture of Earth's temperature over the past centuries.

The record provides a long-term perspective, helping scientists to determine whether the higher temperatures of recent decades may signal a global warming caused by human activity, or may be part of a natural variation. By comparing the tree rings with other evidence, scientists will better understand whether the buildup of industrial greenhouse gases in the atmosphere is causing the warming trend, or whether other factors, such as sunspot or volcanic activity, play critical roles in Earth's climate.

The general trends reflected in the tree-ring record include cooler conditions in the early 1700s, followed by warming that started mid-century. An abrupt cooling occurred in the late 1700s and continued for much of the 1800s. The coldest period was between the 1830s and 1870s, after which a steadily increasing warming trend began.

Temperatures in the 1900s have been higher than in any other period captured by the tree rings from all over the northern hemisphere. The 10 highest growth intervals are all after 1920 and the highest 25-year growth period was between 1944 and 1968.

Scientists are already using the emerging tree-ring record of annual temperature changes to search for the critical factors - working cumulatively or counteractively - that affect Earth's climate. In general, warmer temperatures recorded by the tree rings correspond with periods of increased solar brightness before the Industrial Revolution. But solar brightness alone cannot account for the steady temperature rise and unusually high warming that began in the late 1800s, when large amounts of greenhouse gases began to build up in the atmosphere. Volcanoes play a role by sending up particles that block sunlight and cool the atmosphere. The cold trend in the middle 1800s coincides with several major volcanic eruptions.

To find long-lived trees undisturbed by human activity, the scientists traveled to a remote region of Mongolia. They sampled trees at timber-line sites at 8,000-foot elevations, where temperature is the limiting growth factor. The coring technique, which does not harm the trees, produces a narrow, pencil-thin cross-section extending from the center of the tree, the oldest rings, to the outermost surface, the newest.

The research was supported by the National Science Foundation.