Gordon Jacoby, senior research scientist with the team, takes a pencil-thin core sample from a 550-year-old Mongolian Siberian pine tree. The retrieved core extends from the oldest rings at the center of the tree to the newest rings at the outermost surface of the wood under the bark and does not harm the tree. -- photo by Rosanne D'Arrigo

Columbia University researchers have discovered unusually rapid growth in recent times in trees from the remote alpine treeline forests in Mongolia, indicating that temperatures in that region rose to their highest levels in the past century. This latest study, which provides a detailed record of annual temperature-related growth fluctuations from the third century to today, is the first of its kind for this region of Eurasia.

"The results suggest that the temperatures in Mongolia rose to their highest in the past millennium, reaching their peak in the 20th century. The 1999 ring, the widest, indicates the highest temperature reached in this region in the past thousand years," said head researcher Rosanne D'Arrigo from the Tree Ring Lab at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York.

The study, funded by the National Science Foundation's (NSF) Earth System History and Paleoclimatology programs, is published in the current issue of the journal Geophysical Research Letters.

Sampling ancient Siberian pine trees in the Tarvagatay Mountains of west central Mongolia, the scientists analyzed annual growth rings, which generally grow wider during warmer times and narrower in colder periods in such settings. The researchers developed an exactly dated tree ring chronology, which reflects annual temperatures dating back to 262 AD. This latest discovery helps to fill in a large gap in paleoclimatic data from a remote and previously unsampled region of the globe.

"This is an important piece of the puzzle on global warming," said D'Arrigo, who noted that Mongolia was essentially closed to western researchers until 10 years ago and that very few records of past climates exist for north Asia. "Our results from Mongolia fit into the overall picture of warming indicated for other areas of the globe."

Tree-ring width chronology of Siberian pine from Sol Dav, Mongolia from AD 262-1999. Note unusual warming in the 20th century following the severe cold of the 19th c. "Little Ice Age.

Such records help scientists determine whether higher temperatures in recent decades may be a signature of global warming, possibly caused by human activity rather than the earth's natural variability. By comparing the tree rings with other evidence, scientists will improve our understanding of whether the anthropogenic (human-induced) release of trace gases into the atmosphere is the cause of recent warming, or whether other factors, such as solar or volcanic activity, have played more critical roles in the Earth's climate in recent years.

The temperature fluctuations inferred from the Mongolian tree rings are strikingly similar to those seen in a network of tree ring records from sites at northern and alpine treeline in North America, Europe and Russia, including the Taymir Peninsula in Siberia. These tree ring series, spanning much of the circumpolar northern treeline, have been compiled to create a long-term reconstruction of the Earth's temperature over centuries. The new chronology, in addition to its value as a detailed record of Mongolian climate, provides independent corroboration for such hemispheric and global reconstructions and their indications of unusual warming during the 20th century.

This research indicates that the most severe cold occurred during the 19th century, the latter part of a period known as the "Little Ice Age." Unusual cold and frost also occurred in AD 536-545, coinciding with extremes found in other historical records, including evidence of severe cold in tree rings in areas of North America and Europe, and historical accounts of widespread famine and plague in China and the Middle East. The Mongolia chronology helps confirm that a volcanic or other event caused major climatic effects at this time.

These global climatic changes may have profound effects in Mongolia, which has a largely agrarian-based economy. Livestock and food crops are major enterprises and land management for these purposes is extremely important. The greater understanding of climate extremes and possible causes gleaned from tree rings and other extended records can lead to better planning and agricultural management in the future.
Founded in 1949, The Lamont-Doherty Earth Observatoryis the only research center in the world examining the planet from its core to its atmosphere. This multi-disciplinary approach by more than 200 researchers cuts across every continent and ocean, revolutionizing our understanding of the planet's origin, history and, increasingly, its future.

For more information on Mongolia and other research see the Tree Ring Lab at Columbia University's Lamont-Doherty Earth Observatory website: www.ldeo.columbia.edu/trl/

related story:
Tree-Ring Evidence Confirms Alaskan Inuit Saga of Climate Disaster -- 02/01/00