Tree rings are one of the best natural archives of past growing conditions because trees are sensitive to local climate factors, such as precipitation and temperature. In warm, wet years, tree growth rings are usually wider while they are narrower in cold, dry years. Trees growing in subpolar regions are usually the most sensitive to temperature changes and if a good statistical match exists between recent tree growth and temperature records then the ring widths of ancient trees can be used to estimate past temperatures.
Scientists from the Institute of Plant and Animal Ecology, Ural Branch of the Russian Academy of Science (RAS), and the Ural Federal University (UrFU), have been making expeditions to the region of the Yamal Peninsula, north-western Siberia, for the past 40 years, collecting subfossil wood for analysis. Stepan Shiyatov, a pioneer of dendrochronology in Russia, was the first to recognize the value of the ancient trees found in this region, and to begin collecting samples systematically.
Since then, more than two dozen expeditions have been carried out and more than 5,000 samples of ancient tree trunks have been collected by researchers from the RAS. Dendrochronologists from the RAS have measured and analyzed the rings and, with support from colleagues at the Climatic Research Unit of the University of East Anglia, from the Swiss Federal Institute for Forest, Snow and Landscape Research, and from the University of Geneva, they have been able to reconstruct and characterize the summer temperatures over the past 7,638 years.
Approximately 2,000 samples of larch and spruce subfossils have also been dated using with the cross-dating method, which has allowed the researchers to assign, with absolute accuracy, the exact year of formation to each annual ring over the last 8,800 years. This exceptional sample of tree rings from the Yamal region, which is archived at the Institute of Plant and Animal Ecology at the RAS, is now the longest existing tree-ring chronology of the Polar Regions.
The results of the analysis, published in the journal Nature Communications, show that northwestern Siberia is currently experiencing its warmest summers in the past 7,000 years. While for several millennia the temperature of the region followed a pattern of general cooling, in the 19th century there was an abrupt change and temperatures began to rise rapidly.
“Due to changes in the Earth’s orbit we would have expected a continuous, slow and gradual decrease of incoming summer solar energy, and thus temperature, at the subpolar latitudes of the Northern Hemisphere during last 8–9 millennia. However, [as] recorded by the trees growing in Yamal, this cooling trend has been interrupted in the middle of the 19th century, when temperature began to rise very quickly and reached the highest values in recent decades,” says Rashit Hantemirov, leading researcher of the Laboratories of Dendrochronology of the Ural Branch of the RAS and Natural Science Methods in the Humanities of UrFU.
Independently of the period length considered (from 30 to 170 years), the most recent period has been the warmest in the time period covered by the tree ring record. Not only has the temperature reached unprecedented levels of warmth, but also the rate of temperature increase is greater currently than it has been since the middle of the 19th century.
“The exceptionality of the modern warming is corroborated by observations that the last century was characterized by a total lack of cold extremes contrasted by the occurrence of 27 extreme warm years, 19 of which have fallen in the last 40 years,” specifies Hantemirov.
The authors of this research are confident that human activities have not only influenced climate change, but have become its major determinant, at least for north-western Siberia. They plan to continue research on tree-ring based climate reconstruction in this region as the remains of even older trees, perhaps dating back a further 2,000 years, are still present on the Yamal Peninsula. They state that the remains of such trees that lived thousands of years ago, provide an understanding of the past, which is the best foundation to assess the future.
“Thanks to international cooperation it will also be possible to use other tree-ring parameters to further precise the climate reconstructions. With colleagues from Switzerland, we are working on the analysis of the tree rings cellular structures, and together with the Institute of Geology and Geophysics of the Chinese Academy of Sciences, we intend to perform a climate reconstruction based on the analysis of the oxygen-18 isotope in the annual rings,” adds Hantemirov.