Nov. 20, 1997 Natural causes, not just human-induced forces, played a significant role in an unprecedented warming trend in the Arctic in the last 150 years, according to a study published in the November 14 issue of Science.
The study found that the Arctic experienced its highest temperatures in 400 years between the mid-19th and mid-20th centuries. Contrary to previous assumptions, the evidence indicates that the Arctic is characterized by significant climatic change even without the influence of environmental effects caused by humans.
"Some of the warming that we observed after the time of the Industrial Revolution may be attributed to atmospheric greenhouse gases, but our observation of dramatic environmental change pre-dates this period," said investigator Dr. Marianne Douglas, an assistant professor in the University of Toronto's department of geology. The lead author of the study was Jonathan Overpeck of the National Oceanic and Atmospheric Administration's Paleoclimatology Program in Colorado.
The multi-centred project capitalized on the expertise of 18 North American researchers who studied complementary climate indicators in all parts of the Arctic. The final compilation of paleoenvironmental data included information from glaciers, tree rings and marine, lake and pond sediments. Douglas's work focused on diatoms, a type of algae known to respond in a measurable way to environmental change, in Arctic ponds. "I reconstructed past environmental conditions using the diatom assemblages that are preserved in lake and pond sediments," she said.
Until recently the record of Arctic climate change was geographically and historically limited, but this study contributes to an improved understanding of the area's environmental variability. The findings suggest the Arctic is especially susceptible to global climate change caused by both natural and human sources, and in turn it can influence changes at lower latitudes through mechanisms such as river runoff into the Arctic Ocean and subsequent changes in thermohaline (ocean currents distributing heat) circulation. The period of warming that began in the 1840s ended the Little Ice Age, caused melting of permafrost and sea ice and alterations in land and lake ecosystems.
Douglas's funding came from the Natural Sciences and Engineering Research Council and the Canadian government's Polar Continental Shelf Project. In the United States, the National Science Foundation provided financial support. The other institutions involved in producing this paper were the University of Colorado, the University of Massachusetts, the University of California, the University of Alaska, Bates College, the University of Ottawa, the University of Alberta, the University of New Hampshire and Columbia University.
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