That finding comes from a new study that examined the archaeological record in southwestern France, where reindeer became locally extinct during two earlier episodes of warming roughly 10,000 and 130,000 years ago.
"There will be a direct impact of increases in summer temperature on reindeer well-being if global warming is allowed to proceed," said University of Washington archaeologist Donald Grayson, lead author of the study. "The number of southern reindeer will diminish dramatically as their range will move far to the north, and the number of reindeer in the north also will fall greatly."
Grayson and his colleague, Francoise Delpech, a French paleontologist at the Institut de Prehistoire et de Geologie du Quaternaire at the University of Bordeaux, will report their findings in a forthcoming issue of the journal Conservation Biology.
The pair examined the fossil record left in Grotte XVI, a cave above the Ceou River in the Dordogne region of France. The cave, which was occupied by both Neandertal and later Cro-Magnon people, has a very well-dated archaeological sequence from about 40,000 to 12,000 years ago. The sequence actually extends to about 65,000 years ago, but the older dates are less well documented.
Grayson and Delpech correlated the number of reindeer bones found in the cave with summer climate data from previously published paleobotany studies of pollen counts.
"As summer temperatures went up, the number of reindeer went down," said Grayson. "The warmer the summer, the fewer the reindeer. And when the Pleistocene Epoch ended about 10,000 years ago and summer temperatures soared, reindeer disappeared. Sometime between 11,000 and 10,000 years ago, reindeer became extinct from higher elevations in southwestern France."
He added that the Pleistocene extinction was not the first time that reindeer vanished from the region, noting that the animals were present in the glacial period prior to 130,000 years ago. However, he said there is no record of reindeer in southwest France during the Eemian Interglacial Stage, another period of warming that stretched from 130,000 to about l16,000 years ago.
The range of reindeer, as they are called in the Old World, and caribou, the name used for the same species in the New World, has varied over time. Today they extend from Scandinavia across northern Russia in Europe and roughly along the United States-Canada border in North America, although most of the population is in the far north.
Prior to global warming at the end of the Pleistocene the animals were found as far south as northern Spain and northern Italy in Europe. In North America they ranged into northern Mississippi in the southeastern United States and in into southern Idaho in the West.
Grayson said the idea of looking at summer temperature as a driving factor in declining reindeer populations is important and controversial. Biologists have linked declines in animal populations to a combination of changing climate and vegetation, increased rainfall and even insect harassment. He admits that there is no fossil record of rodent or small mammals to support the findings in Grotte XVI, but contends reindeer biologists have ignored summer temperature.
"Reindeer cannot physiologically tolerate high summer temperatures," he said. "They have almost no sweat glands and keep their insulation, a heavy pelt, in the summer. You would expect them to have trouble in high temperatures. Summer in those conditions would be the worst time for them because they have to eat a great deal to make up for the scarcity of winter food."
The National Science Foundation, the Sous Direction de l'Archeologie et Direction des Musees de France and the Conseil General de la Dordogne funded the research.