Yet another blow to climate stability.
Conditions during that period, the Eemian, interest scientists in part because they may shed light on how easily today's climate might shift. Earth, now in another interglacial stage, has enjoyed remarkably stable temperatures for the past 10,000 years. But some investigators worry that modern pollutants threaten that long record and will cause global changes in climate.
For years, researchers thought that stable, mild temperatures characterized the Emian. Then in 1993, two teams studying ice cored from the Greenland ice cap reported conflicting findings on the Eemian (SN: 12/11/93, p.390). A European group contended that flucturating temperatures marked that period. But U.S. researchers said they could draw no conclusions about the Eemian from their ice, drilled nearby.
Now, pollen records appear to support the Europeans' conclusions, Field's group reports in the Oct. 27 NATURE.
However, "it's too early to say whether there is or isn't a match between specific wiggles in our curve and specific wiggles in their curve," concerning the timing and degree of climate change, warns coauthor Brian Huntley of the University of Durham in England.
"The ice core records tell principally about the mean annual temperature. [Pollen] can tell more about seasonal fluctuations," he adds.
The study "is really exciting," says Kendrick C. Taylor of the Desert Research Institute in Reno, Nev., one of the U.S. ice core investigators. But he remains unconvinced that the Emian had unstable weather. They need more evidence to back up their argument, he says.
Field and his colleagues analyzed data describing the abundance of different types of ancient bush and tree pollen in the sediment below Bispingen in northern Germany and La Grande Pile in eastern France.
Using information on what trees and shrubs grow in different conditions, they found that the climate warmed rapidly at the beginning of the Eemian and remained temperate for about 3,000 years. During that time, the mean temperature of the coldest month was about 5[degrees]C warmer than it is today.
Then the weather, particularly the winters, became cooler, and readings dropped almost to ice age lows, Huntley says. Temperatures also fluctuated more widely, especially in winter. As the weather cooled, conifers became more abundant, the researchers report. The French site experienced less erratic conditions than the German site, however.
Since submitting their article to NATURE, Field's team has analyzed data from 10 additional areas in Europe and England. The results "give some overall support to the pattern we've reported," says Huntley. The group detected "violent fluctuations" in the climate, but more so at the northern and eastern locations than the southern and western ones, he says.
Shifts in the location of the polar front in the north Atlantic, where surface temperatures drop most dramatically, may have caused Eemian fluctuations, says Huntley.
The new work supports the conclusions of other scientists that "the whole [climate] system can readily become rather unstable," says Huntley. Global warming due to pollutants could trigger "some of these inherent unstable mechanisms in the system...but we don't know," he adds.