Tree ring studies from the bristlecone pine, the oldest living tree species, suggest that the Earth has been cooling for at least 6,800 years, Caltech researchers say.
In a study to be published today in the journal Science, two Caltech scientists said measurements of chemical ratios in the trunks of trees grown in the White Mountains of California show that temperatures declined about 5 degrees since 4800 BC.
"This is consistent with other studies, such as the ice cores, and suggests that the cooling may very well have been global," said Samuel Epstein, lead researcher in the study.
But he said the research cannot be related to current theories that the Earth is warming because of the so-called greenhouse effect caused by an increase of carbon dioxide in the atmosphere.
Epstein said the bristlecone pine record of ancient temperatures cannot be related to any mathematical model of future changes because "the modern climate is much more unpredictable."
"In modern times, there seems to be some mechanism that is not well understood," he said. "When the climate gets a little warmer, the models go a little haywire."
In the study, Epstein and a Caltech graduate student, Xiahong Feng, measured the ratio of two chemicals, deuterium and hydrogen, in the annual growth rings of bristlecone pine trunks that are known to be up to 8,000 years old. It is known that the ratio of deuterium and hydrogen increases in tree rings as the climatic temperature rises. A single tree ring is created annually by growth of a wood layer in the trunk.
Based on measurements in 50-year intervals from three trees, the researchers found that temperatures climbed slightly until about 6,800 years ago and then started a long cooling slide.
The trend flattened out from 2,000 to 400 years ago, and then another cold period started. The coldest period was between 1700 and 1900, with a slight warming at the end of the measured period.
Epstein said the last measured season was in 1950, the time at which the trees were cut. Wood from the trees has been preserved for scientific study because the bristlecone pine is thought to be the longest-lived tree on Earth.
Ice cores drilled from Devon Island in far northern Canada also have given long-range temperature trends, but some experts have said this data may have been influenced by the ocean or local effects.
Epstein said the bristlecone pine temperature measurements are in general agreement with those of the ice cores, suggesting that the cooling trends recorded by the two methods may have been worldwide.