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An MIT researcher has discovered that for at least the last 1.5 million years, the Earth has undergone rapid and dramatic climate changes similar to those observed in ice cores from more recent times. These climate swings are so dramatic that if we lived through one today, it would be like New England taking on Miami-like weather within a 25-year period.
"Ten years ago, we had no idea that climate could change this quickly," said Maureen E. Raymo, associate professor of earth, atmospheric and planetary sciences. Publishing their results for the first time in the April 16 issue of Nature, she and her colleagues at MIT and Woods Hole Oceanographic Institute report that millennial-scale climate instabilities -- swings of as much as 10 degrees Celsius within a few decades -- are not restricted to the large glacial periods of the last 700,000 years but existed much further back in time.
For the last five years, a cutting-edge initiative in climate studies has been the search for understanding of millennial-scale climate instabilities. These rapid, large-amplitude climate fluctuations were first identified in ice cores in Greenland and later in ocean sediment cores around the world.
While other researchers have focused on the geologic record of the past 120,000 years, Professor Raymo and a handful of colleagues have undertaken the far more ambitious effort of looking at climate trends as far back as 1.5 million years ago, at the dawn of the human race.
Their work has turned up some unexpected results. "Our results suggest that such millennial-scale climate instability may be a pervasive and long-term characteristic of Earth's climate, rather than just a feature of the strong glacial-interglacial cycles of the past 800,000 years," the authors wrote.
Professor Raymo and her colleagues, including graduate student Susan Carter and research technician Kristen Ganley, also show that these climate changes are tied to changes in the conveyor-like circulation of ocean waters that delivers tropical heat to the northern Atlantic in surface currents while exporting salt-heavy deep waters cooled by Greenland's winds to the south. But while scientists know that climate changes are linked to the changes in the ocean circulation and heat transfer, they don't know what causes these climate and circulation variations in the first place.
The most dramatic instabilities tend to be triggered during the cooler Ice Age periods, most recently the period from about 100,000 years ago to about 10,000 years ago. "We're living at the end of an unusually stable period," Professor Raymo said. After more than 8,000 years of warmth, we're about due for a cooling phase, "but all bets are off because of global warming from the buildup of carbon dioxide or other greenhouse gases in the atmosphere," she said.
To study these climate changes, Professor Raymo retrieved previously untouched sediments from far below the ocean floor, where deep-water currents sweeping down from Greenland have caused sediment to accumulate at unusually high rates for millions of years. By analyzing the composition and chemical structure of fossils in these sediments, Professor Raymo, a marine geologist and paleoceano-grapher, reconstructs past conditions on the Earth.
During a 1995 deep-sea drilling expedition -- two months at sea aboard the JOIDES Resolution, a former British Petroleum oil-drillingrig -- Professor Raymo, leading a team of 24 international scientists, broke the world record for the most sediment recovered in a single ocean-drilling expedition.
Although these sediments have provided a multitude of clues about climate variations over the millennia, scientists remain at a loss to explain why parts of the planet have swung from ice age to warm stage and back again. Is it due to external forces such as variations inthe output of the sun, or internal oscillations caused by instability in the distribution of salt and heat within the ocean?
"What causes climate variations on this time scale is a black box for scientists right now," Professor Raymo said.
A version of this article appeared in MIT Tech Talk on April 29, 1998.