Researchers studying deep deposits of muck on the sea floor have dug out a detailed history of climate that extends nearly 2 million years into the past, confirming that throughout that period, Earth experienced more or less regular cycles of climate change lasting from 1200 to 6000 years. The oscillations are extreme during ice ages. But the long record shows that they are far gentler during warm periods like today's, suggesting that the climate roller coaster of glacial times won't reappear anytime soon.
Researchers had already caught a glimpse of these cycles during the past 100,000 years in records cored from the Greenland Ice Sheet, but paleoceanographer Delia Oppo of Woods Hole Oceanographic Institution in Massachusetts and her colleagues now report in Science that North Atlantic sea-surface temperatures were oscillating on the same schedule half a million years ago, tending to repeat roughly every 6000, 2600, 1800, and 1400 years. They extracted the cycles from corings of an exceptionally thick sediment pile, or drift, off Ireland. Ongoing studies of other cores from North Atlantic drifts show the same pattern of oscillations--greatly enhanced during ice ages and damped during interglacials--throughout the past 2 million years.
The same forces seem to have been driving climate cycles for almost 2 million years, but what those forces are, "we really don't know," says Oppo. Candidates include cyclical variations in the sun's brightness and Earth's regular wobbles on its axis, which could shift climate by changing the distribution of sunlight across the planet. Whatever the cause of the climatic gyrations, the records suggest that the worst climate swing that would naturally occur in the present interglacial is a modest 1°C chilling.
The climate oscillations as Earth warms because of increasing greenhouse gases could be another matter, suggests glaciologist Richard Alley of Pennsylvania State University in University Park. The greater warmth might damp oscillations, the more rapid climate change might aggravate them, or they might stay the same. Which scenario is more likely? For better or worse, the answer will come as human beings continue to pour greenhouse gases into the atmosphere, says Alley: "The experiment to answer that question is the one we're doing now."