A Debate That Could Last An Iceage

SPACEDAILY

New York - March 22, 2000 -
It seems reasonable to think that global ice ages result from climatic forces at work in the Northern Hemisphere. After all, that's where most of the world's ice periodically accumulates into the massive sheets that then grind southward over Europe, northern Asia and North America.

This intuitive assumption underlies most traditional theories for the origin of the ice ages -- not surprising when you realize that the ground in much of North America is still moving in response to the weight of the last great ice sheet, which melted away more than 10,000 years ago. North is where the glacial action is. Isn't it?

Perhaps not.

New evidence based on a technical breakthrough described in the March 2 issue of Nature suggests that the real driving mechanism behind the ice ages must lie in the tropics, or even south of the equator -- far removed from whence the ice sheets have arrived every 100 millennia or so for at least the past million years.

By adapting an established radiological dating technique to pinpoint the age of a major glacial event that occurred more than 100,000 years ago, researchers Gideon Henderson of the Lamont-Doherty Earth Observatory and Niall Slowey, of Texas A&M University have succeeded in both bolstering the general theory of why ice ages occur, and throwing cold water on traditional notions of how the whole thing actually works.

"In our paper we demonstrate that, based on a simple argument of timing, the traditional model of ice ages as forced by climate amplifying mechanisms in the Northern Hemisphere cannot be correct," Henderson, now at Oxford University, said. "The general correspondence between glacial chronology and orbital insolation, however, remains clear."

Scientists generally agree that the ice ages, and indeed most major climate phenomena including the seasons, are somehow linked to insolation -- the intensity of solar energy striking the planet -- and how it varies with latitude and cyclic changes in Earth's orbit around the sun. This theory has been around since the 1870s, and was formalized in the 1930s by Serbian astronomer Milutin Milankovitch, for whom it is now known.

Earth's solar orbit varies from slightly elliptical to nearly circular on cycles of 100,000 and 400,000 years. At the same time, the tilt of Earth's rotational axis toward the sun wobbles between 22 and 25 degrees every 41,000 years, while the hemisphere pointed toward the sun at the closest approach of its orbit cycles every 22,000 years. The interaction of these cycles produces varying insolation patterns thought to influence the relative intensity of the seasons through time, and thus also long-term climate.

Milankovitch's idea was that when summertime insolation in the Northern Hemisphere is at its lowest, the season remains cool enough for snow to persist year-round and eventually accumulate into great ice sheets. Conversely, when summertime insolation peaks, the glaciers retreat.

But if this were true, the middle of the interglacial period prior to the one we're in now would have occurred 127,000 years ago. Henderson and Slowey have demonstrated that the midpoint actually came 135,000 years ago -- 8,000 years too early to have resulted directly from increased insolation in the Northern Hemisphere.

Their finding reinforces recent theories that view insolation as simply the first domino to fall in a complex web of global ocean/atmosphere interactions that amplify the relatively weak, gradually changing solar signal into significant and apparently abrupt climate swings from warm to cold.

Henderson and Slowey propose two possible driving mechanisms for what they term the "penultimate deglaciation" -- polar warming via heat transport from warm tropical oceans, and general atmospheric warming due to CO2 released from the vast southern oceans. Both these mechanisms would be relatively independent of insolation in the upper northern latitudes.

Despite the important ramifications of their conclusions, however, the team's refined dating technique may well prove the greater contribution.

Based on the radioactive decay of uranium to thorium, the technique provides the first reliable method for obtaining independent and reliable ages from marine carbonate sediments more than 30,000 years old. These sediments record the total volume of global ice through time in the changing ratio of oxygen isotopes captured as they accumulated. Peaks in global ice volume correspond to ice ages; valleys correspond to interglacial periods.

The glacial time scale now in general use is an approximation based on assumptions about the rate of sediment accumulation and is inextricably tied to the inferred link between climate events and Earth's orbital cycles -- the timing of which can be back-calculated with extreme accuracy.

"Ultimately, the major contribution of this effort, funded by the National Science Foundation, lies in the technique to pin accurate and precise ages to the oxygen-isotope record of climate preserved in marine sediments," Henderson said. "This technique will allow us to date other climate events and produce a true glacial chronology that will hopefully lead to new insights into the mechanisms that control our climate."

Columbia University

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