soulxtc
July 25th, 2006, 09:29 AM
Ice sheets drive atmospheric carbon dioxide levels, inverting previous ice-age theory
In the early 20th century, Milutin Milankovitch, a leading astronomer and climatologist of the time, proposed that the Earth's ice-age cycles could be predicted because they correspond directly with routine changes in the Earth's orbit and its tilt over cycles of tens of thousands of years.
Because of these changes, there are predictable variations in the amount of solar radiation striking the Earth's surface. Milankovitch argued that low levels of summer radiation permit snow to accumulate as permanent ice, while high levels of solar radiation melt snow and ice.
It all seemed so clean and simple.
And indeed the hypothesis was partially confirmed in the 1970s from marine sediment records extending through 2.75 million years of northern hemisphere ice-age cycles. As Milankovitch predicted, ice grew and melted at cycles of 23,000 and 41,000 years. But two observations were unexpected: from 2.75 until 0.9 million years ago, the ice sheets grew and melted almost entirely at the 41,000-year cycle. Since then, an oscillation near 100,000 years has dominated.
This knocked Milankovitch's theory for a loop.
Scientists have since turned to changes in atmospheric carbon dioxide as a possible explanation. Carbon dioxide concentrations can be measured in ancient air bubbles preserved in sequences of cores drilled into the Antarctic ice sheet. Because some changes in carbon dioxide have been found to occur slightly before changes in ice volume, the prevailing interpretation has been that carbon dioxide is an additional independent 'driver' of the size of ice sheets, along with solar radiation.
Now, a new hypothesis inverts this view.
William Ruddiman, an environmental scientist with the University of Virginia, provides a novel explanation for the rhythms of the ice ages in a paper just published online in the journal Climate of the Past. Ruddiman found that carbon dioxide is a driver of ice sheets only at the relatively small 23,000-year cycle, but not at the much larger ice-volume cycles at 41,000 years and approximately 100,000 years. In those cases he found that ice sheets instead control atmospheric carbon dioxide and drive feedbacks that amplify ice growth and melting. He says his carbon dioxide feedback hypothesis explains why the strongest cycles of ice response are not in correspondence with those in the orbital cycles.
http://www.physorg.com/news72982748.html
In the early 20th century, Milutin Milankovitch, a leading astronomer and climatologist of the time, proposed that the Earth's ice-age cycles could be predicted because they correspond directly with routine changes in the Earth's orbit and its tilt over cycles of tens of thousands of years.
Because of these changes, there are predictable variations in the amount of solar radiation striking the Earth's surface. Milankovitch argued that low levels of summer radiation permit snow to accumulate as permanent ice, while high levels of solar radiation melt snow and ice.
It all seemed so clean and simple.
And indeed the hypothesis was partially confirmed in the 1970s from marine sediment records extending through 2.75 million years of northern hemisphere ice-age cycles. As Milankovitch predicted, ice grew and melted at cycles of 23,000 and 41,000 years. But two observations were unexpected: from 2.75 until 0.9 million years ago, the ice sheets grew and melted almost entirely at the 41,000-year cycle. Since then, an oscillation near 100,000 years has dominated.
This knocked Milankovitch's theory for a loop.
Scientists have since turned to changes in atmospheric carbon dioxide as a possible explanation. Carbon dioxide concentrations can be measured in ancient air bubbles preserved in sequences of cores drilled into the Antarctic ice sheet. Because some changes in carbon dioxide have been found to occur slightly before changes in ice volume, the prevailing interpretation has been that carbon dioxide is an additional independent 'driver' of the size of ice sheets, along with solar radiation.
Now, a new hypothesis inverts this view.
William Ruddiman, an environmental scientist with the University of Virginia, provides a novel explanation for the rhythms of the ice ages in a paper just published online in the journal Climate of the Past. Ruddiman found that carbon dioxide is a driver of ice sheets only at the relatively small 23,000-year cycle, but not at the much larger ice-volume cycles at 41,000 years and approximately 100,000 years. In those cases he found that ice sheets instead control atmospheric carbon dioxide and drive feedbacks that amplify ice growth and melting. He says his carbon dioxide feedback hypothesis explains why the strongest cycles of ice response are not in correspondence with those in the orbital cycles.
http://www.physorg.com/news72982748.html