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Study examines dynamics that drive glacial-interglacial cycles.

Earth's most recent glacial period began about 110,000 years ago and ended about 15,000 years ago, and over the past million years Earth's glacial-interglacial cycle has consistently alternated phases approximately every 100,000 years. That 100,000-year pace "has been difficult to explain," notes Jung-Eun Lee of Brown University, who was part of a recent study published in Geophysical Research Letters. The new research shows that recurring changes in Earth's orbital path known as Milankovitch Cycles cause an increase in Southern Hemisphere sea ice on 100,000-year cycles, and that sunlight reflecting off the increased ice in turn curbs incoming solar radiation and causes global temperatures to cool, kicking off each cyclic phase of significantly expanding glacial ice--or ice age--around the globe. The study also explains why the ice age cycle more than a million years ago was much different--recurring about every 40,000 years instead.

Lee and colleagues used computer simulations to more closely examine the Milankovitch Cycles, which occur every 100,000,41,000, and 21,000 years. The 100,000-year cycle of eccentricity--how much Earth's orbit deviates from a circle--actually has the least impact of the three on solar radiation, which makes the glacial-interglacial cycles more perplexing. The new study revealed that the 21,000-year cycle works in tandem with the 100,000year cycle to drive the glacial tempo. The 21,000-year cycle involves precession, which is the change in the orientation of Earth's tilted rotational axis and what generates the temperature changes on Earth we call seasons. The direction that Earth's axis points gradually precesses, or changes, with respect to Earth's orbit, and the orbital position where the seasons change migrates marginally from year to year. Because Earth's orbit is also elliptical, varying in its eccentricity from nearly circular to elliptical and back every 100,000 years, the distance between Earth and the sun varies depending on the orbital ellipse, and as a result, the intensity of seasons is dependent on whether Earth is closest to or farthest from the sun, or at some point in-between.

This is significant to the current research because there is a period during the 21,000-year cycle when the Southern Hemisphere summer occurs with Earth at its most distant point from the sun, making the summers there cooler than normal. This means there is less melting of sea ice over a series of summers, which allows the ice to expand dramatically, causing solar radiation to be reflected away from Earth rather than being absorbed by the ocean, which cools the planet. (The same effect doesn't happen in the Northern Hemisphere because there isn't as much oceanic space there for sea ice to expand.)

"What we show is that even if the total incoming energy is the same throughout the whole precession cycle, the amount of energy the Earth actually absorbs does change with precession," Lee explains. "The large Southern Hemispheric sea ice that forms when summers are cooler reduces the energy absorbed."

At the same time, the 100,000-year cycle is influencing the precession cycle. A larger eccentricity (i.e., when the orbit is more elliptical) corresponds to a significant difference between Earth's farthest point from the sun and its closest, and as a result, "when eccentricity is small, precession doesn't matter," Lee says. "Precession only matters when eccentricity is large. That's why we see a stronger 100,000-year pace than a 21,000-year pace."

The models used in the study determined that cool summers in the Southern Hemisphere can cut down by as much as 17% the amount of solar radiation absorbed by Earth. This creates favorable conditions for the onset of an ice age.

The study explains ice age cycles of the last million years, and the researchers state that their model has also solved the mystery of why ice ages cycled at closer to 40,000 years more than a million years ago. The third Milankovitch Cycle was dominant then.

The third or 41,000-year Milankovitch Cycle has to do with the tilt--or obliquity--of Earth's axis, which varies over that period from about 22[degrees] to about 25[degrees]. The greater the obliquity, the more sunlight reaches Earth's poles and warms the planet. The 41,000-year cycle was more significant more than one million years ago because Earth then was generally warmer than it has been since, and according to the new study's models, sea ice expansion due to precession in the Southern Hemisphere is less likely to occur on a warmer planet--thus favoring the obliquity cycle to be the greatest influence on the global temperature signature. As Earth cooled since then, the precession/eccentricity signal became dominant.

Additionally, the research indicated that the increase in Southern Hemisphere sea ice may also decrease carbon dioxide levels in the atmosphere and trigger energy feedbacks, exacerbating the cooling effect in the region. [Source: Brown University]
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Title Annotation:NOWCAST: NEWS AND NOTES
Publication:Bulletin of the American Meteorological Society
Article Type:Report
Date:Jun 1, 2017
Words:808
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