Orbital Forcing on ClimateFinish Climates of Geologic TimeIntroduction to Orbital FactorsAxial TiltAxial PrecessionChanges in Earth’s eccentric orbitPrecession of the Ellipse

For Next Class: Read Ruddiman Ch. 9 (AsUlearn)Exam 2 on October 8th!

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Climates of the Pleistocene Ice Age

Chronology and Temperature Trends Last major glacial climatic episode began

about 27,000 years ago and reached its peak about 20,000 to 18,000 years ago

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Climates of the Pleistocene Ice Age

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Climates of the Holocene

The Holocene Epoch is the present interglacial Glacial ice withdrew from North American

Great Lakes region about 10,500 years ago• Civilization and agriculture developed

Holocene has had spatially and temporally variable temperature and precipitation

• Global climate transitioned from a cold glacial (Pleistocene) to a warmer interglacial (Holocene) approximately 15,000 years ago primarily as a result of orbital forcing.

• However, two prominent periods of abrupt cooling are evident at the end of the Pleistocene and beginning of Holocene.

Abrupt Climate ChangesTheory, models, and paleoclimatic reconstructions have shown that changes in the climate system can be abrupt and widespread.Abrupt climate change: “. . . occurs when the climate system is forced to cross some threshold, triggering a transition to a new state . . .” (Alley et al. 2002)“Climate surprises” often used to refer to these abrupt transitions

Responses of a Climate Variable to Forcing

Younger DryasAbout 14,500 years ago, the global climate began to transition to a warmer interglacial.Around 13,000 years ago, the climate suddenly or abruptly returned to glacial conditions, cooling 3-8 °C (even colder across portions of Europe).This period is called the Younger Dryas, named after an Arctic flower that became common across Europe.

NOAA 2011

8.2 ka Cooling EventAnother period of abrupt climate change occurred approximately 8,200 years ago and is hence termed the 8.2 ka cooling event. Temperatures were lower and much of the world was much drier.Climate returned to warmer conditions characteristic of the present interglacial after about 150 years.

8.2 ka Event in Paleoarchives

NOAA 2011, from Morrill and Jacobson (2005)

Possible Causes of 8.2 ka Event

Day After Tomorrow

• http://www.youtube.com/watch?v=SnvqsWVluCE

Causal MechanismsSignificant influx of freshwater into the North Atlantic appears to have caused both the Younger Dryas cooling and the 8.2 ka cooling event. Freshwater influx changed the density structure associated with meridional overturning circulation (or thermohaline circulation) and caused a slowing or stopping of Gulf Stream circulation.

Orbital Influences on InsolationVariations in axial tilt (41,000 year cycle)Axial precession (25,700 year cycle)Changes in Earth’s eccentric orbit (100,000 year cycle)Precession of the ellipse (23,000 year cycle)

Earth’s Tilt Actually VariesVaries between 22 and 24°, on a 41,000 year cycle

No Tilt vs. 90° TiltHow does the annual pattern of solar radiation vary at the Equator and at the Poles in the two examples on the right?

Which scenario would be most favorable for polar ice sheet growth? Tropical glacier advance?

Axial Precession

A rotating body like the Earth is subject to a slow wobble.

Changes in Eccentric OrbitEccentricity, or the shape of Earth’s path around the sun, changes on a 100,000 year cycle

Causes changes to the distance away from the sun during theperihelion and aphelion, resulting in subtle differences in insolation.

Precession of the EllipseThe entire ellipse is also subject to slow rotation.

Precession of the EquinoxesAxial precession and precession of the ellipse results in the precession of the equinoxes. A strong cycle is evident on a 23,000 year cycle.

Influences on Solar Radiation

Departure from Modern Insolation