Lecture 20: Orbital Variations in Ice SheetsLecture 20: Orbital Variations in Ice Sheets(Milankovitch Cycles)(Milankovitch Cycles)

Chapter 9 (p. 163-174)

Milankovitch Theory

Milutin Milankovitch first proposed the following idea in the 1930s.

Changes in climatic cycles of glacial-interglacial periods were initiated by variations in the Earth’s orbital parameters (Earth-Sun geometry factors)

High summer insolation heats land glacier ablatesLow summer insolation keeps land cool glacier persists or grows

Milankovitch Theory of Climate ChangeMilankovitch Theory of Climate Change

Climate change may be driven by Climate change may be driven by changes in earth's a) orbit changes in earth's a) orbit ((eccentricityeccentricity), from ellipse to circle at ), from ellipse to circle at 100,000 year cycles, b) wobble 100,000 year cycles, b) wobble ((precessionprecession), from the north pole ), from the north pole pointing toward or away from the sun pointing toward or away from the sun in June at 23,000 year cycles, and in June at 23,000 year cycles, and c) tilt (c) tilt (obliquityobliquity), from 22.2° to 24.5° ), from 22.2° to 24.5° at 41,000 year cycles.at 41,000 year cycles.

The precession of the The precession of the earth’s axis changes earth’s axis changes seasonal variations.seasonal variations.

Presently the earth is Presently the earth is closestclosest to the sun to the sun ((perihelionperihelion) in January ) in January (the N.H. winter), (the N.H. winter), most most distantdistant from the sun from the sun ((aphelionaphelion) in July.) in July.

In about 11,500 years, the In about 11,500 years, the earth will be closer to the earth will be closer to the sun in July (the N.H. sun in July (the N.H. summer), most distant summer), most distant from the sun in January.from the sun in January.

Milankovitch Theory of Climate ChangeMilankovitch Theory of Climate Change

Return to modern position after 23,000 years

Testing the Milankovitch Theory

Kump et al., 1999

Calculated June insolation at 65˚N

Measured Ice volume

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Testing the Milankovitch Theory

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-Preglaciation PhaseSmall Glaciation Phase

Large Glaciation PhasePermanent Glaciation Phase

Ocean sediments have 2 key indicators of past glaciations.

Last Glacial and Interglacial Cycle

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> 50 glacial maxima

Slow shift toward a colder, more glacial world

Climate surprises:Dominate 100,000 yr cycleSawtoothed features:

Slow glaciation & rapid deglaciation

The Earth’s Climate History1. Over the last century, the earth’s surface temperature has increased by

about 0.75°C (about 1.35°F).

2. Little Ice Age = 1350 A.D. – 1850 A.D. (N.H. temperature was lower by 0.5°C, alpine glaciers increased; few sunspots, low solar output)

3. Medieval Warm Period = 950 A.D. – 1,250 A.D. (N.H. warm and dry, Vikings colonized Iceland & Greenland)

4. Holocene Maximum = 5,000-6,000 ybp (1°C warmer than now, warmest of the current interglacial period)

5. Younger-Dryas Event = 11,000 ybp (sudden drop in temperature and portions of N.H. reverted back to glacial conditions)

6. Last Glacial Maximum = 18,000 ybp (maximum North American continental glaciers, lower sea level exposed Bering land bridge allowing human migration from Asia to North America)

7. We are presently living in a long-term Icehouse climate period, which is comprised of shorter-term glacial (e.g., 18,000 ybp) and interglacial (e.g., today) periods. There were four periods of Icehouse prior to the current one.

8. For most of the earth’s history, the climate was much warmer than today.