Lecture 20: Orbital Variations in Ice Sheets (Milankovitch Cycles) Chapter 9 (p. 163-174) This lecture was prepared on 1/23/2005 (Sunday). Took me 5 hours (from 8:00pm to 1:00am)

Milankovitch Theory Milutin Milankovitch first proposed the following idea in the 1930s. High summer insolation heats land  glacier ablates Low summer insolation keeps land cool  glacier persists or grows Changes in climatic cycles of glacial-interglacial periods were initiated by variations in the Earth’s orbital parameters (Earth-Sun geometry factors)

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

Milankovitch Theory of Climate Change The precession of the earth’s axis changes seasonal variations. Presently the earth is closest to the sun (perihelion) in January (the N.H. winter), most distant from the sun (aphelion) in July. In about 11,500 years, the earth will be closer to the sun in July (the N.H. summer), most distant from the sun in January. Return to modern position after 23,000 years

Testing the Milankovitch Theory Calculated June insolation at 65˚N Kump et al., 1999 - - Measured Ice volume

Testing the Milankovitch Theory Permanent Glaciation Phase Large Glaciation Phase - Preglaciation Phase - Small Glaciation Phase Ocean sediments have 2 key indicators of past glaciations.

Last Glacial and Interglacial Cycle Climate surprises: > 50 glacial maxima Dominate 100,000 yr cycle Sawtoothed features: Slow glaciation & rapid deglaciation - - Slow shift toward a colder, more glacial world

The Earth’s Climate History Over the last century, the earth’s surface temperature has increased by about 0.75°C (about 1.35°F). 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) Medieval Warm Period = 950 A.D. – 1,250 A.D. (N.H. warm and dry, Vikings colonized Iceland & Greenland) Holocene Maximum = 5,000-6,000 ybp (1°C warmer than now, warmest of the current interglacial period) Younger-Dryas Event = 11,000 ybp (sudden drop in temperature and portions of N.H. reverted back to glacial conditions) 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) 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. For most of the earth’s history, the climate was much warmer than today.