Physics of Planetary Climate Cors221: Physics in Everyday Life Fall 2010 Module 3 Lecture 5: Medium-Term Climate History: The Ice Ages

From Last Time Long-term (> 10s of millions of years to 4.6Gyr) climate history information can come from geology. Oxygen isotopes (  18 O) from carbonate (CaCO 3 ) can be used as a paleotemperature proxy. Can also use similar techniques to measure ancient carbon dioxide levels. Earth has only had polar ice for ~15% of its history; frequently there is sufficient equator-to-pole heat transport to allow palm trees at the poles. Specific example: Snowball Earth, when Earth froze over 600 Myr ago. Snowball Earths are reversed by build up of carbon dioxide, and are followed by global hothouses when the ice melts. Earth’s climate history is a total roller coaster on 10 million year to billion year timescales.

Climate Since the Dinosaurs

Climate Over the Past 5 Myr

Climate Over the Past 450 kyr

Glaciated Earth

Glaciers and Sea Level Change

Antarctic Drill Cores

From Last Time

Eccentricity Varies on ~100,000 year timescales

Obliquity Varies on ~41,000 year timescales

Longitude of Periapsis Varies on ~23,000 year timescales

Net Solar Flux at 65N

Glacial-Interglacial Cycles

Milankovic Cycles

Key Points Earth was hot in the Eocene 50Myr ago (+12 K or +21F), but Antarctica glaciated over 13 Myr ago and stayed that way. For the past 2.5 Myr, Earth has been in an Ice Age, characterized by semiperiodic glacial and interglacial cycles with ~100,000 yr period. With so much of Earth's water bound up in 2-mile-thick glaciers over North America and Siberia, global sea level was 120 m lower. Earth’s orbital eccentricity, a measure of how far the orbit is from non-circular, changes from 0.0 to 0.05 on 100,000 year timescales (presently 0.017) and 400,000 year timescales. Earth's obliquity, the angle of tilt of the rotation axis, changes from 22.1 to 24.5 degrees on 41,000 year timescales (presently 23.44). Day of summer solstice relative to periapsis varies on 21,000 & 26,000 year timescales. The natural variations in climate over the past 2.5 Myr, i.e. during the ice age, are well-explained by changes in insolation due to orbit and obliquity changes. These are known as Milankovich cycles.