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•  Large Ice sheets covered Northwestern Europe and Northern North America

•  Due to orbitally induced changes in northern hemisphere summer insolation –  Ice volume changes are coordinated with CO2

changes (shift of carbon between atmosphere and oceans)

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Last Glacial Maximum Present

What does an ice age look like?

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Vocabulary: Isotopes!

atomic mass: number of protons plus neutrons H normally 1+0 = 1 C normally 6+6 = 12 O normally 8+8 = 16

an atom (or element) is defined by number of protons H(1), C(6), O(8), Pb(82)

isotope: same element, different atomic mass >> that is, different number of neutrons

1H (normal), 2H (deuterium), 3H (tritium; radioactive) 12C (normal), 13C, 14C (radioactive) 16O (normal), 18O

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16O vs 18O "light"

(normal) "heavy"

•  evaporation selects for "light" •  condensation (precipitation) selects for "heavy"

18O in ocean sediments records glacial ice volume: More "light" water in ice-sheets means remaining ocean water is "heavier". 18O in ice-cores indicates local temperature: Colder conditions means more precipitation en route so "lighter" snow.

Kasting et al"

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Oxygen Isotope Concentration in Shells of Organisms Growing in the Deep Ocean

TIME

"heavy" water! = more ice!

"light" water! = less ice!

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Kasting et al"

"heavy" water! = more ice!

"light" water! = less ice!

TIME

Short (10,000 yr) Warm Episodes

Ocean sediment record !

of Global Ice Volume"

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Trigger with feedback causes ice-sheets... to grow and keep growing

or to melt and keep melting

Ice-albedo feedback

- Other feedbacks are needed to explain the magnitude of the changes. - Greenhouse gases (e.g. CO2 and CH4) seem to be involved.