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Factors Influencing IS Presence (Obliquity (41 kyr) (~ Axial tilt,…
Factors Influencing IS Presence
Factors:
Major ∆ Ocean Circulation (long term)
CO2 Drawdown (long term)
Variation Earth Orbit (short term)
1. CO2 Drawdown
Climate slowly cooled over past 60 My
due to decreasing atmospheric CO2
(overall trend)
= conducive to ice sheet growth
Timing Ice Sheets
NH ~ 2.5 Myr (much later)
SH ~ 34 Myr if not earlier (Antarctic IS)
2. Major ∆ Ocean Circulation
Gulf stream
--> warms NH
NH would be much colder if not for Gulf Stream
~Atlantic Boundary Current
Transports heat poleward from Equator
Continental barriers deflect warm current NE e.g. E coast USA
Antarctic Circumpolar Current (ACC)
Isolates Antarctica from warm currents
Previously: Warm water deflected toward S. Pole
--> by continental barrier btwn Antarctica & S America
Drake's Passage Opening ~35-45 Ma
Allowed ACC flow all around Antarctica
Isolates Antarctica from warm currents
Conducive to/cause AIS nucleation?
Antarctic IS Nucleated ~34 Ma
--> coincides with Drake's Passage opening
--> cooled continent?
3. Orbital Variations
Earth's orbit - not fixed
i.e. varies cyclically over tens kyr
Glacial-interglacial & orbital cycles match to some extent
3 important variations Earth's orbit
Obliquity (41 kyr)
Precession (22.5 kyr)
Eccentricity (100 kyr)
Each affect solar radiation distribution
Obliquity
(41 kyr)
~ Axial tilt
Earth's axis = tilted out of plane of solar orbit (ecliptic)
Degree of tilt varies (by 2.4°)
Influences extremity of seasons
i.e. high obliquity = more extreme seasons
Low obliquity favours ice sheets
-->milder summers thus
lower summer melt
Ellipticity
Earth's orbit around sun not circular --> elliptic
∴ Seasonal cycles differ btwn hemispheres
more extreme in one
less extreme in other
Effect = modulated by precession & eccentricity
Precession
(22.5 kyr)
= Rotation of tilt axis
i.e. which hemisphere points toward sun
Dictates which hemisphere has strong seasonal cycle
Aphelian favours NH ice sheet growth
i.e. NH has weaker cycle ∴ lower summer melt
Eccentricity
(100 kyr)
= how elliptic the orbit is
Controls strength of precession effect
Insolation & Glacials
Insolation = energy per m2 from sun (Wm-2)
65°N often used as an indicator of glaciation
Ellipticity
Modulates insolation received over time
In-phase with glacial cycle (longterm temp record)
N.B. Glacial cycle = more pronounced, less symmetric
than orbital forcing on insolation