Transcript Climates of the Past 200 my PART 1

Orbital Control of Climate
The last 600,000 years
Objective
To understand
orbital control of
natural climate
change during the
Quaternary
(late Tertiary),
the last 2.5My
2.5My-0.65My (weak ice ages, every 50Ky),
0.65My-present (strong ice ages, every 100Ky)
Broecker, p.167
LGM
Last Glacial
Maximum
18Ky
The last 600ky…
Orbital Cycles
Orbital Cycle Changes
1• Shape of Earth’s yearly path of revolution around the Sun
ECCENTRICITY
2• Tilt of Earth’s axis
TILT
3• Changing positions of the seasons along that path
PRECESSION
Various records of climate (ice cores, deep ocean circulation,
greenhouse gas concentrations, cyclic stratigraphy in sediments)
have these same orbital cycles.
1. Eccentricity
• 3% range of
variation of E-S
distance due to elliptical
orbit around Sun,
Pull of large planets
• Now, perihelion
occurs during northern
Winter (3 January)
• 100ky and 413ky
periods
• Now, e =.0167
(relatively circular)
2. Tilt - Reason
for Seasons
• Now tilt is 23.5°, and
is decreasing.
• Higher the tilt, higher
the seasonal differences
• Has gone from 24.5°
-22.2°, varied by pull
of large planets
• 41 ky period
3. Precession - Wobble
Varies E-S distance with season
• Variation of the
direction in which the E
axis leans.
• Caused by the pull of
Sun and moon on Earth
Now
• Now Earth’s axis points
to Polaris
• 23 ky stronger period combined wobble and
slow turning of elliptical
orbit.
11Kya
On the long term…the
precessional index
Eccentricity variations at 100Ky and 413Ky
modulate the amplitude of the precession cycle.
This may be why we have the 100Ky ice age cycle
Variation of insolation with time - Big changes at high northern latitudes during
deglaciations (arrows, June solstice at perihelion)
How orbital theory,
climate records (18O)
and northern
hemisphere insolation
(i.e. melting in N.
Atlantic) compare over
the past 600ky.
now
LGM
18ky
Cold
warm
Orbital Cycles
Isotopic and spectral evidence of transition to 100ky ice age periodicity…
Figs 10-15 and 10-17
Milankovitch Theory of ice ages
What triggers an ice age? Low
insolation:
• when tilt is small (~22°)(seasonality differences are
smallest)
• Eccentricity is large (very elliptical orbit)
• Perihelion occurs during the northern hemisphere’s
winter
Deglaciation can be triggered when:
• perihelion occurs in July (e.g. 11K ya)
• the Earth’s tilt is near its maximum
What are the mechanisms that amplify
orbital variations in insolation?
(1) Ice-albedo feedback
(2) ocean circulation/CO2 feedback (Shackleton 2000)
– ice lags CO2 and orbital changes
20-30W
Crucial for
deglaciation melting
25-90W
Relative effects of
orbital cycles on
insolation at 65°N
Broecker, p.159
25W
Cycle of ice sheet growth FACTS:
Ice accumulation rates (0-.5m/y) << Ablation rates (0-4m/y)
Summer insolation is most important for ice sheet growth and
decay
Fig. 10-12
Cycle of ice sheet growth What makes ice sheets grow
and shrink?
Equilibrium line =
Boundary between
areas of net ice ablation
and accumulation.
Cooling at higher altitudes,
6.5°C cooling per km of altitude. = Ice Elevation Feedback
D.* Ice growth continues because
Insolation levels are still relatively
Low and most of ice is at high
elevation
P = Climate Point, where equilibrium line intersects surface
accumulation
ablation
Cycle of ice sheet growth -
3 Factors control ice sheets:
1. Insolation control of IS size
2. Initial lag of vol behind
Insolation
accumulation
ablation
3. Subsequent lag of bedrock
depression and rebound behind
ice loading and unloading.
Milankovitch