EAE 13 - Climate proxies

Question 1

What does palaeoclimate science try to do?

4 points.

Answer 1

• Identify the nature of past climate changes
• Synthesize a coherent, falsifiable narrative describing major palaeoclimate events
• Understand the dynamics shaping these events
• Inform the evaluation and development of climate models

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Question 2

What is Palaeoclimatology at the Tectonic-scale?

4 points.

Answer 2

• Chemical weathering is an atmospheric CO₂ sink and its variation over time is a hypothesised long-term thermostat of the Earth
• Plate tectonics have large impact on topography and ocean/atmosphere circulation
• Distribution of sensible/latent heat
• Solid Earth is a huge carbon reservoir but slow exchange rate
• Immense changes have occurred in the past: e.g.
  • 5 mass extinctions
  • Snowball and Greenhouse Earth
  • Closure of Tethys sea 50 Ma
  • Opening of Drake’s passage (25 Ma)
  • Global cooling

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Question 3

What are the main pieces of evidence for ice sheet
history?

2 points.

Answer 3

• Foraminifera δ¹⁸O indicate ice volume and ocean temperature
• Ice-rafted debris: sediments from melting icebergs calved from ice-sheet margins

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Question 4

What is the quaternary glaciation activity over last 2.75Ma?

5 points.

Answer 4

• Slow drift towards more ice, change in glaciation threshold
• δ¹⁸O lags behind orbital changes to summer insolation
• Change from 41,000 to 100,000 yr cycles (see Huybers, 2007)
• ~Fifty glacial maxima!
• Slow trend part of 50My trend

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Question 5

What are the orbital-scale change cycles?

3 points.

Answer 5

Three key orbital cycles

• Eccentricity
• Obliquity
• Precession

Change the distribution of incoming solar radiation

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Question 6

What are the cycles of the orbital scale changes?

3 points.

Answer 6

Periodic cycles:

• ~100-400 ky
• 41 ky
• 23 ky

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Question 7

What are the effects of orbital scale changes?

7 points.

Answer 7

• Ice sheet extent
• Atmospheric greenhouse gases
• Sea level (~120m lower!)
• Monsoons
• Circulation
• Vegetation
• Global temperature

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Question 8

What other things influce orbital scale changes?

2 points.

Answer 8

• Immense ice sheets affect global climate
• Positive and negative feedbacks (e.g. icealbedo), delays (e.g. bedrock), nonlinearities (e.g. accumulation / melt)

_{EAE 16ah}

Question 9

What was the climate at Last Glacial Maximum?

3 points.

Answer 9

LGM at 21-26 ka

• ~4-7°C colder
• Sea level 110-125m lower
• Ice sheets ~2-3km thick

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Question 10

What has been the climate since LGM?

4 points.

Answer 10

• Summer insolation at 10 ka was at a maximum (Obliquity and Precession), drives ice sheet melt, GHG rise and further melting due to ice-albedo effect
• Brief interruptions to warming (Younger Dryas 13 ka, Antarctic Cold Reversal 14 ka)
• Millennial Oscillations: non-cyclical, high amp Nhem, ice sheet instability, thermohaline and atmos circ.
• Pleistocene (2.6 Ma to 12 ka) epoch of glacial-interglacials ended, gave way to stable, warm Holocene

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Question 11

What is a positive feedback loop?

4 points.

Answer 11

1. Initial climate forcing action
2. Intial climate response
3. Response amplified by climate system
4. Response becomes new input (i.e. Goes back into 1).

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Question 12

What does the symbol ‘‰’ mean?

Answer 12

‰ is “per mil” i.e. per thousand

Just as % is “per cent” i.e. per hundred

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Question 13

What is δ¹⁸O?

Answer 13

δ¹⁸O is a measure of the departure from a standard reference ratio of ¹⁸O/¹⁶O

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Question 14

What are typical values for δ¹⁸O?

3 points.

Answer 14

• Surface tropics 0 to -2‰
• Deep ocean +3 to +4‰
• Polar Ice -30 to -55‰

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Question 15

5 points.

Answer 15

• Fractionation of the isotopes relate mostly to temperature of ocean water
• Lighter ¹⁶O evaporates preferentially
• Heavier ¹⁸O condenses to precipitation preferentially
• Atmospheric circulation transports water vapour towards poles
• For a ~4°C increase in temperature, δ¹⁸O reduces by about 1‰

_{EAE 16ao}

Question 16

What is the δ¹⁸O in seawater?

Answer 16

In seawater δ¹⁸O ∝ Temperature of seawater

Planktic forams (upper 100m)

Benthic forams (sea floor)

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Question 17

What is a ‘foram’?

Answer 17

Foraminifera

From Wikipedia:

Single-celled organisms characterized by streaming granular ectoplasm for catching food and other uses and commonly an external shell (called a “test”) of diverse forms and materials.

Most foraminifera are marine, the majority of which live on or within the seafloor sediment (i.e., are benthic), while a smaller number float in the water column at various depths.

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Question 18

What is the δ¹⁸O in ice sheets?

Answer 18

In ice sheets δ¹⁸O ∝ Temperature of snow

Other factors also impact such as transport, seasonality, elevation

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Question 19

What is a Speleothem?

Answer 19

Speleothems commonly known as cave formations, are secondary mineral deposits formed in a cave. Speleothems typically form in limestone or dolomite solutional caves.

The definition of speleothem, in most publications, specifically excludes secondary mineral deposits in mines, tunnels, and other man-made structures.

From Wikipedia

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Question 20

What is the δ¹⁸O in speleothems?

2 points.

Answer 20

• In speleothems δ¹⁸O ∝ Monsoon strength (precip)
• ¹⁶O higher when lots of evap/precip

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Question 21

How does the δ¹⁸O change in glacial climates?

3 points.

Answer 21

Ice/Marine responses are opposite; In glacial climates:

• Ice core δ¹⁸O ↓
• Marine δ¹⁸O ↑

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Question 22

What is obliquity of orbit?

Answer 22

Tilt of the poles relative to the plane of orbit?

Note this is true poles (i.e. axis of rotation) not magnetic poles.

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Question 23

What is eccentricity of orbit?

4 points.

Answer 23

How elliptical is the orbit?

Distance between Earth and Sun:

• Orbit is not a perfect circle
• Due to gravity of other planets
• Small effect on solar radiation
• Interval between equinoxes not equal (one is 7 days longer)

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Question 24

When are the earth’s solstices?

Answer 24

~21 Jun/Dec

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Question 25

When are the earth’s equinoxes?

Answer 25

~21 Mar/Sep

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Question 26

When is the earth’s perihelion?

Answer 26

4 July

Perihelion = Nearest point of a body’s direct orbit around the Sun

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Question 27

When is the earth’s Aphelion?

Answer 27

3 January

Aphelion = Furthest point of a body’s direct orbit around the Sun

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Question 28

How does the sunlight vary for the earth?

Answer 28

Overall 6% more sunlight reaches Earth at aphelion vs perihelion

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Question 29

How does obliquity change?

8 points.

Answer 29

• 1,000 year regular cycle
• Present tilt 23.5°
• Tilt currently decreasing
• Small variations in amplitude
• Range is 22.2° to 24.5°
• Last max: 8700 BCE
• Next min: 11,800 CE
• One of the ‘Milankovitch’ cycles

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Question 30

What is the effect of obliquity?

2 points.

Answer 30

• Amplify/suppress seasons
• Biggest impacts at the poles

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Question 31

What is the earth’s orbital eccentricity?

4 points.

Answer 31

How elliptical is the earth’s orbit?

• Currently 0.0167 (nearer to circular)
• Ranges between 0.005 and 0.0607
• Periods of 100,000 yr and 413,000 yr
• The second of the Milankovitch cycles

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Question 32

What is the precession of the equinoxes?

4 points.

Answer 32

• Wobbling motion
• Change in the direction of the lean (not the angle itself that’s Obliquity!)
• Minor and major axes shift slowly in time
• We can understand precession as a variation in the solstices/equinoxes relative to the perihelion

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Question 33

What is the precession of the equinoxes?

6 points.

Answer 33

• Axial precession and orbital precession combine to create the precession of the equinoxes
• The major periodicity is at about 23,000 years
• In addition to these two effects, eccentricity modulates precession
• The precessional index represents the overall affect of precession and eccentricity: 𝛆 sin(𝛚)
• Angle 𝛚 helps simplify equations of motion and is the angle between these the imaginary lines of the perigelion axis and the March 20 equinox
• This is the third Milankovitch cycle

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Question 34

What is the Impact of Eccentricity on Precession?

5 points.

Answer 34

• Changes in eccentricity magnify/suppress contrasts
• Combined effects cause the distance from Earth to Sun to vary by season
• High eccentricity → highest contrasts in Earth-sun distance
• Low eccentricity → lowest contrasts in Earth-sun distance
• Precession influences the seasonality of arriving radiation

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Question 35

What is solar insolation?

Answer 35

Radiation received at the top of atmosphere

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Question 36

What are the long term changes in insolation?

4 points.

Answer 36

• Orbital changes affect the amount of solar insolation
• Influence of cycles of precession (P) and tilt (T) vary with latitude
• Precession: influences insolation at low-mid latitudes, and high latitudes in summer only
• Obliquity (tilt): influences winter insolation at higher latitudes (60° N/S)

_{EAE 16bj}

Question 37

What would have helped the melting along once orbital cycles kick things off?

3 points.

Answer 37

• Summer insolation: Melting began due to increased summer insolation (obliquity and precession)
• Greenhouse gases: As ice sheets melted, CO2 and CH4 rose, enhancing the greenhouse effect: Positive Feedback
• Surface albedo: changed as ice sheets melted: Positive Feedback

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