EAE 13 - Climate proxies Flashcards
What does palaeoclimate science try to do?
4 points.
- 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
EAE 16aa
What is Palaeoclimatology at the Tectonic-scale?
4 points.
- 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
EAE 16ab
What are the main pieces of evidence for ice sheet
history?
2 points.
- Foraminifera δ¹⁸O indicate ice volume and ocean temperature
- Ice-rafted debris: sediments from melting icebergs calved from ice-sheet margins
EAE 16ac
What is the quaternary glaciation activity over last 2.75Ma?
5 points.
- 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
EAE 16ad
What are the orbital-scale change cycles?
3 points.
Three key orbital cycles
- Eccentricity
- Obliquity
- Precession
Change the distribution of incoming solar radiation
EAE 16ae
What are the cycles of the orbital scale changes?
3 points.
Periodic cycles:
- ~100-400 ky
- 41 ky
- 23 ky
EAE 16af
What are the effects of orbital scale changes?
7 points.
- Ice sheet extent
- Atmospheric greenhouse gases
- Sea level (~120m lower!)
- Monsoons
- Circulation
- Vegetation
- Global temperature
EAE 16ag
What other things influce orbital scale changes?
2 points.
- Immense ice sheets affect global climate
- Positive and negative feedbacks (e.g. icealbedo), delays (e.g. bedrock), nonlinearities (e.g. accumulation / melt)
EAE 16ah
What was the climate at Last Glacial Maximum?
3 points.
LGM at 21-26 ka
- ~4-7°C colder
- Sea level 110-125m lower
- Ice sheets ~2-3km thick
EAE 16ai
What has been the climate since LGM?
4 points.
- 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
EAE 16aj
What is a positive feedback loop?
4 points.
- Initial climate forcing action
- Intial climate response
- Response amplified by climate system
- Response becomes new input (i.e. Goes back into 1).
EAE 16ak
What does the symbol ‘‰’ mean?
‰ is “per mil” i.e. per thousand
Just as % is “per cent” i.e. per hundred
EAE 16al
What is δ¹⁸O?
δ¹⁸O is a measure of the departure from a standard reference ratio of ¹⁸O/¹⁶O
EAE 16am
What are typical values for δ¹⁸O?
3 points.
- Surface tropics 0 to -2‰
- Deep ocean +3 to +4‰
- Polar Ice -30 to -55‰
EAE 16an
5 points.
- 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
What is the δ¹⁸O in seawater?
In seawater δ¹⁸O ∝ Temperature of seawater
Planktic forams (upper 100m)
Benthic forams (sea floor)
EAE 16ap
What is a ‘foram’?
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.
EAE 16aq
What is the δ¹⁸O in ice sheets?
In ice sheets δ¹⁸O ∝ Temperature of snow
Other factors also impact such as transport, seasonality, elevation
EAE 16ar
What is a Speleothem?
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
EAE 16as
What is the δ¹⁸O in speleothems?
2 points.
- In speleothems δ¹⁸O ∝ Monsoon strength (precip)
- ¹⁶O higher when lots of evap/precip
EAE 16at
How does the δ¹⁸O change in glacial climates?
3 points.
Ice/Marine responses are opposite; In glacial climates:
- Ice core δ¹⁸O ↓
- Marine δ¹⁸O ↑
EAE 16au
What is obliquity of orbit?
Tilt of the poles relative to the plane of orbit?
Note this is true poles (i.e. axis of rotation) not magnetic poles.
EAE 16av
What is eccentricity of orbit?
4 points.
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)
EAE 16aw
When are the earth’s solstices?
~21 Jun/Dec
EAE 16ax
When are the earth’s equinoxes?
~21 Mar/Sep
EAE 16ay
When is the earth’s perihelion?
4 July
Perihelion = Nearest point of a body’s direct orbit around the Sun
EAE 16az
When is the earth’s Aphelion?
3 January
Aphelion = Furthest point of a body’s direct orbit around the Sun
EAE 16ba
How does the sunlight vary for the earth?
Overall 6% more sunlight reaches Earth at aphelion vs perihelion
EAE 16bb
How does obliquity change?
8 points.
- 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
EAE 16bc
What is the effect of obliquity?
2 points.
- Amplify/suppress seasons
- Biggest impacts at the poles
EAE 16bd
What is the earth’s orbital eccentricity?
4 points.
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
EAE 16be
What is the precession of the equinoxes?
4 points.
- 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
EAE 16bf
What is the precession of the equinoxes?
6 points.
- 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
EAE 16bg
What is the Impact of Eccentricity on Precession?
5 points.
- 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
EAE 16bh
What is solar insolation?
Radiation received at the top of atmosphere
EAE 16bi
What are the long term changes in insolation?
4 points.
- 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
What would have helped the melting along once orbital cycles kick things off?
3 points.
- 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
EAE 16bk
