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Orbital Forcing Notes

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ESD: Past Climate Change- Orbital Forcing Comprehensive notes on Orbital Forcing and the uncertainties and debates about orbital forcing impact on climate. Contents:

1. Nature of Quaternary Period Climate

2. Orbital Forcing Theory- Theory, Importance of latitude and seasons, Milankovitch Theory

3. Impacts of Orbital Forcing on the Earth System - Insolation, Temperature, Greenhouse Gases, Ice Volume, Monsoon Strength

4. Role of Feedbacks

5. Evidence that Orbital Forcing matches Climate Change

6. Difficulties in understanding Causes - Feedbacks, Non Linear Responses, Lags

1. Nature of Quaternary period climate Nature of Quaternary (Q) period climate:

Climate since the last glaciation The orbital changes produced summer insolation maximum at all latitudes of the northern hemisphere
 An increase in atmospheric CO2 from 190 - 280 ppm and methane levels doubled
 C14 dating has issues because carbon production in the atmosphere isn't produced at that same consistent rate due to solar variations with the N concentration and cosmic rays Younger Dryas- Possible cooling due to thinning of the ice. No analogue vegetation was seen because vegetation had a lagged response.

Orbital forcing:

In the first 2/3rds of the Q (Quaternary)- ice in the Northern Hemisphere advanced/retreated on 42ka cycles= similar temporal scale to changes in TILT
= Low tilt= Cool summer= Ice persists

800ka years ago, these cycles changed to 100ka cycles= Combined impact of Orbital forcing and Milankovitch cycles Dominant 42ka cycles replaced by 100ka cycles under the SAME orbital forcing WHY?
o Explanation probably lies in the ice mass-balance relationship (Ruddiman, 2006). Ruddiman's theory is explained below (remember Ruddiman is a controversial scientist, and there is still uncertainty over the reason for the switch) o 2.75m-0.9m ya = reduced ablation (melting/evaporation) as it was COLD= growth of ice sheets every 41k years, but the next insolation maximum melted most of the ice o After 0.9m y= further cooling and further reduction in ablation=
permitted ice sheets to survive weaker insolation maxima and persist longer so switching them to 100kya (Ruddiman, 2006) o Alternative idea from Elkibbi and Rial (2001)- maybe ocean carbonchemistry, or reorganisation of ocean circulation caused the switch

2. Orbital forcing theory

Theory The variability in climate within the Quaternary is faster than can be accounted for by tectonics. Orbital Forcing Theory suggests that these cycles of the Quaternary are a response to changes in the Earth's orbital configuration. 3 ways in which orbital configuration changes:

1. Eccentricity

2. Axial tilt or obliquity

3. Precession


Eccentricity= the amount the orbit moves away from a perfect circle (e.g. low eccentricity= circular) Dominant 100,000 year cycle, but also larger superimposed cycle of 413,000 year cycle

Eccentricity changes from 0 (almost circular) to 0.06 Impact= affects proximity of Earth to Sun= affects strength insolation in each season (Image Source: Ruddiman 2006)


Axial tilt/obliquity
 Axial tilt= angle of earth's tilt, increasing tilt= more tilted
41, 000 year cycle
 Impact= affects seasonality ie) the difference between summer and winter insolation o HIGH TILT= more extreme difference between the seasons (NH (northern hemisphere) even closer to sun in summer, NH even further from sun in winter- imagine a bigger tilt in picture)= Warmer summer, colder winter (for BOTH HEMISPHERES, but greater effect at HIGHER LATITUDES) o Milankovitch= low tilt= cooler summer
 Dominant effect on high latitudes- greater impact as this is most tilted towards sun or away..
 If compare within one point in time, NH and SH are out of phase- high tilt that turns NH toward Sun in NH summer, turns SH away from Sun (winter)= tilt causes opposite insolation effects at the North and South for a given point in the orbit (Ruddiman, 2006)


Precession includes precession of the axis (wobble of the axis) and precession of the ellipse (shift of shortest distance/perihelion. Impact = cyclic change in where on the orbit the seasons occur Axial precession= 25.7ka cycle Precession of the equinoxes = 23 ka cycle o Perihelion at December solstice= shorter distance in December=
warmer winter (cooler summer)

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