1. Mechanisms of change: External forcing
External forcing mechanisms concentrated around:
Changes in gravitational effects on fluids on earth (e.g., tides, spring/neap tides (moon circles earth every 29 days))
Changes in the solar constant (i.e., solar energy output – e.g., sun-spot cycle)
Changes in the position of earth relative to sun (e.g., orbital changes, seasons, day-night alternation)

2. The physical basis of climate and climate modelling: report of the international study conference in Stockholm, 29 July - 10 August 1974
Issue 16 of GARP publications series, Global Atmospheric Research ProgramGlobal Atmospheric Research Programme. Joint Organizing Committee, World Meteorological Organization, International Council of Scientific Unions, United Nations Environment Programme

3. External forcing: a very common effect: tides (N. B
External forcing: a very common effect: tides (N.B. not only these short-term effects, but also an 1,800 year periodicity – may be important for palaeo-records)
University of Southampton

4. Seasonal cycle, the cycle controlling the greatest periodic energy changes. Itself only rarely resolved in palaeo-records, but it plays a crucial role in precession cycle
University of Southampton

5. First a reminder on what determines the seasonal cycle
No tilt ==> No seasons!
University of Southampton

6. Tilt of earth’s axis determines differences in time of exposure to the sun and angle of incidence of sunlight
Angle of incidence
through the seasons
University of Southampton

7. More sunspots (cool areas) associated with more faculae (or plages), which are hot areas. Hot areas “win”, giving net result of enhanced solar output at times of sunspot maxima
Sunspot cycle further organised on longer periods (reported: 22, 50+, ~80 years). Most prominent one defines ‘grand minima” - times of persistent low/no sunspot activity that re-occur every ~200 years
Courtesy of NASA: click here for link to original image for the sunspots

8. Cosmogenic isotopes (e. g
Cosmogenic isotopes (e.g.,14C and 10Be) produced by cosmic radiation in upper atmosphere. Elevated solar output ==> more shielding of atmosphere against cosmic radiation ==> lower cosmogenic isotope production
Reduced solar output
Reproduced figure, Data source is NOAA, GISP2 ice core
A (quasi-)periodical cycle of ~2500 year repeated “grand minima” can be recognised during the last 10,000 years.
We do not know at all how these weak fluctuations in the solar output could have become important in the climate system, but it’s one of the main topics in climatology at the moment.

9. Astronomical cycles
The main energy supply to climate is the solar radiation - greenhouse gases merely determine how much of it is retained. Hence, greenhouse studies are only one side of the coin. We need to know how the earth-sun variables affect climate.
1930s - Serbian engineer Milutin Milankovitch develops the first detailed calculation of impacts on insolation per latitude circle of changes in earth-sun orbital parameters, in a bid to explain the Pleistocene ice ages:
eccentricity (main periods ~ 100,000 and 400,000 years)
obliquity (main period of about 41,000 years)
precession (main periods 19,000 and 23,000 years)

10. University of Southampton

11. Precession: the “wobble of the spinning top
University of Southampton

12. Precession (‘wobble):
main periods of impact on insolation:19 and 23 kyrs
Eccentricity (1-6%) and precession have a closely interdependent impact on insolation distribution
University of Southampton

13. University of Southampton

14. The degree of orbital eccentricity “modulates” the impact of precession
Courtesy of Cambridge university press: Burroughs, W.J., 1992, Weather Cycles: Real or Imaginary,
Cambridge University Press, Cambridge, U.K.; New York, U.S.A.

15. Sea-breeze “model” for monsoon variations
University of Southampton

16. Obliquity or Tilt (varies between 22.5 and 24.5 deg) over 41,000 years
Obliquity especially important at high latitudes
Strong tilt  poles receive more sunlight & sun’s rays reach the polar surface at
less shallow angle, decreasing reflection and increasing heat absorption
University of Southampton

17. Orbital parameters can be used to calculate insolation per latitude band, and those insolation records can be compared with relevant observations of climate (proxy) change. This is what Milankovitch pioneered.
Alternatively, the orbital parameters can be compared directly with relevant proxy records.
In both cases, the orbital parameters (with their well-known variations through time) are used as a template for variability, and proxy records can thus be “dated” using the orbitally controlled variations.
This underlies the practice of “orbital tuning” or “astronomical tuning”.

18. Orbital tuning of timescales 1
Orbital cycles and their timings well understood, and they affect insolation distribution. These variations very similar to observed climate fluctuations ==> calculated timings can be used to fine-tune time-frames of observed variations (records) = “orbital tuning”
Reprinted by permission from Macmillan Publishers Ltd: Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.M., Basile, I.,
Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C.,
Pepin, L., Ritz, C., Saltzman, E., Stievenard, M., Climate and atmospheric history of the past 420,000 years from the Vostok ice core,
Antarctica. Nature, v. 399, p Copyright (1999). Not under CC licence.

19. Orbital tuning of timescales 2
Reproduced by permission of American Geophysical Union: Hilgen, F.J., Lourens, L.J., Berger, A.., Loutre, M.F.,
Evaluation of the Astronomically Calibrated Time Scale for the Late Pliocene and Earliest Pleistocene
Paleoceanography, v. 8, no. 5, p. 549–565. October Copyright [1993] American Geophysical Union.

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