Lecture 34: Orbital (Milankovitch) Theory of the Ice Ages During the Quaternary, climate alternated between glacial and interglacial states, but we still don’t completely understand many of the details about how this occurred.

warmer, less ice colder, more ice Interglacial Glacial

Orbital forcing of Earth’s climate Changes in Earth’s orbital geometry (eccentricity, tilt, precession) Changes in the Earth's orbit can be precisely calculated using principles of celestial mechanics. Changes result from gravitational interactions between the Earth, the moon, and the larger planets in our solar system. Changes in Earth’s orbital geometry give rise to changes in the seasonal distribution of insolation as a function of latitude. The total insolation averaged over the year doesn’t change much, but the seasonal distribution does. These changes in solar radiaiton influence the growth and decay of glaciers that, together with other feedback processes (e.g., greenhouse gases) results in glacial-interglacial cycles. Changes in the seasonal distribution of Insolation (heat) as a function of latitude Amplified by other processes CO2, albedo feedback Glacial-interglacial climate change

James Croll Scottish (1821-1890) Millwright, carpenter, tea shopkeeper, electrical sales, hotelkeeper, insurance salesman, janitor, Geologic Survey, Fellow of the Royal Society In the 19th century, James Croll developed a theory of the effects of variations of the earth's orbit on climate cycles. His idea was that decreases in winter sunlight would favour snow accumulation. He was the first to recognize the importance of a positive ice-albedo feedback to amplify solar variations. decreases in winter radiation would favor snow accumulation, coupled this to the idea of a positive ice-albedo feedback to amplify the solar variations.

Milutin Milankovitch, Serbian mathematician (1879-1958) 1941, “Canon of Insolation of the Earth and Its Application to the Problem of the Ice Ages,” 626 pages improved upon Croll's work partly by making more precise calculations (all done by hand!) Emphasized the importance of decreases in summer radiation which favors snow accumulation and glacial advance Milutin Milankovitch improved upon Croll's work partly by the use of improved calculations of the earth's orbit then recently published by Ludwig Pilgrim in 1904. In 1941, Milankovitch published his great work “Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitenproblem” (Canon of Insolation of the Earth and Its Application to the Problem of the Ice Ages) was completed, 626 pages Milankovitch correctly hypothesized that summer radiation is the determining factor whether ice sheets grow or melt. It is always cold enough in winter for snow to be preserved, but summer temperature determines whether the snow from the previous season will be preserved or will melt.

The equilibrium line of a glacier is the location where accumulation equals ablation If accum>ablation, glacier advances If ablation>accum glacier retreats Changes in summer insolation controls the position of the equilibrium line and determines whether a glacier has a net positive or negative mass balance. Decreased summer insolation lowers the equilibrium line and glacier advances.

in northern hemisphere Cool summers in northern hemisphere 60 65 70 Latitude of equivalent insolation 75 300 250 200 150 100 50 Milankovitch (1941) predicted that glaciations should correspond with summer insolation minima. Unfortunately, the known history of glaciations was too coarse in 1941 to provide an adequate test of Milankovitch’s hypothesis. The hypothesis was shelved for more than 30 years. 550 500 450 400 350 300 600

Earth’s orbital parameters that affect incoming solar radiation: Precessions (seasonal distribution) Obliquity (seasonal distribution) Eccentricity (only parameter to affect total radiation but effects are small)

Eccentricity grossly exaggerated 152.5 x 106 km 147.5 x 106 km July

Obliquity is responsible for seasons Obliquity is the tilt of the Earth’s rotational axis relative to the plane of the ecliptic. Obliquity, which is presently at 23.5o, is responsible for seasonality.

Obliquity Current value: 23.5o Range: 22o-24.5o Period: 41,000 yrs Obliquity varies from a minimum of 22.1o to a maximum of 24.5 o with a period of 41,000 years. Current value: 23.5o Range: 22o-24.5o Period: 41,000 yrs

Effect of Obliquity on Insolation difference in obliquity from 22 to 24.5o with other parameters held at present values This figure shows changes in the seasonal contrast of insolation in W m-2 caused by an increase in the obliquity from 22.0o to 24.5o with eccentricity and precession set at present values. This represents the maximum effect of obliquity. Note that the high latitudes are affected more than the low latitudes. Obliquity is in phase between the two hemisphere for the same season. For example, an increase in obliquity will result in more summer insolation for both the northern and southern hemipshere polar regions. Effect on insolation is greatest at high latitudes Same sign for respective summer season (hemispheric response is in phase).

varies at a period of 41 kyrs. The top figure shows variations in obliquity for the last million years along with its affect on insolation at the poles and equator. Note the effect is greater at the poles than the equator. Below is shown the periods of variation. By far the dominant period is 41,0000 years. varies at a period of 41 kyrs. Obliquity frequency [1/ky] Period [ky] Amplitude 0.02439 40.996 0.011168 0.02522 39.657 0.004401 0.02483 40.270 0.003010 0.01862 53.714 0.002912 0.03462 28.889 0.001452

Eccentricity Current value: 0.017 Range: 0-0.06 p The eccentricity of the Earth’s orbit is currently 0.017. In the past, it has varied near 0 (circular) to 0.06. a - p Current value: 0.017 Range: 0-0.06 Period(s): ~100,000 yrs ~400,000 yrs Eccentricity = a + p a = apehlion distance p = perihelion distance

Eccentricity changes the total insolation received by the Earth but the difference is small! 0.5 W m-2/342.7 W m-2 = 0.15% Variations in eccentricity for the past million years. Eccentricity changes the global mean insolation but only by a small amount (~0.5 W m-2 between max and min eccentricity values) The power spectrum of eccentricity is complex with dominant periods at ~406, 95 and 123 kyrs. Dominant periods are at ~400 and near 100 kyrs Eccentricity frequency [1/ky] Period [ky] Amplitude 0.00246 406.182 0.010851 0.01055 94.830 0.009208 0.00807 123.882 0.007078 0.01014 98.607 0.005925 0.00769 130.019 0.005295

Precession (axial) Precession is the “wobble” of the earth’s rotational axis with time.

Axial Precession Precession of the axis of the earth Year: The projection of the earth’s rotational axis maps a circle in space that takes about 25,700 years to complete. This means the north star (Polaris presently) changes through time.

Precession of the Ellipse This animation shows precession of the ellipse at constant eccentricity. Elliptical shape of Earth’s orbit rotates Precession of ellipse is slower than axial precession Both motions shift position of the solstices and equinoxes

Today The effect of precession is to change the time (season) of year of perihelion and aphelion. If perihelion occurs during northern hemisphere winter as it does today, then winters will be slightly milder. Similarly, when aphelion occurs in northern hemisphere summer, summers will be a little cooler.

Precession of the Equinoxes Earth’s wobble and rotation of its elliptical orbit produce precession of the solstices and equinoxes One cycles takes 23,000 years Simplification of complex angular motions in three-dimensional space The effect of axial and elliptical precession is to change the relation of the equinoxes and solstices relative to perhelion and aphelion. This is also known as precession of the equinoxes.

Extreme Solstice Positions Today June 21 solstice is near aphelion (July 4) Solar radiation a bit lower making summers a bit cooler Configuration reversed ~11,500 years ago Precession moves June solstice to perihelion Solar radiation a bit higher, summers are warmer Assumes no change in eccentricity For example, perihelion (Jan 4) occurs close to winter solstice today (Dec. 21). 11,500 years ago, perihelion coincided with summer solstice and aphelion occurred during winter solstice.

maximum value (boreal summer) Effect of precession from its minimum value (boreal winter at perihelion) to its maximum value (boreal summer) Effect of precession from its minimum value (boreal winter at perihelion) to its maximum value (boreal summer at perihelion) with present-day values for eccentricity and obliquity. Contour interval is 10 W m-2. The brown areas correspond to zone with a zero insolation. Note that precession affects all latitudes and the sign is opposite for northern and southern hemipsheres for the same season. E.g., when northern hemisphere summer is warm, southern hemisphere summer is cool. The hemispheres are out of phase for precession. Precession affects insolation at both high and low latitudes. Opposite sign in northern and southern hemispheres for respective season (out of phase)

Dominant periods at ~19, 22 and 24 kyrs Precession Precession for the past million years. Note that precession affects incoming solar radiation at all latitudes. The dominant frequencies are at ~19, 22 and 24 kyrs. Often these are not distinguishable in paleoclimate records and precession is expressed as a broad peak centered on ~21 kyrs. Dominant periods at ~19, 22 and 24 kyrs Precession frequency [1/ky] Period [ky] Amplitude 0.04221 23.690 0.018839 0.04467 22.385 0.016981 0.05275 18.956 0.014792 0.05236 19.097 0.010121 0.04326 23.114 0.004252

Eccentricity modulates precession The amplitude of the precessional cycle is modulated by eccentricity. If eccentricity is 0, there is no difference between perihelion and aphelion so the affect of precession is minimized. The greater the eccentricity, the larger the effect of precession on seasonal insolation.

today The three orbital parameters (eccentricity, obliquity, precession) for the last million years and 100 kyrs into the future.

Difference in Northern Hemisphere insolation at summer solstice relative to today How do orbital parameters combine to produce changes in insolation? Visualization of Northern Hemisphere insolation anomaly (relative to today) at summer solstice from 25,000 years (LGM) to 10,000 (early Holocene). Note large negative anomaly 25,000 year ago and large positive anomaly at 11,000 yrs bp in the early Holocene.

Insolation anomaly at 10 ka relative to present Deviations from present-day values of the calendar 24h mean solar irradiance (daily insolation) around 10 000 years ago (i.e. at 10ka BP) (in Wm-2).

Ice Growth (glacial) Configuration (cool summers) Low obliquity (low seasonal contrast) High eccentricity and NH summers during aphelion (cold summers in the north) Glaciation can only develop if the summer high northern latitudes are cold enough to prevent the winter snow from melting, thereby allowing a positive annual balance of snow and ice. Ice growth occurs during cool summers. What orbital configuration corresponds to cool summers? Minimum tilt, June 21 aphelion, and highly eccentric orbit

Ice Decay (Deglaciation) High obliquity (high seasonal contrast) High eccentricity and NH summers during perihelion (hot summers in the north) Ice decay configuration would correspond to minimum northern hemisphere summer temperatures. High obliquity, perihelion during summer, and high eccentricity.

Classic Milankovitch Forcing: Insolation at 65°N, June 21 Ice decay Insolation forcing at 65oN on summer solstice for the past 400 kyrs. Milankovitch theory predicts ice growth when summer insolation is low, and ice decay when it is high. Ice growth

Milankovitch Theory Revived The Milankovitch Theory was not well received until the early 1970s when investigations of the deep-sea sediments brought widespread acceptance of Milanković's view, because the periodicities observed in the marine record matched the predictions by Milankovitch. Pacific deep core V28-238 Shackelton & Opdyke, 1972

Variations in the Earth's Orbit: Pacemaker of the Ice Ages J. D. Hays, John Imbrie, N. J. Shackleton Science, 194, No. 4270, (Dec. 10, 1976), pp. 1121-1132 (eccentricity) (obliquity) (precession) The “smoking gun” came in a publication to Science in 1976 by Jim Hays, John Imbrie and Nick Shackleton (pictured from right to left). Marine oxygen isotope record shows the same periodicities predicted by orbital forcing.

Problems for the Milankovitch Theory 1. 100,000-year problem So many problems 2. The Mid Pleistocene transition problem 3. Stage 11 (Termination V) problem 4. Causality Problem: Timing of Term II Although orbital forcing of Earth’s climate is well accepted, the details of how orbitally-induced insolation changes affect climate are debated. Several challenges have been raised to the “Classical” Milankovitch Theory that summer insolation at 65oN is the main driver. Three problems with classical astronomical theory: Cyclicity is close to 100 kyrs but there is insignificant changes in radiative forcing at this period (100-kyr problem) Mid Pleistocene transition change from 41 to 100 kyr periodicity occurred without major changes in orbital forcing Stage 11 problem, the most prominent glacial-interglacial transition of the Pleistocene (Term V) occurs at a time of minimal forcing Causality problem -- supposed effect occurs before the casue Milutin Milankovitch 1879-1958

I II III IV V VI Ice decay Ice growth The oxygen isotope record (ice volume) doesn’t look exactly like the insolation forcing so clearly other factors must come into play. Comparison between (a) the SPECMAP stacked δ18O composite and (b) insolation curve for 65°N, June 21. The vertical red lines denote the Terminations (glacial-to-interglacial transitions). Ice growth

Power spectrum of June Insolation at 65oN precession Power spectrum of June insolation at 65oN (classical Milankovitch forcing). Note the obliquity and precessional frequencies, but absence of eccentricity at 100 kyrs. obliquity Note absence of 100 Kyr power

Insolation Benthic 18O 65oN 100 ky 41 ky 23 ky The power spectrum for oxygen isotopes looks very different from that of insolation for the past 450 kyrs. It is dominated by 100-kyr (eccentricty) power, 41-kyr obliquity and little precessional power.

? 1. The 100-kyr Problem Forcing small big Response Classic Milankovitch forcing (might consider alternatives) Forcing small Why does the climate system have so much 100-kyr power when the supposed forcing does not? Traditionally explained by non-linear response involving internal feedbacks. ? big Response Why does the climate system have so much 100-kyr power when the forcing is so small?

2. The Middle Pleistocene Transition Problem 32 16 The MPT is very apparent in power spectra of the benthic d18O record of the last 2 million years. Note that there is no 100-kyr power in the early Pleistocene and the spectra is dominated by obliquity. This has been referred to as the so-called “41-kyr” world. The power spectrum of the late Pleistocene is dominated by 100-kyr cycle. Note however that the power of the 41-kyr cycle remains about the same as during the early Pleistocene. Schulz and Zeebe (2006)

Insolation (June 65oN ) Milankovitch Forcing According to Classic Milankovitch theory, glacial-interglacial cycles are controlled by summer insolaiton at 65oN that controls The power spectra for the forcing (June 65oN insolation) according to classic Milankovitch theory has a very different character than the benthic d18O record. It is dominated by precession with some 41-kyr power, bu contains no 100-kyr power. There is also no apparent change in orbital forcing (I.e., summer insolation 65oN) across the MPT. This suggests that the MPT must be driven by processes internal to the climate system Schulz and Zeebe (2006) 100 kyr 100 kyr

? 100-kyr world 18O 41-kyr world Insolation Comparison of power spectra of benthic d18O and insolation (June 65oN) before and after the MPT. ?

3. Stage 11/Termination V Problem II III IV V VI Ice decay 11 Strong Response 12 The Stage 11 problem refers to the fact that insolation forcing is very weak about 400,000 years ago, yet termination V (transition from MIS 12 to 11) represents one of the largest deglaciations of the late Pleistocene. How can the response be so strong with such a weak forcing? Weak Forcing Ice growth

sea level begins to rise before boreal summer insolation 4. Causality Problem sea level begins to rise before boreal summer insolation Termination II/Stage 5 Problem The “causality problem” refers to the timing of the start of deglaciaiton relative to boreal summer insolation forcing. U/Th of corals from Tahiti drilling clearly shows the rise in sea level at Termination II began at about 142,000 yrs BP, significantly earlier than the rise in boreal summer insolation. A. L. Thomas, G. M. Henderson, et al., 2009. Penultimate Deglacial Sea-Level Timing from U/Th Dating of Tahitian Corals. Science doi:10.1126/science.1168754 (23 April) 2009