The main focus in understanding Glacial cycles has been on the maximum summer insolation at 65N, yet prior to MPT (Mid Pleistocene Transition) all glaciations simply followed the obliquity cycle. To understand why obliquity is the underlying metronome, we need to consider the integrated summer insolation. Increases in the earth’s tilt boost insolation at both poles. The precession of the equinoxes only affects maximum summer insolation at one pole at a time because the earth has an elliptical orbit. Once every 23ooo years the Northern hemisphere summer coincides with perihelion and maximum insolation occurs at the summer equinox (June 21). Eccentricity of the earth’s orbit is the crucial factor in modulating just how strong this maximum value becomes, because eccentricity determines the distance at perihelion between the earth and the sun. If the earth’s orbit were circular then precession would have no effect at all. When eccentricity is low like it is today, then the effect is small , but 220,000 years ago a much larger eccentricity increased NH summer maximum insolation by 4 times greater than today. Despite this varying precession/eccentricity effect I would argue that the underlying influence on summer insolation is still obliquity, because it affects both poles equally. If obliquity were zero there would be no seasons on earth, while larger obliquity increases the size of climate zones.
Figure 1a compares the relative strength of summer insolation at both poles with the modulating eccentricity cycle over the last 600,000 years. If you simply average the North and South maximum insolation together then you can see a perfect obliquity signal – the red dashed curve. This is what insolation would be at both poles if the earth’s orbit were perfectly circular.
Figure1. A) Calculations of insolation at both poles, their average, and the integrated insolation over a full year. B) Ice Volume data(LRO4) compared to a parameterisation of obliquity and eccentricity. Click to expand.
The solid red curve in Figure 1a is the integral of annual insolation received at the North Pole (~5GJ/m2). The precession signal now vanishes! Total summer insolation simply follows obliquity at both poles. The reason for this is because angular momentum is conserved during an elliptical orbit. This means that the earth speeds up near perihelion and slows down at aphelion. Perihelion summers may have a high insolation on June 21st but as a result these summers can be up to 20 days shorter than aphelion summers! Currently the SH summer is about 4 days shorter than the NH summer. The end result of this effect is that the total summer energy received from the sun in either hemisphere does not depend on precession at all. It depends only on obliquity.
The dashed red curve in Figure 1a is the average insolation over both poles. Raymo et al (1) have proposed that prior to the MPT, the Earth was warm enough that cycles in ice volume for the Northern and the Southern hemisphere were out of phase and acted independently, depending only on local solar input. So the global effect on ice volume was the average of the two hemispheres and this caused ice volume to follows obliquity (Figure 2). This was possible before MPT because the East Antarctic Ice sheet melted back to land each cycle and so contributing to global ice volume. As the earth cooled beyond MPT so Antarctic Ice permanently saturated the continent shelf, breaking the N-S symmetry and Ice ages then became dominated by NH insolation alone. Another possibility proposed by Huybers (2) is simply that total summer insolation has always driven glacial cycles before MTP and even today is the dominant term.
Figure 2: Obliquity dominated glacial cycles before MPT: 900-1500 ya
Figure 1a shows that eccentricity modulates the NH summer maximum. Figure 1b shows the Ice Volume data LRO4 from 600,000 years ago. Can eccentricity and obliquity alone describe post MTP ice ages, or is 65N insolation now the driver? The blue curve in figure 1B is simply a combination of eccentricity and obliquity, both inverted to match ice volume. The curve is not based on any physics, but is simply chosen ad hoc. Here it is.
Surprisingly the main features of the last 6 glacial cycles in Figure 1b are reasonably well reproduced by this simple formula. This demonstrates how eccentricity still plays the hidden role after MPT by modulating the NH summer maximum insolation. Yet mysteriously only some NH summer maxima significantly reduce ice volume.
However, none of this yet explains why the deepest glaciations, which always occur at low eccentricity, suddenly end with a bang, even though NH summer maximum insolation maxima remain small. The LGM is a classic example.
Figure 3. Last Glacial Cycle. Note how larger NH insolation peaks before LGM had little effect on Ice Volume, although both Antarctic temperatures and CO2 both recovered slightly. The correlation of low CO2 with dust deposits in the ice however is striking. The largest deposits are just before rising temperatures ended the last glacial cycle.
The next glaciation also looks to be even more severe than the last one as eccentricity falls to an all time low, last seen 2.8 million years ago! You can also see the 400 ky eccentricity cycle as well below in Figure 4.
Figure 4. LA2010 calculations of the earth’s eccentricity over a 4 million year period spanning the present day.
Glacial termination at low eccentricity
So what causes the deepest glaciations like the LGM to terminate? I think the most promising proposal so far is that increased Dust due to CO2 starvation reduces albedo and primes the ice sheets for rapid melting at the next NH summer maximum. This idea was originally proposed by Ralf Ellis(3). As low eccentricity glaciations deepen, so sea levels drop and CO2 levels fall below 200ppm. This CO2 starvation, combined with arid conditions causes boreal forests and vegetation to die back inducing dust storms that cover the ice sheets with dust over thousands of years. This occurs especially near their southern edges.
Figure 5. The last 800,000 years of glacial cycles. The current interglacial is most similar to that 400,000 years ago. Click to expand
During the LGM CO2 levels reached dangerously low levels of ~180 ppm causing arid desertification as temperate trees and savannah died off. The resulting dust storms then deposited huge amounts of dust onto the ice sheets increasing its albedo. The consequent NH summer insolation maximum, coinciding with maximum eccentricity, finally melted back the ice sheets through reduced albedo, aided by increasing CO2 and H2O feedbacks.
When all else fails GAIA ends ice ages !
References
- (Raymo, Maureen & Lisiecki, Lorraine & Nisancioglu, Kerim. (2006). Plio-Pleistocene Ice Volume, Antarctic Climate, and the Global 18O Record. Science (New York, N.Y.). 313. 492-5. 10.1126/science.1123296)
- Peter Huybers, Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing, Science June 2006
- Ralf Ellis, Michael Palmer, Modulation of ice ages via precession and dust-albedo feedbacks, Geoscience Frontiers, Volume 7, Issue 6, November 2016, Pages 891-909
