Milankovitch Cycles and Climate: Part I – Axial Tilt and Precession

The theory of Milankovitch cycles is named after Serbian astronomer and geophysicist, Milutin Milanković, who in the 1920s postulated three cyclical movement patterns related to Earth’s orbit and rotation and their resultant effects on the Earth’s climate. These cycles include axial tilt (obliquity), elliptical eccentricity, and axial precession. In aggregate, these cycles contribute to profound long term changes in earth’s climate via orbital forcing.

Axial Obliquity: The Earth’s rotational axis is always tilted slightly; currently, its axis is about 23.4 degrees from the vertical. Alternatively, you could say that its equatorial plane is tilted about 23.4 degrees relative to its orbital plane. This tilt is responsible for Earth’s seasons. During the Northern Hemisphere (NH) summer, Earth is further away from the Sun than it is during the NH winter due to its slightly elliptical orbit, yet it receives more sunlight because it’s tilted towards the Sun. During this same time period, the Southern Hemisphere (SH) is tilted away from the Sun, which is why NH summer coincides with SH Winter and vice versa. Contrastingly, during the NH winter, the Earth is closer to the Sun, yet receives less sunlight because it’s tilted away from it. During that same period, the SH is tilted towards the Sun, and is thus experiencing summer.

However, that axial tilt slowly varies between about 22.1 degrees and 24.5 degrees over long quasi-periodic cycles of roughly 41,000 years. The last maximum is estimated to have occurred around 8,700 BCE, and the next minimum should occur roughly around the year 11,800 CE. A more exaggerated tilt corresponds to more severe seasons: warmer summers and colder winters. As you may have guessed, less exaggerated tilt corresponds to milder seasons: cooler summers and warmer winters. The latter phases can lead to increased glaciation. This is because cooler summers mean less ice loss per year, and warmer winters mean more precipitation (rain or snow) to build up ice sheets. Now, you might be wondering why exaggerated tilt wouldn’t build ice sheets with its extra cold winters, but remember that the freezing point of water at 1 ATM of pressure is still going to be 0 degrees Celsius. Reaching negative 50 degrees C in the winter isn’t likely to facilitate much greater glaciation than reaching negative 10 degrees C, and those extra cold winters would involve less precipitation. To add insult to injury, the extra hot summers would melt greater portions of the existing ice each year. That’s why smaller axial tilt values are thought to correspond to increased glaciation and larger tilt values to deglaciation. Moreover, greater surface areas of ice cover can function to resist warming via the ice-albedo feedback (or snow-albedo feedback), which I mentioned briefly in my article on how continental drift affects climate (here and here).

Axial Tilt (photo credit).

Axial Tilt (photo credit).

Axial Precession: At any given obliquity, the direction of the earth’s rotational axis can “wobble” around the vertical in its own cycles (called precession) even while maintaining a more or less constant angle between the rotational axis and the vertical. This is caused by gravitational influences on the earth from the sun and moon. It takes roughly just under 26,000 years for the earth to complete an entire cycle of precession. Estimates differ from different sources, in part due to the fact that the rate of precession is not constant. This is also the reason earth’s axis points either towards Polaris or Vega as the “North Star” roughly every 13,000 years.

Axial Precession.

Axial Precession.

In the contrasting case (i.e. precession in the opposite phase of its current configuration), NH Winters would occur when Earth was furthest away from the sun and summers would occur when it was closest. That would mean extra hot summers, and thus more glacial melting. It would also mean extra cold winters, but those colder winters also correspond to less precipitation. That’s why our current precession should be more conducive to building the NH ice sheets, but the opposite is occurring due to reasons we’ll delve into soon enough. Again, loss of ice also means less help from the ice-albedo feedback effect, which could otherwise help resist further warming. Although axial precession does not affect total annual insolation, it can have a profound effect on where and when that solar energy is distributed, and consequently on the formation or disintegration of ice sheets. Right now, the northern hemisphere is closer to the sun in the NH Winter and further away in the NH Summer. This makes NH Summers less hot and NH Winters less cold than would be the case if Earth were in the opposite phase in its precession cycle. Presumably, the warmer NH Winters should be conducive to more precipitation (snow fall), which would contribute to glaciation, whereas the moderate summers would be conducive to less glacial melting than if the precession were in the opposite configuration from its current phase.

Keep in mind that the magnitude of these seasonal effects also depends on how eccentric our orbit is around the sun, and neither obliquity nor precession affects the total amount of energy coming in from the sun. Their immediate warming or cooling effects are only regional, but regional warming can lead to global warming by altering ocean circulation patterns, redistributing heat throughout the oceans, and consequently causing the oceans to release stored CO2, by decreasing its solubility, which can drive additional warming via the greenhouse effect.

In part II, we’ll look at three other orbital cycles: orbital eccentricity, apsidal precession and orbital inclination. After that, we’ll look at their combined effects on climate in part III, and then discuss the limitations of our current knowledge by examining some unsolved problems regarding the relationship between these cycles and Earth’s glaciation cycles.

References:

Capitaine, N., Wallace, P. T., & Chapront, J. (2003). Expressions for IAU 2000 precession quantities. Astronomy & Astrophysics412(2), 567-586.

Hays, J. D., Imbrie, J., & Shackleton, N. J. (1976, December). Variations in the Earth’s orbit: pacemaker of the ice ages. American Association for the Advancement of Science.

Martin, P., Archer, D., & Lea, D. W. (2005). Role of deep sea temperature in the carbon cycle during the last glacial. Paleoceanography20(2).

Schmittner, A., & Galbraith, E. D. (2008). Glacial greenhouse-gas fluctuations controlled by ocean circulation changes. Nature456(7220), 373-376.

Skinner, L. C., Fallon, S., Waelbroeck, C., Michel, E., & Barker, S. (2010). Ventilation of the deep Southern Ocean and deglacial CO2 rise. Science,328(5982), 1147-1151.

Toggweiler, J. R., Russell, J. L., & Carson, S. R. (2006). Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages.Paleoceanography21(2).

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How Continental Drift affects Climate: Part I – Plate Tectonics, Albedo, and the SnowBall Earth Hypothesis

About 700 – 800 MYA, during the Neoproterozoic era (in the late Precambrian), it has been proposed that the Supercontinent, Rodinia occupied an equatorial position on the Earth. Perhaps counter-intuitively, this tropical arrangement of the continents may have set the stage for a massive glaciation known as “Snowball Earth” during the Cryogenian period, despite equatorial regions receiving the most sunlight due to the orientation of the Earth with respect to the sun. Indeed, the fact that equatorial regions receive the most sunlight is part of the reason why tropical rainforest biomes exist where they do. So how could this have happened? Why would an equatorial continental arrangement render such a glaciation more likely? More on the Snowball Earth hypothesis in a moment: More generally however, what factors are capable of inducing changes in the climate?

There are several of them. First of all though, what do we mean by climate, and how does it differ from weather?

Weather refers to short term local fluctuations in temperature, precipitation, humidity, sunlight, atmospheric pressure, cloud cover and wind patterns, whereas climate refers to the average distribution of weather patterns over longer periods of time (usually decades or longer).

It’s no secret that average global temperatures have increased appreciably since the beginning of the industrial revolution. It’s a subject of endless and vehement political controversy, particularly surrounding the subject of anthropogenic causation via increased atmospheric concentrations greenhouse gases – primarily through the burning of fossil fuels – and what it might mean for policy makers.

Weather station temperature records, paleoclimatological data from ice cores and dendroclimatology (tree ring data), loss of Arctic glaciers and sea ice, and rising sea levels all converge on the conclusion that both land and sea surface temperatures have risen significantly in the last 150 years or so, and are breaking new records even as I type this.

I can delve into the current global warming phenomenon, its causes, why it’s considered a problem, and the public debate associated with it in a later post. Today however, I want to talk about something else. So for now, suffice it to say that global warming is expected to lead to broader climate change more generally. But we know that climate has changed naturally in the past, so what are the factors which can cause it?

There are several variables capable of changing earth’s climate, including the following: Plate tectonics (continental drift), changes in earth’s orbit (Milankovitch cycles), changes in solar output (sun spots and solar flares), volcanism, asteroid, comet and meteoroid impacts, as well as changes in the composition of the atmosphere (i.e. the greenhouse effect).

Plate Tectonics:

Plate tectonics is the scientific theory that the Earth’s outer shell consists of a network of rigid plates which glide slowly over the Earth’s mantle, interact with each other a various ways, and which are responsible for continental drift. The lithosphere includes the Earth’s rigid outer crust as well as the top layer of the mantle. The plates glide over the layer beneath that, a viscoelastic layer known as the asthenosphere, which is elastic and ductile under short term stress, and capable of convection and flow under long term stress. The motion of lithospheric plates results from a combination of influences such as gravity, convection currents and various forces associated with the rotation of the earth.

Tectonic plates can interact by pushing together along convergent boundaries, whereby one plate can slide beneath another other in a process called subduction. They can also pull apart at divergent boundaries; this is what occurs in seafloor spreading along oceanic ridges. They can also undergo lateral motion at what are called transform boundaries, which is when two adjacent plates slide past each other horizontally. Transform boundaries are usually associated with ocean floors where they offset the divergent sea floor spreading at ocean ridges. However, they can also occur on land, such as the San Andreas Fault in California, for example. In actuality, plates can also deform and interact in more complicated ways, but these are the Classical Plate Tectonics interactions covered in your typical 101 treatment.

plate_tectonics

U.S. Geological Survey/ map by Jose F. Vigil

Plate tectonics have been shaping the earth’s geography for billions of years via continental drift, and are responsible for the current layout of the continents (see these animated reconstructions here and here). Continental drift via the movement of tectonic plates can affect earth’s climate by changing the sizes and locations of both land masses and ice caps, and by altering ocean circulation patterns, which are responsible for transporting heat around the earth, which in turn affect atmospheric circulation processes. For instance, changes in continental area at higher latitude can lead to corresponding changes in the area of permanent ice cover, which can lead to what’s called ice-albedo feedback (or snow-albedo feedback). Earth’s albedo is simply the proportion of light from the sun that gets reflected back into the atmosphere. An albedo of 0 corresponds to a perfect black body (total light absorption), whereas an albedo of 1 corresponds to a perfect white body (total reflection).

So how does this relate to the Rodinia Supercontinent, and the Snowball Earth hypothesis? The idea here is that tropical continents reflect more light than Open Ocean, thus absorbing less heat. Today, by contrast, most of the heat is absorbed by the tropical oceans. Since the equator receives more sunlight than other latitudes, that means that a significant portion of incoming sunlight would have been reflected rather than absorbed.

Snow and ice have even higher albedos than rock and soil, so all that was needed was some sort of triggering mechanism to set a runaway ice age effect in motion: cooling leads to more snow and ice, which reflects more light, which leads to more cooling, which leads to more snow and ice, and so on and so forth.

Credit: MIT Artist concept of a planet-wide Ice Age on Earth. Credit: iStockphoto

Credit: MIT Artist concept of a planet-wide Ice Age on Earth. Credit: iStockphoto

I should mention that although the presence of glaciers during the Cryogenian is not disputed, the snowball earth hypothesis is far from settled science. There’s even a less extreme variant that has been dubbed the Slush Ball Earth hypothesis. Regardless of its details, or whether it’s even correct, the point of this section is that a tropical distribution of continents makes such a self-reinforcing feedback loop possible, and it serves as an example of what a dramatic effect continental drift can have on climate change over the long term.

In part 2, I’ll finish up the Rodinia Supercontinent and Snowball Earth example by discussing candidates for a possible triggering mechanism that could have initiated such a reinforcing albedo feedback loop, and discuss how climate can be affected by perhaps a more familiar result of plate tectonics: mountain ranges.

References:

CNRS (Délégation Paris Michel-Ange). (2011, October 12). ‘Snowball Earth’ hypothesis challenged. ScienceDaily. Retrieved October 2, 2016 from

Fairchild, I. J., & Kennedy, M. J. (2007). Neoproterozoic glaciation in the Earth System. Journal of the Geological Society164(5), 895-921.

Hoffman, P. F. (2005). 28th DeBeers Alex. Du Toit Memorial Lecture, 2004. On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record. South African Journal of Geology108(4), 557-577.

Jacobsen, S. B. (2001). Earth science: Gas hydrates and deglaciations.Nature412(6848), 691-693.

Li, Z. and Bogdanova, S. and Collins, A. and Davidson, A. and De Waele, B. and Ernst, R. and Fitzsimons, I. et al. 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research. 160: pp. 179-210.

Riguzzi, F., et al., Can Earth’s rotation and tidal despinning drive plate tectonics? Tectonophysics (2009).

Tsvetsinskaya, E. A., C. B. Schaaf, F. Gao, A. H. Strahler, R. E. Dickinson, X. Zeng, and W. Lucht, Relating MODIS derived surface albedo to soils and landforms over Northern Africa and the Arabian peninsula, Geophys. Res. Lett.,29(9), doi:10.1029/2001GL014096, 2002.

Watson C.S., King M.A., White N.J., Church J.A., Legresy B., & Burgette R.J. (2015). Unabated global mean sea-level rise over the satellite altimeter era. Nature Climate Change,5(6), 565-568. doi:10.1038/nclimate2635

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